.::pth(3)pthsem Portable Threads .::pth(3)NAMEpthsem - GNU Portable Threads
VERSIONpthsem 2.0.8 based on GNU Pth
SYNOPSIS
Global Library Management
pth_init, pth_kill, pth_ctrl, pth_version.
Thread Attribute Handling
pth_attr_of, pth_attr_new, pth_attr_init, pth_attr_set,
pth_attr_get, pth_attr_destroy.
Thread Control
pth_spawn, pth_once, pth_self, pth_suspend, pth_resume, pth_yield,
pth_nap, pth_wait, pth_cancel, pth_abort, pth_raise, pth_join,
pth_exit.
Utilities
pth_fdmode, pth_time, pth_timeout, pth_int_time, pth_sfiodisc.
Cancellation Management
pth_cancel_point, pth_cancel_state.
Event Handling
pth_event, pth_event_typeof, pth_event_extract, pth_event_concat,
pth_event_isolate, pth_event_walk, pth_event_status,
pth_event_free.
Key-Based Storage
pth_key_create, pth_key_delete, pth_key_setdata, pth_key_getdata.
Message Port Communication
pth_msgport_create, pth_msgport_destroy, pth_msgport_find,
pth_msgport_pending, pth_msgport_put, pth_msgport_get,
pth_msgport_reply.
Thread Cleanups
pth_cleanup_push, pth_cleanup_pop.
Process Forking
pth_atfork_push, pth_atfork_pop, pth_fork.
Synchronization
pth_mutex_init, pth_mutex_acquire, pth_mutex_release,
pth_rwlock_init, pth_rwlock_acquire, pth_rwlock_release,
pth_cond_init, pth_cond_await, pth_cond_notify, pth_barrier_init,
pth_barrier_reach.
Semaphore support
pth_sem_init, pth_sem_dec, pth_sem_dec_value, pth_sem_inc,
pth_sem_inc_value, pth_sem_set_value, pth_sem_get_value.
User-Space Context
pth_uctx_create, pth_uctx_make, pth_uctx_switch, pth_uctx_destroy.
Generalized POSIX Replacement API
pth_sigwait_ev, pth_accept_ev, pth_connect_ev, pth_select_ev,
pth_poll_ev, pth_read_ev, pth_readv_ev, pth_write_ev,
pth_writev_ev, pth_recv_ev, pth_recvfrom_ev, pth_send_ev,
pth_sendto_ev.
Standard POSIX Replacement API
pth_nanosleep, pth_usleep, pth_sleep, pth_waitpid, pth_system,
pth_sigmask, pth_sigwait, pth_accept, pth_connect, pth_select,
pth_pselect, pth_poll, pth_read, pth_readv, pth_write, pth_writev,
pth_pread, pth_pwrite, pth_recv, pth_recvfrom, pth_send,
pth_sendto.
DESCRIPTION
____ _ _
| _ \| |_| |__
| |_) | __| '_ \ ``Only those who attempt
| __/| |_| | | | the absurd can achieve
|_| \__|_| |_| the impossible.''
Pth is a very portable POSIX/ANSI-C based library for Unix platforms
which provides non-preemptive priority-based scheduling for multiple
threads of execution (aka `multithreading') inside event-driven
applications. All threads run in the same address space of the
application process, but each thread has its own individual program
counter, run-time stack, signal mask and "errno" variable.
The thread scheduling itself is done in a cooperative way, i.e., the
threads are managed and dispatched by a priority- and event-driven non-
preemptive scheduler. The intention is that this way both better
portability and run-time performance is achieved than with preemptive
scheduling. The event facility allows threads to wait until various
types of internal and external events occur, including pending I/O on
file descriptors, asynchronous signals, elapsed timers, pending I/O on
message ports, thread and process termination, and even results of
customized callback functions.
Pth also provides an optional emulation API for POSIX.1c threads
(`Pthreads') which can be used for backward compatibility to existing
multithreaded applications. See Pth's pthread(3) manual page for
details.
Threading Background
When programming event-driven applications, usually servers, lots of
regular jobs and one-shot requests have to be processed in parallel.
To efficiently simulate this parallel processing on uniprocessor
machines, we use `multitasking' -- that is, we have the application ask
the operating system to spawn multiple instances of itself. On Unix,
typically the kernel implements multitasking in a preemptive and
priority-based way through heavy-weight processes spawned with fork(2).
These processes usually do not share a common address space. Instead
they are clearly separated from each other, and are created by direct
cloning a process address space (although modern kernels use memory
segment mapping and copy-on-write semantics to avoid unnecessary
copying of physical memory).
The drawbacks are obvious: Sharing data between the processes is
complicated, and can usually only be done efficiently through shared
memory (but which itself is not very portable). Synchronization is
complicated because of the preemptive nature of the Unix scheduler (one
has to use atomic locks, etc). The machine's resources can be exhausted
very quickly when the server application has to serve too many long-
running requests (heavy-weight processes cost memory). And when each
request spawns a sub-process to handle it, the server performance and
responsiveness is horrible (heavy-weight processes cost time to spawn).
Finally, the server application doesn't scale very well with the load
because of these resource problems. In practice, lots of tricks are
usually used to overcome these problems - ranging from pre-forked sub-
process pools to semi-serialized processing, etc.
One of the most elegant ways to solve these resource- and data-sharing
problems is to have multiple light-weight threads of execution inside a
single (heavy-weight) process, i.e., to use multithreading. Those
threads usually improve responsiveness and performance of the
application, often improve and simplify the internal program structure,
and most important, require less system resources than heavy-weight
processes. Threads are neither the optimal run-time facility for all
types of applications, nor can all applications benefit from them. But
at least event-driven server applications usually benefit greatly from
using threads.
The World of Threading
Even though lots of documents exists which describe and define the
world of threading, to understand Pth, you need only basic knowledge
about threading. The following definitions of thread-related terms
should at least help you understand thread programming enough to allow
you to use Pth.
o process vs. thread
A process on Unix systems consists of at least the following
fundamental ingredients: virtual memory table, program code, program
counter, heap memory, stack memory, stack pointer, file descriptor
set, signal table. On every process switch, the kernel saves and
restores these ingredients for the individual processes. On the other
hand, a thread consists of only a private program counter, stack
memory, stack pointer and signal table. All other ingredients, in
particular the virtual memory, it shares with the other threads of
the same process.
o kernel-space vs. user-space threading
Threads on a Unix platform traditionally can be implemented either
inside kernel-space or user-space. When threads are implemented by
the kernel, the thread context switches are performed by the kernel
without the application's knowledge. Similarly, when threads are
implemented in user-space, the thread context switches are performed
by an application library, without the kernel's knowledge. There also
are hybrid threading approaches where, typically, a user-space
library binds one or more user-space threads to one or more kernel-
space threads (there usually called light-weight processes - or in
short LWPs).
User-space threads are usually more portable and can perform faster
and cheaper context switches (for instance via swapcontext(2) or
setjmp(3)/longjmp(3)) than kernel based threads. On the other hand,
kernel-space threads can take advantage of multiprocessor machines
and don't have any inherent I/O blocking problems. Kernel-space
threads are usually scheduled in preemptive way side-by-side with the
underlying processes. User-space threads on the other hand use either
preemptive or non-preemptive scheduling.
o preemptive vs. non-preemptive thread scheduling
In preemptive scheduling, the scheduler lets a thread execute until a
blocking situation occurs (usually a function call which would block)
or the assigned timeslice elapses. Then it detracts control from the
thread without a chance for the thread to object. This is usually
realized by interrupting the thread through a hardware interrupt
signal (for kernel-space threads) or a software interrupt signal (for
user-space threads), like "SIGALRM" or "SIGVTALRM". In non-preemptive
scheduling, once a thread received control from the scheduler it
keeps it until either a blocking situation occurs (again a function
call which would block and instead switches back to the scheduler) or
the thread explicitly yields control back to the scheduler in a
cooperative way.
o concurrency vs. parallelism
Concurrency exists when at least two threads are in progress at the
same time. Parallelism arises when at least two threads are executing
simultaneously. Real parallelism can be only achieved on
multiprocessor machines, of course. But one also usually speaks of
parallelism or high concurrency in the context of preemptive thread
scheduling and of low concurrency in the context of non-preemptive
thread scheduling.
o responsiveness
The responsiveness of a system can be described by the user visible
delay until the system responses to an external request. When this
delay is small enough and the user doesn't recognize a noticeable
delay, the responsiveness of the system is considered good. When the
user recognizes or is even annoyed by the delay, the responsiveness
of the system is considered bad.
o reentrant, thread-safe and asynchronous-safe functions
A reentrant function is one that behaves correctly if it is called
simultaneously by several threads and then also executes
simultaneously. Functions that access global state, such as memory
or files, of course, need to be carefully designed in order to be
reentrant. Two traditional approaches to solve these problems are
caller-supplied states and thread-specific data.
Thread-safety is the avoidance of data races, i.e., situations in
which data is set to either correct or incorrect value depending upon
the (unpredictable) order in which multiple threads access and modify
the data. So a function is thread-safe when it still behaves
semantically correct when called simultaneously by several threads
(it is not required that the functions also execute simultaneously).
The traditional approach to achieve thread-safety is to wrap a
function body with an internal mutual exclusion lock (aka `mutex').
As you should recognize, reentrant is a stronger attribute than
thread-safe, because it is harder to achieve and results especially
in no run-time contention between threads. So, a reentrant function
is always thread-safe, but not vice versa.
Additionally there is a related attribute for functions named
asynchronous-safe, which comes into play in conjunction with signal
handlers. This is very related to the problem of reentrant functions.
An asynchronous-safe function is one that can be called safe and
without side-effects from within a signal handler context. Usually
very few functions are of this type, because an application is very
restricted in what it can perform from within a signal handler
(especially what system functions it is allowed to call). The reason
mainly is, because only a few system functions are officially
declared by POSIX as guaranteed to be asynchronous-safe.
Asynchronous-safe functions usually have to be already reentrant.
User-Space Threads
User-space threads can be implemented in various way. The two
traditional approaches are:
1. Matrix-based explicit dispatching between small units of execution:
Here the global procedures of the application are split into small
execution units (each is required to not run for more than a few
milliseconds) and those units are implemented by separate functions.
Then a global matrix is defined which describes the execution (and
perhaps even dependency) order of these functions. The main server
procedure then just dispatches between these units by calling one
function after each other controlled by this matrix. The threads are
created by more than one jump-trail through this matrix and by
switching between these jump-trails controlled by corresponding
occurred events.
This approach gives the best possible performance, because one can
fine-tune the threads of execution by adjusting the matrix, and the
scheduling is done explicitly by the application itself. It is also
very portable, because the matrix is just an ordinary data
structure, and functions are a standard feature of ANSI C.
The disadvantage of this approach is that it is complicated to write
large applications with this approach, because in those applications
one quickly gets hundreds(!) of execution units and the control flow
inside such an application is very hard to understand (because it is
interrupted by function borders and one always has to remember the
global dispatching matrix to follow it). Additionally, all threads
operate on the same execution stack. Although this saves memory, it
is often nasty, because one cannot switch between threads in the
middle of a function. Thus the scheduling borders are the function
borders.
2. Context-based implicit scheduling between threads of execution:
Here the idea is that one programs the application as with forked
processes, i.e., one spawns a thread of execution and this runs from
the begin to the end without an interrupted control flow. But the
control flow can be still interrupted - even in the middle of a
function. Actually in a preemptive way, similar to what the kernel
does for the heavy-weight processes, i.e., every few milliseconds
the user-space scheduler switches between the threads of execution.
But the thread itself doesn't recognize this and usually (except for
synchronization issues) doesn't have to care about this.
The advantage of this approach is that it's very easy to program,
because the control flow and context of a thread directly follows a
procedure without forced interrupts through function borders.
Additionally, the programming is very similar to a traditional and
well understood fork(2) based approach.
The disadvantage is that although the general performance is
increased, compared to using approaches based on heavy-weight
processes, it is decreased compared to the matrix-approach above.
Because the implicit preemptive scheduling does usually a lot more
context switches (every user-space context switch costs some
overhead even when it is a lot cheaper than a kernel-level context
switch) than the explicit cooperative/non-preemptive scheduling.
Finally, there is no really portable POSIX/ANSI-C based way to
implement user-space preemptive threading. Either the platform
already has threads, or one has to hope that some semi-portable
package exists for it. And even those semi-portable packages usually
have to deal with assembler code and other nasty internals and are
not easy to port to forthcoming platforms.
So, in short: the matrix-dispatching approach is portable and fast, but
nasty to program. The thread scheduling approach is easy to program,
but suffers from synchronization and portability problems caused by its
preemptive nature.
The Compromise of Pth
But why not combine the good aspects of both approaches while avoiding
their bad aspects? That's the goal of Pth. Pth implements easy-to-
program threads of execution, but avoids the problems of preemptive
scheduling by using non-preemptive scheduling instead.
This sounds like, and is, a useful approach. Nevertheless, one has to
keep the implications of non-preemptive thread scheduling in mind when
working with Pth. The following list summarizes a few essential points:
o Pth provides maximum portability, but NOT the fanciest features.
This is, because it uses a nifty and portable POSIX/ANSI-C approach
for thread creation (and this way doesn't require any platform
dependent assembler hacks) and schedules the threads in non-
preemptive way (which doesn't require unportable facilities like
"SIGVTALRM"). On the other hand, this way not all fancy threading
features can be implemented. Nevertheless the available facilities
are enough to provide a robust and full-featured threading system.
o Pth increases the responsiveness and concurrency of an event-driven
application, but NOT the concurrency of number-crunching
applications.
The reason is the non-preemptive scheduling. Number-crunching
applications usually require preemptive scheduling to achieve
concurrency because of their long CPU bursts. For them, non-
preemptive scheduling (even together with explicit yielding) provides
only the old concept of `coroutines'. On the other hand, event driven
applications benefit greatly from non-preemptive scheduling. They
have only short CPU bursts and lots of events to wait on, and this
way run faster under non-preemptive scheduling because no unnecessary
context switching occurs, as it is the case for preemptive
scheduling. That's why Pth is mainly intended for server type
applications, although there is no technical restriction.
o Pth requires thread-safe functions, but NOT reentrant functions.
This nice fact exists again because of the nature of non-preemptive
scheduling, where a function isn't interrupted and this way cannot be
reentered before it returned. This is a great portability benefit,
because thread-safety can be achieved more easily than reentrance
possibility. Especially this means that under Pth more existing
third-party libraries can be used without side-effects than it's the
case for other threading systems.
o Pth doesn't require any kernel support, but can NOT benefit from
multiprocessor machines.
This means that Pth runs on almost all Unix kernels, because the
kernel does not need to be aware of the Pth threads (because they are
implemented entirely in user-space). On the other hand, it cannot
benefit from the existence of multiprocessors, because for this,
kernel support would be needed. In practice, this is no problem,
because multiprocessor systems are rare, and portability is almost
more important than highest concurrency.
The life cycle of a thread
To understand the Pth Application Programming Interface (API), it helps
to first understand the life cycle of a thread in the Pth threading
system. It can be illustrated with the following directed graph:
NEW
|
V
+---> READY ---+
| ^ |
| | V
WAITING <--+-- RUNNING
|
: V
SUSPENDED DEAD
When a new thread is created, it is moved into the NEW queue of the
scheduler. On the next dispatching for this thread, the scheduler picks
it up from there and moves it to the READY queue. This is a queue
containing all threads which want to perform a CPU burst. There they
are queued in priority order. On each dispatching step, the scheduler
always removes the thread with the highest priority only. It then
increases the priority of all remaining threads by 1, to prevent them
from `starving'.
The thread which was removed from the READY queue is the new RUNNING
thread (there is always just one RUNNING thread, of course). The
RUNNING thread is assigned execution control. After this thread yields
execution (either explicitly by yielding execution or implicitly by
calling a function which would block) there are three possibilities:
Either it has terminated, then it is moved to the DEAD queue, or it has
events on which it wants to wait, then it is moved into the WAITING
queue. Else it is assumed it wants to perform more CPU bursts and
immediately enters the READY queue again.
Before the next thread is taken out of the READY queue, the WAITING
queue is checked for pending events. If one or more events occurred,
the threads that are waiting on them are immediately moved to the READY
queue.
The purpose of the NEW queue has to do with the fact that in Pth a
thread never directly switches to another thread. A thread always
yields execution to the scheduler and the scheduler dispatches to the
next thread. So a freshly spawned thread has to be kept somewhere until
the scheduler gets a chance to pick it up for scheduling. That is what
the NEW queue is for.
The purpose of the DEAD queue is to support thread joining. When a
thread is marked to be unjoinable, it is directly kicked out of the
system after it terminated. But when it is joinable, it enters the DEAD
queue. There it remains until another thread joins it.
Finally, there is a special separated queue named SUSPENDED, to where
threads can be manually moved from the NEW, READY or WAITING queues by
the application. The purpose of this special queue is to temporarily
absorb suspended threads until they are again resumed by the
application. Suspended threads do not cost scheduling or event handling
resources, because they are temporarily completely out of the
scheduler's scope. If a thread is resumed, it is moved back to the
queue from where it originally came and this way again enters the
schedulers scope.
APPLICATION PROGRAMMING INTERFACE (API)
In the following the Pth Application Programming Interface (API) is
discussed in detail. With the knowledge given above, it should now be
easy to understand how to program threads with this API. In good Unix
tradition, Pth functions use special return values ("NULL" in pointer
context, "FALSE" in boolean context and "-1" in integer context) to
indicate an error condition and set (or pass through) the "errno"
system variable to pass more details about the error to the caller.
Global Library Management
The following functions act on the library as a whole. They are used
to initialize and shutdown the scheduler and fetch information from it.
int pth_init(void);
This initializes the Pth library. It has to be the first Pth API
function call in an application, and is mandatory. It's usually
done at the begin of the main() function of the application. This
implicitly spawns the internal scheduler thread and transforms the
single execution unit of the current process into a thread (the
`main' thread). It returns "TRUE" on success and "FALSE" on error.
int pth_kill(void);
This kills the Pth library. It should be the last Pth API function
call in an application, but is not really required. It's usually
done at the end of the main function of the application. At least,
it has to be called from within the main thread. It implicitly
kills all threads and transforms back the calling thread into the
single execution unit of the underlying process. The usual way to
terminate a Pth application is either a simple `"pth_exit(0);"' in
the main thread (which waits for all other threads to terminate,
kills the threading system and then terminates the process) or a
`"pth_kill(); exit(0)"' (which immediately kills the threading
system and terminates the process). The pth_kill() return
immediately with a return code of "FALSE" if it is not called from
within the main thread. Else it kills the threading system and
returns "TRUE".
long pth_ctrl(unsigned long query, ...);
This is a generalized query/control function for the Pth library.
The argument query is a bitmask formed out of one or more
"PTH_CTRL_"XXXX queries. Currently the following queries are
supported:
"PTH_CTRL_GETTHREADS"
This returns the total number of threads currently in
existence. This query actually is formed out of the
combination of queries for threads in a particular state, i.e.,
the "PTH_CTRL_GETTHREADS" query is equal to the OR-combination
of all the following specialized queries:
"PTH_CTRL_GETTHREADS_NEW" for the number of threads in the new
queue (threads created via pth_spawn(3) but still not scheduled
once), "PTH_CTRL_GETTHREADS_READY" for the number of threads in
the ready queue (threads who want to do CPU bursts),
"PTH_CTRL_GETTHREADS_RUNNING" for the number of running threads
(always just one thread!), "PTH_CTRL_GETTHREADS_WAITING" for
the number of threads in the waiting queue (threads waiting for
events), "PTH_CTRL_GETTHREADS_SUSPENDED" for the number of
threads in the suspended queue (threads waiting to be resumed)
and "PTH_CTRL_GETTHREADS_DEAD" for the number of threads in the
new queue (terminated threads waiting for a join).
"PTH_CTRL_GETAVLOAD"
This requires a second argument of type `"float *"' (pointer to
a floating point variable). It stores a floating point value
describing the exponential averaged load of the scheduler in
this variable. The load is a function from the number of
threads in the ready queue of the schedulers dispatching unit.
So a load around 1.0 means there is only one ready thread (the
standard situation when the application has no high load). A
higher load value means there a more threads ready who want to
do CPU bursts. The average load value updates once per second
only. The return value for this query is always 0.
"PTH_CTRL_GETPRIO"
This requires a second argument of type `"pth_t"' which
identifies a thread. It returns the priority (ranging from
"PTH_PRIO_MIN" to "PTH_PRIO_MAX") of the given thread.
"PTH_CTRL_GETNAME"
This requires a second argument of type `"pth_t"' which
identifies a thread. It returns the name of the given thread,
i.e., the return value of pth_ctrl(3) should be casted to a
`"char *"'.
"PTH_CTRL_DUMPSTATE"
This requires a second argument of type `"FILE *"' to which a
summary of the internal Pth library state is written to. The
main information which is currently written out is the current
state of the thread pool.
"PTH_CTRL_FAVOURNEW"
This requires a second argument of type `"int"' which specified
whether the GNU Pth scheduler favours new threads on startup,
i.e., whether they are moved from the new queue to the top
(argument is "TRUE") or middle (argument is "FALSE") of the
ready queue. The default is to favour new threads to make sure
they do not starve already at startup, although this slightly
violates the strict priority based scheduling.
The function returns "-1" on error.
long pth_version(void);
This function returns a hex-value `0xVRRTLL' which describes the
current Pth library version. V is the version, RR the revisions, LL
the level and T the type of the level (alphalevel=0, betalevel=1,
patchlevel=2, etc). For instance Pth version 1.0b1 is encoded as
0x100101. The reason for this unusual mapping is that this way the
version number is steadily increasing. The same value is also
available under compile time as "PTH_VERSION".
Thread Attribute Handling
Attribute objects are used in Pth for two things: First
stand-alone/unbound attribute objects are used to store attributes for
to be spawned threads. Bounded attribute objects are used to modify
attributes of already existing threads. The following attribute fields
exists in attribute objects:
"PTH_ATTR_PRIO" (read-write) ["int"]
Thread Priority between "PTH_PRIO_MIN" and "PTH_PRIO_MAX". The
default is "PTH_PRIO_STD".
"PTH_ATTR_NAME" (read-write) ["char *"]
Name of thread (up to 40 characters are stored only), mainly for
debugging purposes.
"PTH_ATTR_DISPATCHES" (read-write) ["int"]
In bounded attribute objects, this field is incremented every time
the context is switched to the associated thread.
"PTH_ATTR_JOINABLE" (read-write> ["int"]
The thread detachment type, "TRUE" indicates a joinable thread,
"FALSE" indicates a detached thread. When a thread is detached,
after termination it is immediately kicked out of the system
instead of inserted into the dead queue.
"PTH_ATTR_CANCEL_STATE" (read-write) ["unsigned int"]
The thread cancellation state, i.e., a combination of
"PTH_CANCEL_ENABLE" or "PTH_CANCEL_DISABLE" and
"PTH_CANCEL_DEFERRED" or "PTH_CANCEL_ASYNCHRONOUS".
"PTH_ATTR_STACK_SIZE" (read-write) ["unsigned int"]
The thread stack size in bytes. Use lower values than 64 KB with
great care!
"PTH_ATTR_STACK_ADDR" (read-write) ["char *"]
A pointer to the lower address of a chunk of malloc(3)'ed memory
for the stack.
"PTH_ATTR_TIME_SPAWN" (read-only) ["pth_time_t"]
The time when the thread was spawned. This can be queried only
when the attribute object is bound to a thread.
"PTH_ATTR_TIME_LAST" (read-only) ["pth_time_t"]
The time when the thread was last dispatched. This can be queried
only when the attribute object is bound to a thread.
"PTH_ATTR_TIME_RAN" (read-only) ["pth_time_t"]
The total time the thread was running. This can be queried only
when the attribute object is bound to a thread.
"PTH_ATTR_START_FUNC" (read-only) ["void *(*)(void *)"]
The thread start function. This can be queried only when the
attribute object is bound to a thread.
"PTH_ATTR_START_ARG" (read-only) ["void *"]
The thread start argument. This can be queried only when the
attribute object is bound to a thread.
"PTH_ATTR_STATE" (read-only) ["pth_state_t"]
The scheduling state of the thread, i.e., either "PTH_STATE_NEW",
"PTH_STATE_READY", "PTH_STATE_WAITING", or "PTH_STATE_DEAD" This
can be queried only when the attribute object is bound to a thread.
"PTH_ATTR_EVENTS" (read-only) ["pth_event_t"]
The event ring the thread is waiting for. This can be queried only
when the attribute object is bound to a thread.
"PTH_ATTR_BOUND" (read-only) ["int"]
Whether the attribute object is bound ("TRUE") to a thread or not
("FALSE").
The following API functions can be used to handle the attribute
objects:
pth_attr_t pth_attr_of(pth_t tid);
This returns a new attribute object bound to thread tid. Any
queries on this object directly fetch attributes from tid. And
attribute modifications directly change tid. Use such attribute
objects to modify existing threads.
pth_attr_t pth_attr_new(void);
This returns a new unbound attribute object. An implicit
pth_attr_init() is done on it. Any queries on this object just
fetch stored attributes from it. And attribute modifications just
change the stored attributes. Use such attribute objects to pre-
configure attributes for to be spawned threads.
int pth_attr_init(pth_attr_t attr);
This initializes an attribute object attr to the default values:
"PTH_ATTR_PRIO" := "PTH_PRIO_STD", "PTH_ATTR_NAME" := `"unknown"',
"PTH_ATTR_DISPATCHES" := 0, "PTH_ATTR_JOINABLE" := "TRUE",
"PTH_ATTR_CANCELSTATE" := "PTH_CANCEL_DEFAULT",
"PTH_ATTR_STACK_SIZE" := 64*1024 and "PTH_ATTR_STACK_ADDR" :=
"NULL". All other "PTH_ATTR_*" attributes are read-only attributes
and don't receive default values in attr, because they exists only
for bounded attribute objects.
int pth_attr_set(pth_attr_t attr, int field, ...);
This sets the attribute field field in attr to a value specified as
an additional argument on the variable argument list. The following
attribute fields and argument pairs can be used:
PTH_ATTR_PRIO int
PTH_ATTR_NAME char *
PTH_ATTR_DISPATCHES int
PTH_ATTR_JOINABLE int
PTH_ATTR_CANCEL_STATE unsigned int
PTH_ATTR_STACK_SIZE unsigned int
PTH_ATTR_STACK_ADDR char *
int pth_attr_get(pth_attr_t attr, int field, ...);
This retrieves the attribute field field in attr and stores its
value in the variable specified through a pointer in an additional
argument on the variable argument list. The following fields and
argument pairs can be used:
PTH_ATTR_PRIO int *
PTH_ATTR_NAME char **
PTH_ATTR_DISPATCHES int *
PTH_ATTR_JOINABLE int *
PTH_ATTR_CANCEL_STATE unsigned int *
PTH_ATTR_STACK_SIZE unsigned int *
PTH_ATTR_STACK_ADDR char **
PTH_ATTR_TIME_SPAWN pth_time_t *
PTH_ATTR_TIME_LAST pth_time_t *
PTH_ATTR_TIME_RAN pth_time_t *
PTH_ATTR_START_FUNC void *(**)(void *)
PTH_ATTR_START_ARG void **
PTH_ATTR_STATE pth_state_t *
PTH_ATTR_EVENTS pth_event_t *
PTH_ATTR_BOUND int *
int pth_attr_destroy(pth_attr_t attr);
This destroys a attribute object attr. After this attr is no longer
a valid attribute object.
Thread Control
The following functions control the threading itself and make up the
main API of the Pth library.
pth_t pth_spawn(pth_attr_t attr, void *(*entry)(void *), void *arg);
This spawns a new thread with the attributes given in attr (or
"PTH_ATTR_DEFAULT" for default attributes - which means that thread
priority, joinability and cancel state are inherited from the
current thread) with the starting point at routine entry; the
dispatch count is not inherited from the current thread if attr is
not specified - rather, it is initialized to zero. This entry
routine is called as `pth_exit(entry(arg))' inside the new thread
unit, i.e., entry's return value is fed to an implicit pth_exit(3).
So the thread can also exit by just returning. Nevertheless the
thread can also exit explicitly at any time by calling pth_exit(3).
But keep in mind that calling the POSIX function exit(3) still
terminates the complete process and not just the current thread.
There is no Pth-internal limit on the number of threads one can
spawn, except the limit implied by the available virtual memory.
Pth internally keeps track of thread in dynamic data structures.
The function returns "NULL" on error.
int pth_once(pth_once_t *ctrlvar, void (*func)(void *), void *arg);
This is a convenience function which uses a control variable of
type "pth_once_t" to make sure a constructor function func is
called only once as `func(arg)' in the system. In other words: Only
the first call to pth_once(3) by any thread in the system succeeds.
The variable referenced via ctrlvar should be declared as
`"pth_once_t" variable-name = "PTH_ONCE_INIT";' before calling this
function.
pth_t pth_self(void);
This just returns the unique thread handle of the currently running
thread. This handle itself has to be treated as an opaque entity
by the application. It's usually used as an argument to other
functions who require an argument of type "pth_t".
int pth_suspend(pth_t tid);
This suspends a thread tid until it is manually resumed again via
pth_resume(3). For this, the thread is moved to the SUSPENDED queue
and this way is completely out of the scheduler's event handling
and thread dispatching scope. Suspending the current thread is not
allowed. The function returns "TRUE" on success and "FALSE" on
errors.
int pth_resume(pth_t tid);
This function resumes a previously suspended thread tid, i.e. tid
has to stay on the SUSPENDED queue. The thread is moved to the NEW,
READY or WAITING queue (dependent on what its state was when the
pth_suspend(3) call were made) and this way again enters the event
handling and thread dispatching scope of the scheduler. The
function returns "TRUE" on success and "FALSE" on errors.
int pth_raise(pth_t tid, int sig)
This function raises a signal for delivery to thread tid only.
When one just raises a signal via raise(3) or kill(2), its
delivered to an arbitrary thread which has this signal not blocked.
With pth_raise(3) one can send a signal to a thread and its
guarantees that only this thread gets the signal delivered. But
keep in mind that nevertheless the signals action is still
configured process-wide. When sig is 0 plain thread checking is
performed, i.e., `"pth_raise(tid, 0)"' returns "TRUE" when thread
tid still exists in the PTH system but doesn't send any signal to
it.
int pth_yield(pth_t tid);
This explicitly yields back the execution control to the scheduler
thread. Usually the execution is implicitly transferred back to
the scheduler when a thread waits for an event. But when a thread
has to do larger CPU bursts, it can be reasonable to interrupt it
explicitly by doing a few pth_yield(3) calls to give other threads
a chance to execute, too. This obviously is the cooperating part
of Pth. A thread has not to yield execution, of course. But when
you want to program a server application with good response times
the threads should be cooperative, i.e., when they should split
their CPU bursts into smaller units with this call.
Usually one specifies tid as "NULL" to indicate to the scheduler
that it can freely decide which thread to dispatch next. But if
one wants to indicate to the scheduler that a particular thread
should be favored on the next dispatching step, one can specify
this thread explicitly. This allows the usage of the old concept of
coroutines where a thread/routine switches to a particular
cooperating thread. If tid is not "NULL" and points to a new or
ready thread, it is guaranteed that this thread receives execution
control on the next dispatching step. If tid is in a different
state (that is, not in "PTH_STATE_NEW" or "PTH_STATE_READY") an
error is reported.
The function usually returns "TRUE" for success and only "FALSE"
(with "errno" set to "EINVAL") if tid specified an invalid or still
not new or ready thread.
int pth_nap(pth_time_t naptime);
This functions suspends the execution of the current thread until
naptime is elapsed. naptime is of type "pth_time_t" and this way
has theoretically a resolution of one microsecond. In practice you
should neither rely on this nor that the thread is awakened exactly
after naptime has elapsed. It's only guarantees that the thread
will sleep at least naptime. But because of the non-preemptive
nature of Pth it can last longer (when another thread kept the CPU
for a long time). Additionally the resolution is dependent of the
implementation of timers by the operating system and these usually
have only a resolution of 10 microseconds or larger. But usually
this isn't important for an application unless it tries to use this
facility for real time tasks.
int pth_wait(pth_event_t ev);
This is the link between the scheduler and the event facility (see
below for the various pth_event_xxx() functions). It's modeled like
select(2), i.e., one gives this function one or more events (in the
event ring specified by ev) on which the current thread wants to
wait. The scheduler awakes the thread when one ore more of them
occurred or failed after tagging them as such. The ev argument is a
pointer to an event ring which isn't changed except for the
tagging. pth_wait(3) returns the number of occurred or failed
events and the application can use pth_event_status(3) to test
which events occurred or failed.
int pth_cancel(pth_t tid);
This cancels a thread tid. How the cancellation is done depends on
the cancellation state of tid which the thread can configure
itself. When its state is "PTH_CANCEL_DISABLE" a cancellation
request is just made pending. When it is "PTH_CANCEL_ENABLE" it
depends on the cancellation type what is performed. When its
"PTH_CANCEL_DEFERRED" again the cancellation request is just made
pending. But when its "PTH_CANCEL_ASYNCHRONOUS" the thread is
immediately canceled before pth_cancel(3) returns. The effect of a
thread cancellation is equal to implicitly forcing the thread to
call `"pth_exit(PTH_CANCELED)"' at one of his cancellation points.
In Pth thread enter a cancellation point either explicitly via
pth_cancel_point(3) or implicitly by waiting for an event.
int pth_abort(pth_t tid);
This is the cruel way to cancel a thread tid. When it's already
dead and waits to be joined it just joins it (via `"pth_join("tid",
NULL)"') and this way kicks it out of the system. Else it forces
the thread to be not joinable and to allow asynchronous
cancellation and then cancels it via `"pth_cancel("tid")"'.
int pth_join(pth_t tid, void **value);
This joins the current thread with the thread specified via tid.
It first suspends the current thread until the tid thread has
terminated. Then it is awakened and stores the value of tid's
pth_exit(3) call into *value (if value and not "NULL") and returns
to the caller. A thread can be joined only when it has the
attribute "PTH_ATTR_JOINABLE" set to "TRUE" (the default). A thread
can only be joined once, i.e., after the pth_join(3) call the
thread tid is completely removed from the system.
void pth_exit(void *value);
This terminates the current thread. Whether it's immediately
removed from the system or inserted into the dead queue of the
scheduler depends on its join type which was specified at spawning
time. If it has the attribute "PTH_ATTR_JOINABLE" set to "FALSE",
it's immediately removed and value is ignored. Else the thread is
inserted into the dead queue and value remembered for a subsequent
pth_join(3) call by another thread.
Utilities
Utility functions.
int pth_fdmode(int fd, int mode);
This switches the non-blocking mode flag on file descriptor fd.
The argument mode can be "PTH_FDMODE_BLOCK" for switching fd into
blocking I/O mode, "PTH_FDMODE_NONBLOCK" for switching fd into non-
blocking I/O mode or "PTH_FDMODE_POLL" for just polling the current
mode. The current mode is returned (either "PTH_FDMODE_BLOCK" or
"PTH_FDMODE_NONBLOCK") or "PTH_FDMODE_ERROR" on error. Keep in mind
that since Pth 1.1 there is no longer a requirement to manually
switch a file descriptor into non-blocking mode in order to use it.
This is automatically done temporarily inside Pth. Instead when
you now switch a file descriptor explicitly into non-blocking mode,
pth_read(3) or pth_write(3) will never block the current thread.
pth_time_t pth_time(long sec, long usec);
This is a constructor for a "pth_time_t" structure which is a
convenient function to avoid temporary structure values. It returns
a pth_time_t structure which holds the absolute time value
specified by sec and usec.
pth_time_t pth_timeout(long sec, long usec);
This is a constructor for a "pth_time_t" structure which is a
convenient function to avoid temporary structure values. It
returns a pth_time_t structure which holds the absolute time value
calculated by adding sec and usec to the current time.
void pth_int_time(struct timespec *sp);
Returns the current time of the pthsem internal time base.
Sfdisc_t *pth_sfiodisc(void);
This functions is always available, but only reasonably usable when
Pth was built with Sfio support ("--with-sfio" option) and
"PTH_EXT_SFIO" is then defined by "pth.h". It is useful for
applications which want to use the comprehensive Sfio I/O library
with the Pth threading library. Then this function can be used to
get an Sfio discipline structure ("Sfdisc_t") which can be pushed
onto Sfio streams ("Sfio_t") in order to let this stream use
pth_read(3)/pth_write(2) instead of read(2)/write(2). The benefit
is that this way I/O on the Sfio stream does only block the current
thread instead of the whole process. The application has to free(3)
the "Sfdisc_t" structure when it is no longer needed. The Sfio
package can be found at http://www.research.att.com/sw/tools/sfio/.
Cancellation Management
Pth supports POSIX style thread cancellation via pth_cancel(3) and the
following two related functions:
void pth_cancel_state(int newstate, int *oldstate);
This manages the cancellation state of the current thread. When
oldstate is not "NULL" the function stores the old cancellation
state under the variable pointed to by oldstate. When newstate is
not 0 it sets the new cancellation state. oldstate is created
before newstate is set. A state is a combination of
"PTH_CANCEL_ENABLE" or "PTH_CANCEL_DISABLE" and
"PTH_CANCEL_DEFERRED" or "PTH_CANCEL_ASYNCHRONOUS".
"PTH_CANCEL_ENABLE|PTH_CANCEL_DEFERRED" (or "PTH_CANCEL_DEFAULT")
is the default state where cancellation is possible but only at
cancellation points. Use "PTH_CANCEL_DISABLE" to complete disable
cancellation for a thread and "PTH_CANCEL_ASYNCHRONOUS" for
allowing asynchronous cancellations, i.e., cancellations which can
happen at any time.
void pth_cancel_point(void);
This explicitly enter a cancellation point. When the current
cancellation state is "PTH_CANCEL_DISABLE" or no cancellation
request is pending, this has no side-effect and returns
immediately. Else it calls `"pth_exit(PTH_CANCELED)"'.
Event Handling
Pth has a very flexible event facility which is linked into the
scheduler through the pth_wait(3) function. The following functions
provide the handling of event rings.
pth_event_t pth_event(unsigned long spec, ...);
This creates a new event ring consisting of a single initial event.
The type of the generated event is specified by spec. The following
types are available:
"PTH_EVENT_FD"
This is a file descriptor event. One or more of
"PTH_UNTIL_FD_READABLE", "PTH_UNTIL_FD_WRITEABLE" or
"PTH_UNTIL_FD_EXCEPTION" have to be OR-ed into spec to specify
on which state of the file descriptor you want to wait. The
file descriptor itself has to be given as an additional
argument. Example:
`"pth_event(PTH_EVENT_FD|PTH_UNTIL_FD_READABLE, fd)"'.
"PTH_EVENT_SELECT"
This is a multiple file descriptor event modeled directly after
the select(2) call (actually it is also used to implement
pth_select(3) internally). It's a convenient way to wait for a
large set of file descriptors at once and at each file
descriptor for a different type of state. Additionally as a
nice side-effect one receives the number of file descriptors
which causes the event to be occurred (using BSD semantics,
i.e., when a file descriptor occurred in two sets it's counted
twice). The arguments correspond directly to the select(2)
function arguments except that there is no timeout argument
(because timeouts already can be handled via "PTH_EVENT_TIME"
events).
Example: `"pth_event(PTH_EVENT_SELECT, &rc, nfd, rfds, wfds,
efds)"' where "rc" has to be of type `"int *"', "nfd" has to be
of type `"int"' and "rfds", "wfds" and "efds" have to be of
type `"fd_set *"' (see select(2)). The number of occurred file
descriptors are stored in "rc".
"PTH_EVENT_SIGS"
This is a signal set event. The two additional arguments have
to be a pointer to a signal set (type `"sigset_t *"') and a
pointer to a signal number variable (type `"int *"'). This
event waits until one of the signals in the signal set
occurred. As a result the occurred signal number is stored in
the second additional argument. Keep in mind that the Pth
scheduler doesn't block signals automatically. So when you
want to wait for a signal with this event you've to block it
via sigprocmask(2) or it will be delivered without your notice.
Example: `"sigemptyset(&set); sigaddset(&set, SIGINT);
pth_event(PTH_EVENT_SIG, &set, &sig);"'.
"PTH_EVENT_TIME"
This is a time point event. The additional argument has to be
of type "pth_time_t" (usually on-the-fly generated via
pth_time(3)). This events waits until the specified time point
has elapsed. Keep in mind that the value is an absolute time
point and not an offset. When you want to wait for a specified
amount of time, you've to add the current time to the offset
(usually on-the-fly achieved via pth_timeout(3)). Example:
`"pth_event(PTH_EVENT_TIME, pth_timeout(2,0))"'.
"PTH_EVENT_RTIME"
This is a time interval event. The additional argument has to
be of type "pth_time_t" (usually on-the-fly generated via
pth_time(3)), containing a time interval. During creation, it
is converted into PTH_EVENT_TIME. It has the advantage, that it
only uses the pthsem internal clock. Example:
`"pth_event(PTH_EVENT_TIME, pth_timeout(2,0))"'. is equal to
`"pth_event(PTH_EVENT_RTIME, pth_time(2,0))"'.
"PTH_EVENT_MSG"
This is a message port event. The additional argument has to be
of type "pth_msgport_t". This events waits until one or more
messages were received on the specified message port. Example:
`"pth_event(PTH_EVENT_MSG, mp)"'.
"PTH_EVENT_TID"
This is a thread event. The additional argument has to be of
type "pth_t". One of "PTH_UNTIL_TID_NEW",
"PTH_UNTIL_TID_READY", "PTH_UNTIL_TID_WAITING" or
"PTH_UNTIL_TID_DEAD" has to be OR-ed into spec to specify on
which state of the thread you want to wait. Example:
`"pth_event(PTH_EVENT_TID|PTH_UNTIL_TID_DEAD, tid)"'.
"PTH_EVENT_FUNC"
This is a custom callback function event. Three additional
arguments have to be given with the following types: `"int
(*)(void *)"', `"void *"' and `"pth_time_t"'. The first is a
function pointer to a check function and the second argument is
a user-supplied context value which is passed to this function.
The scheduler calls this function on a regular basis (on his
own scheduler stack, so be very careful!) and the thread is
kept sleeping while the function returns "FALSE". Once it
returned "TRUE" the thread will be awakened. The check interval
is defined by the third argument, i.e., the check function is
polled again not until this amount of time elapsed. Example:
`"pth_event(PTH_EVENT_FUNC, func, arg, pth_time(0,500000))"'.
"PTH_EVENT_SEM"
This is a semaphore event. It waits for a semaphore, until it
can be decremented. By default 1 is used for this, with the
flag "PTH_UNTIL_COUNT" other values can be used. If the flag
"PTH_UNTIL_DECREMENT" is used, the semaphore value is
decremented (so the lock is obtained), else the event is
signaled, if it would be possible. Examples:
* pth_event(PTH_EVENT_SEM|PTH_UNTIL_DECREMENT|PTH_UNTIL_COUNT,
&sem,2): event waits, utils the value of the semaphore is >= 2
and subtracts then two from it
* pth_event(PTH_EVENT_SEM|PTH_UNTIL_COUNT, &sem,2): event
waits, util the value of the semaphore is >= 2
* pth_event(PTH_EVENT_SEM|PTH_UNTIL_DECREMENT, &sem): event
waits, util the value of the semaphore is >= 1 and subtracts
then 1 from it
* pth_event(PTH_EVENT_SEM, &sem): event waits, util the value
of the semaphore is >= 1
unsigned long pth_event_typeof(pth_event_t ev);
This returns the type of event ev. It's a combination of the
describing "PTH_EVENT_XX" and "PTH_UNTIL_XX" value. This is
especially useful to know which arguments have to be supplied to
the pth_event_extract(3) function.
int pth_event_extract(pth_event_t ev, ...);
When pth_event(3) is treated like sprintf(3), then this function is
sscanf(3), i.e., it is the inverse operation of pth_event(3). This
means that it can be used to extract the ingredients of an event.
The ingredients are stored into variables which are given as
pointers on the variable argument list. Which pointers have to be
present depends on the event type and has to be determined by the
caller before via pth_event_typeof(3).
To make it clear, when you constructed ev via `"ev =
pth_event(PTH_EVENT_FD, fd);"' you have to extract it via
`"pth_event_extract(ev, &fd)"', etc. For multiple arguments of an
event the order of the pointer arguments is the same as for
pth_event(3). But always keep in mind that you have to always
supply pointers to variables and these variables have to be of the
same type as the argument of pth_event(3) required.
pth_event_t pth_event_concat(pth_event_t ev, ...);
This concatenates one or more additional event rings to the event
ring ev and returns ev. The end of the argument list has to be
marked with a "NULL" argument. Use this function to create real
events rings out of the single-event rings created by pth_event(3).
pth_event_t pth_event_isolate(pth_event_t ev);
This isolates the event ev from possibly appended events in the
event ring. When in ev only one event exists, this returns "NULL".
When remaining events exists, they form a new event ring which is
returned.
pth_event_t pth_event_walk(pth_event_t ev, int direction);
This walks to the next (when direction is "PTH_WALK_NEXT") or
previews (when direction is "PTH_WALK_PREV") event in the event
ring ev and returns this new reached event. Additionally
"PTH_UNTIL_OCCURRED" can be OR-ed into direction to walk to the
next/previous occurred event in the ring ev.
pth_status_t pth_event_status(pth_event_t ev);
This returns the status of event ev. This is a fast operation
because only a tag on ev is checked which was either set or still
not set by the scheduler. In other words: This doesn't check the
event itself, it just checks the last knowledge of the scheduler.
The possible returned status codes are: "PTH_STATUS_PENDING" (event
is still pending), "PTH_STATUS_OCCURRED" (event successfully
occurred), "PTH_STATUS_FAILED" (event failed).
int pth_event_free(pth_event_t ev, int mode);
This deallocates the event ev (when mode is "PTH_FREE_THIS") or all
events appended to the event ring under ev (when mode is
"PTH_FREE_ALL").
Key-Based Storage
The following functions provide thread-local storage through unique
keys similar to the POSIX Pthread API. Use this for thread specific
global data.
int pth_key_create(pth_key_t *key, void (*func)(void *));
This created a new unique key and stores it in key. Additionally
func can specify a destructor function which is called on the
current threads termination with the key.
int pth_key_delete(pth_key_t key);
This explicitly destroys a key key.
int pth_key_setdata(pth_key_t key, const void *value);
This stores value under key.
void *pth_key_getdata(pth_key_t key);
This retrieves the value under key.
Message Port Communication
The following functions provide message ports which can be used for
efficient and flexible inter-thread communication.
pth_msgport_t pth_msgport_create(const char *name);
This returns a pointer to a new message port. If name name is not
"NULL", the name can be used by other threads via
pth_msgport_find(3) to find the message port in case they do not
know directly the pointer to the message port.
void pth_msgport_destroy(pth_msgport_t mp);
This destroys a message port mp. Before all pending messages on it
are replied to their origin message port.
pth_msgport_t pth_msgport_find(const char *name);
This finds a message port in the system by name and returns the
pointer to it.
int pth_msgport_pending(pth_msgport_t mp);
This returns the number of pending messages on message port mp.
int pth_msgport_put(pth_msgport_t mp, pth_message_t *m);
This puts (or sends) a message m to message port mp.
pth_message_t *pth_msgport_get(pth_msgport_t mp);
This gets (or receives) the top message from message port mp.
Incoming messages are always kept in a queue, so there can be more
pending messages, of course.
int pth_msgport_reply(pth_message_t *m);
This replies a message m to the message port of the sender.
Thread Cleanups
Per-thread cleanup functions.
int pth_cleanup_push(void (*handler)(void *), void *arg);
This pushes the routine handler onto the stack of cleanup routines
for the current thread. These routines are called in LIFO order
when the thread terminates.
int pth_cleanup_pop(int execute);
This pops the top-most routine from the stack of cleanup routines
for the current thread. When execute is "TRUE" the routine is
additionally called.
Process Forking
The following functions provide some special support for process
forking situations inside the threading environment.
int pth_atfork_push(void (*prepare)(void *), void (*)(void *parent),
void (*)(void *child), void *arg);
This function declares forking handlers to be called before and
after pth_fork(3), in the context of the thread that called
pth_fork(3). The prepare handler is called before fork(2)
processing commences. The parent handler is called after fork(2)
processing completes in the parent process. The child handler is
called after fork(2) processing completed in the child process. If
no handling is desired at one or more of these three points, the
corresponding handler can be given as "NULL". Each handler is
called with arg as the argument.
The order of calls to pth_atfork_push(3) is significant. The parent
and child handlers are called in the order in which they were
established by calls to pth_atfork_push(3), i.e., FIFO. The prepare
fork handlers are called in the opposite order, i.e., LIFO.
int pth_atfork_pop(void);
This removes the top-most handlers on the forking handler stack
which were established with the last pth_atfork_push(3) call. It
returns "FALSE" when no more handlers couldn't be removed from the
stack.
pid_t pth_fork(void);
This is a variant of fork(2) with the difference that the current
thread only is forked into a separate process, i.e., in the parent
process nothing changes while in the child process all threads are
gone except for the scheduler and the calling thread. When you
really want to duplicate all threads in the current process you
should use fork(2) directly. But this is usually not reasonable.
Additionally this function takes care of forking handlers as
established by pth_fork_push(3).
Synchronization
The following functions provide synchronization support via mutual
exclusion locks (mutex), read-write locks (rwlock), condition variables
(cond) and barriers (barrier). Keep in mind that in a non-preemptive
threading system like Pth this might sound unnecessary at the first
look, because a thread isn't interrupted by the system. Actually when
you have a critical code section which doesn't contain any pth_xxx()
functions, you don't need any mutex to protect it, of course.
But when your critical code section contains any pth_xxx() function the
chance is high that these temporarily switch to the scheduler. And this
way other threads can make progress and enter your critical code
section, too. This is especially true for critical code sections which
implicitly or explicitly use the event mechanism.
int pth_mutex_init(pth_mutex_t *mutex);
This dynamically initializes a mutex variable of type
`"pth_mutex_t"'. Alternatively one can also use static
initialization via `"pth_mutex_t mutex = PTH_MUTEX_INIT"'.
int pth_mutex_acquire(pth_mutex_t *mutex, int try, pth_event_t ev);
This acquires a mutex mutex. If the mutex is already locked by
another thread, the current threads execution is suspended until
the mutex is unlocked again or additionally the extra events in ev
occurred (when ev is not "NULL"). Recursive locking is explicitly
supported, i.e., a thread is allowed to acquire a mutex more than
once before its released. But it then also has be released the same
number of times until the mutex is again lockable by others. When
try is "TRUE" this function never suspends execution. Instead it
returns "FALSE" with "errno" set to "EBUSY".
int pth_mutex_release(pth_mutex_t *mutex);
This decrements the recursion locking count on mutex and when it is
zero it releases the mutex mutex.
int pth_rwlock_init(pth_rwlock_t *rwlock);
This dynamically initializes a read-write lock variable of type
`"pth_rwlock_t"'. Alternatively one can also use static
initialization via `"pth_rwlock_t rwlock = PTH_RWLOCK_INIT"'.
int pth_rwlock_acquire(pth_rwlock_t *rwlock, int op, int try,
pth_event_t ev);
This acquires a read-only (when op is "PTH_RWLOCK_RD") or a read-
write (when op is "PTH_RWLOCK_RW") lock rwlock. When the lock is
only locked by other threads in read-only mode, the lock succeeds.
But when one thread holds a read-write lock, all locking attempts
suspend the current thread until this lock is released again.
Additionally in ev events can be given to let the locking timeout,
etc. When try is "TRUE" this function never suspends execution.
Instead it returns "FALSE" with "errno" set to "EBUSY".
int pth_rwlock_release(pth_rwlock_t *rwlock);
This releases a previously acquired (read-only or read-write) lock.
int pth_cond_init(pth_cond_t *cond);
This dynamically initializes a condition variable variable of type
`"pth_cond_t"'. Alternatively one can also use static
initialization via `"pth_cond_t cond = PTH_COND_INIT"'.
int pth_cond_await(pth_cond_t *cond, pth_mutex_t *mutex, pth_event_t
ev);
This awaits a condition situation. The caller has to follow the
semantics of the POSIX condition variables: mutex has to be
acquired before this function is called. The execution of the
current thread is then suspended either until the events in ev
occurred (when ev is not "NULL") or cond was notified by another
thread via pth_cond_notify(3). While the thread is waiting, mutex
is released. Before it returns mutex is reacquired.
int pth_cond_notify(pth_cond_t *cond, int broadcast);
This notified one or all threads which are waiting on cond. When
broadcast is "TRUE" all thread are notified, else only a single
(unspecified) one.
int pth_barrier_init(pth_barrier_t *barrier, int threshold);
This dynamically initializes a barrier variable of type
`"pth_barrier_t"'. Alternatively one can also use static
initialization via `"pth_barrier_t barrier =
PTH_BARRIER_INIT("threadhold")"'.
int pth_barrier_reach(pth_barrier_t *barrier);
This function reaches a barrier barrier. If this is the last thread
(as specified by threshold on init of barrier) all threads are
awakened. Else the current thread is suspended until the last
thread reached the barrier and this way awakes all threads. The
function returns (beside "FALSE" on error) the value "TRUE" for any
thread which neither reached the barrier as the first nor the last
thread; "PTH_BARRIER_HEADLIGHT" for the thread which reached the
barrier as the first thread and "PTH_BARRIER_TAILLIGHT" for the
thread which reached the barrier as the last thread.
Semaphore support
The interface provides functions to set/get the value of a semaphore,
increment it with arbitrary values, wait, until the value becomes
bigger than a given value (without or with decrementing, if the
condition becomes true.
The data-type for the semaphore is names "pth_sem_t" and it has an
initializer like "pth_cond_t".
int pth_sem_init(pth_sem_t *sem);
This dynamically initializes a semaphore variable of type
`"pth_sem_t"'. Alternatively one can also use static
initialization via `"pth_sem_t semaphore = PTH_SEM_INIT"'.
int pth_sem_dec(pth_sem_t *sem);
waits, until the value of "sem" is >= 1 and decrement it.
int pth_sem_dec_value(pth_sem_t *sem, unsigned value);
waits, until the value of "sem" is >= "value" and subtracts
"value".
int pth_sem_inc(pth_sem_t *sem, int notify);
increments "sem". The scheduler is started, if "notify" is not
null.
int pth_sem_inc_value(pth_sem_t *sem, unsigned value, int notify);
adds value to "sem". The scheduler is started, if "notify" is not
null.
int pth_sem_set_value(pth_sem_t *sem, unsigned value);
sets the value of "sem" to "value".
int pth_sem_get_value(pth_sem_t *sem, unsigned *value);
stores the value of "sem" in *"value".
User-Space Context
The following functions provide a stand-alone sub-API for user-space
context switching. It internally is based on the same underlying
machine context switching mechanism the threads in GNU Pth are based
on. Hence these functions you can use for implementing your own simple
user-space threads. The "pth_uctx_t" context is somewhat modeled after
POSIX ucontext(3).
The time required to create (via pth_uctx_make(3)) a user-space context
can range from just a few microseconds up to a more dramatical time
(depending on the machine context switching method which is available
on the platform). On the other hand, the raw performance in switching
the user-space contexts is always very good (nearly independent of the
used machine context switching method). For instance, on an Intel
Pentium-III CPU with 800Mhz running under FreeBSD 4 one usually
achieves about 260,000 user-space context switches (via
pth_uctx_switch(3)) per second.
int pth_uctx_create(pth_uctx_t *uctx);
This function creates a user-space context and stores it into uctx.
There is still no underlying user-space context configured. You
still have to do this with pth_uctx_make(3). On success, this
function returns "TRUE", else "FALSE".
int pth_uctx_make(pth_uctx_t uctx, char *sk_addr, size_t sk_size, const
sigset_t *sigmask, void (*start_func)(void *), void *start_arg,
pth_uctx_t uctx_after);
This function makes a new user-space context in uctx which will
operate on the run-time stack sk_addr (which is of maximum size
sk_size), with the signals in sigmask blocked (if sigmask is not
"NULL") and starting to execute with the call
start_func(start_arg). If sk_addr is "NULL", a stack is dynamically
allocated. The stack size sk_size has to be at least 16384 (16KB).
If the start function start_func returns and uctx_after is not
"NULL", an implicit user-space context switch to this context is
performed. Else (if uctx_after is "NULL") the process is terminated
with exit(3). This function is somewhat modeled after POSIX
makecontext(3). On success, this function returns "TRUE", else
"FALSE".
int pth_uctx_switch(pth_uctx_t uctx_from, pth_uctx_t uctx_to);
This function saves the current user-space context in uctx_from for
later restoring by another call to pth_uctx_switch(3) and restores
the new user-space context from uctx_to, which previously had to be
set with either a previous call to pth_uctx_switch(3) or initially
by pth_uctx_make(3). This function is somewhat modeled after POSIX
swapcontext(3). If uctx_from or uctx_to are "NULL" or if uctx_to
contains no valid user-space context, "FALSE" is returned instead
of "TRUE". These are the only errors possible.
int pth_uctx_destroy(pth_uctx_t uctx);
This function destroys the user-space context in uctx. The run-time
stack associated with the user-space context is deallocated only if
it was not given by the application (see sk_addr of
pth_uctx_create(3)). If uctx is "NULL", "FALSE" is returned
instead of "TRUE". This is the only error possible.
Generalized POSIX Replacement API
The following functions are generalized replacements functions for the
POSIX API, i.e., they are similar to the functions under `Standard
POSIX Replacement API' but all have an additional event argument which
can be used for timeouts, etc.
int pth_sigwait_ev(const sigset_t *set, int *sig, pth_event_t ev);
This is equal to pth_sigwait(3) (see below), but has an additional
event argument ev. When pth_sigwait(3) suspends the current threads
execution it usually only uses the signal event on set to awake.
With this function any number of extra events can be used to awake
the current thread (remember that ev actually is an event ring).
int pth_connect_ev(int s, const struct sockaddr *addr, socklen_t
addrlen, pth_event_t ev);
This is equal to pth_connect(3) (see below), but has an additional
event argument ev. When pth_connect(3) suspends the current threads
execution it usually only uses the I/O event on s to awake. With
this function any number of extra events can be used to awake the
current thread (remember that ev actually is an event ring).
int pth_accept_ev(int s, struct sockaddr *addr, socklen_t *addrlen,
pth_event_t ev);
This is equal to pth_accept(3) (see below), but has an additional
event argument ev. When pth_accept(3) suspends the current threads
execution it usually only uses the I/O event on s to awake. With
this function any number of extra events can be used to awake the
current thread (remember that ev actually is an event ring).
int pth_select_ev(int nfd, fd_set *rfds, fd_set *wfds, fd_set *efds,
struct timeval *timeout, pth_event_t ev);
This is equal to pth_select(3) (see below), but has an additional
event argument ev. When pth_select(3) suspends the current threads
execution it usually only uses the I/O event on rfds, wfds and efds
to awake. With this function any number of extra events can be used
to awake the current thread (remember that ev actually is an event
ring).
int pth_poll_ev(struct pollfd *fds, unsigned int nfd, int timeout,
pth_event_t ev);
This is equal to pth_poll(3) (see below), but has an additional
event argument ev. When pth_poll(3) suspends the current threads
execution it usually only uses the I/O event on fds to awake. With
this function any number of extra events can be used to awake the
current thread (remember that ev actually is an event ring).
ssize_t pth_read_ev(int fd, void *buf, size_t nbytes, pth_event_t ev);
This is equal to pth_read(3) (see below), but has an additional
event argument ev. When pth_read(3) suspends the current threads
execution it usually only uses the I/O event on fd to awake. With
this function any number of extra events can be used to awake the
current thread (remember that ev actually is an event ring).
ssize_t pth_readv_ev(int fd, const struct iovec *iovec, int iovcnt,
pth_event_t ev);
This is equal to pth_readv(3) (see below), but has an additional
event argument ev. When pth_readv(3) suspends the current threads
execution it usually only uses the I/O event on fd to awake. With
this function any number of extra events can be used to awake the
current thread (remember that ev actually is an event ring).
ssize_t pth_write_ev(int fd, const void *buf, size_t nbytes,
pth_event_t ev);
This is equal to pth_write(3) (see below), but has an additional
event argument ev. When pth_write(3) suspends the current threads
execution it usually only uses the I/O event on fd to awake. With
this function any number of extra events can be used to awake the
current thread (remember that ev actually is an event ring).
ssize_t pth_writev_ev(int fd, const struct iovec *iovec, int iovcnt,
pth_event_t ev);
This is equal to pth_writev(3) (see below), but has an additional
event argument ev. When pth_writev(3) suspends the current threads
execution it usually only uses the I/O event on fd to awake. With
this function any number of extra events can be used to awake the
current thread (remember that ev actually is an event ring).
ssize_t pth_recv_ev(int fd, void *buf, size_t nbytes, int flags,
pth_event_t ev);
This is equal to pth_recv(3) (see below), but has an additional
event argument ev. When pth_recv(3) suspends the current threads
execution it usually only uses the I/O event on fd to awake. With
this function any number of extra events can be used to awake the
current thread (remember that ev actually is an event ring).
ssize_t pth_recvfrom_ev(int fd, void *buf, size_t nbytes, int flags,
struct sockaddr *from, socklen_t *fromlen, pth_event_t ev);
This is equal to pth_recvfrom(3) (see below), but has an additional
event argument ev. When pth_recvfrom(3) suspends the current
threads execution it usually only uses the I/O event on fd to
awake. With this function any number of extra events can be used to
awake the current thread (remember that ev actually is an event
ring).
ssize_t pth_send_ev(int fd, const void *buf, size_t nbytes, int flags,
pth_event_t ev);
This is equal to pth_send(3) (see below), but has an additional
event argument ev. When pth_send(3) suspends the current threads
execution it usually only uses the I/O event on fd to awake. With
this function any number of extra events can be used to awake the
current thread (remember that ev actually is an event ring).
ssize_t pth_sendto_ev(int fd, const void *buf, size_t nbytes, int
flags, const struct sockaddr *to, socklen_t tolen, pth_event_t ev);
This is equal to pth_sendto(3) (see below), but has an additional
event argument ev. When pth_sendto(3) suspends the current threads
execution it usually only uses the I/O event on fd to awake. With
this function any number of extra events can be used to awake the
current thread (remember that ev actually is an event ring).
Standard POSIX Replacement API
The following functions are standard replacements functions for the
POSIX API. The difference is mainly that they suspend the current
thread only instead of the whole process in case the file descriptors
will block.
int pth_nanosleep(const struct timespec *rqtp, struct timespec *rmtp);
This is a variant of the POSIX nanosleep(3) function. It suspends
the current threads execution until the amount of time in rqtp
elapsed. The thread is guaranteed to not wake up before this time,
but because of the non-preemptive scheduling nature of Pth, it can
be awakened later, of course. If rmtp is not "NULL", the "timespec"
structure it references is updated to contain the unslept amount
(the request time minus the time actually slept time). The
difference between nanosleep(3) and pth_nanosleep(3) is that that
pth_nanosleep(3) suspends only the execution of the current thread
and not the whole process.
int pth_usleep(unsigned int usec);
This is a variant of the 4.3BSD usleep(3) function. It suspends the
current threads execution until usec microseconds (= usec*1/1000000
sec) elapsed. The thread is guaranteed to not wake up before this
time, but because of the non-preemptive scheduling nature of Pth,
it can be awakened later, of course. The difference between
usleep(3) and pth_usleep(3) is that that pth_usleep(3) suspends
only the execution of the current thread and not the whole process.
unsigned int pth_sleep(unsigned int sec);
This is a variant of the POSIX sleep(3) function. It suspends the
current threads execution until sec seconds elapsed. The thread is
guaranteed to not wake up before this time, but because of the non-
preemptive scheduling nature of Pth, it can be awakened later, of
course. The difference between sleep(3) and pth_sleep(3) is that
pth_sleep(3) suspends only the execution of the current thread and
not the whole process.
pid_t pth_waitpid(pid_t pid, int *status, int options);
This is a variant of the POSIX waitpid(2) function. It suspends the
current threads execution until status information is available for
a terminated child process pid. The difference between waitpid(2)
and pth_waitpid(3) is that pth_waitpid(3) suspends only the
execution of the current thread and not the whole process. For
more details about the arguments and return code semantics see
waitpid(2).
int pth_system(const char *cmd);
This is a variant of the POSIX system(3) function. It executes the
shell command cmd with Bourne Shell ("sh") and suspends the current
threads execution until this command terminates. The difference
between system(3) and pth_system(3) is that pth_system(3) suspends
only the execution of the current thread and not the whole process.
For more details about the arguments and return code semantics see
system(3).
int pth_sigmask(int how, const sigset_t *set, sigset_t *oset)
This is the Pth thread-related equivalent of POSIX sigprocmask(2)
respectively pthread_sigmask(3). The arguments how, set and oset
directly relate to sigprocmask(2), because Pth internally just uses
sigprocmask(2) here. So alternatively you can also directly call
sigprocmask(2), but for consistency reasons you should use this
function pth_sigmask(3).
int pth_sigwait(const sigset_t *set, int *sig);
This is a variant of the POSIX.1c sigwait(3) function. It suspends
the current threads execution until a signal in set occurred and
stores the signal number in sig. The important point is that the
signal is not delivered to a signal handler. Instead it's caught by
the scheduler only in order to awake the pth_sigwait() call. The
trick and noticeable point here is that this way you get an
asynchronous aware application that is written completely
synchronously. When you think about the problem of asynchronous
safe functions you should recognize that this is a great benefit.
int pth_connect(int s, const struct sockaddr *addr, socklen_t addrlen);
This is a variant of the 4.2BSD connect(2) function. It establishes
a connection on a socket s to target specified in addr and addrlen.
The difference between connect(2) and pth_connect(3) is that
pth_connect(3) suspends only the execution of the current thread
and not the whole process. For more details about the arguments
and return code semantics see connect(2).
int pth_accept(int s, struct sockaddr *addr, socklen_t *addrlen);
This is a variant of the 4.2BSD accept(2) function. It accepts a
connection on a socket by extracting the first connection request
on the queue of pending connections, creating a new socket with the
same properties of s and allocates a new file descriptor for the
socket (which is returned). The difference between accept(2) and
pth_accept(3) is that pth_accept(3) suspends only the execution of
the current thread and not the whole process. For more details
about the arguments and return code semantics see accept(2).
int pth_select(int nfd, fd_set *rfds, fd_set *wfds, fd_set *efds,
struct timeval *timeout);
This is a variant of the 4.2BSD select(2) function. It examines
the I/O descriptor sets whose addresses are passed in rfds, wfds,
and efds to see if some of their descriptors are ready for reading,
are ready for writing, or have an exceptional condition pending,
respectively. For more details about the arguments and return code
semantics see select(2).
int pth_pselect(int nfd, fd_set *rfds, fd_set *wfds, fd_set *efds,
const struct timespec *timeout, const sigset_t *sigmask);
This is a variant of the POSIX pselect(2) function, which in turn
is a stronger variant of 4.2BSD select(2). The difference is that
the higher-resolution "struct timespec" is passed instead of the
lower-resolution "struct timeval" and that a signal mask is
specified which is temporarily set while waiting for input. For
more details about the arguments and return code semantics see
pselect(2) and select(2).
int pth_poll(struct pollfd *fds, unsigned int nfd, int timeout);
This is a variant of the SysV poll(2) function. It examines the I/O
descriptors which are passed in the array fds to see if some of
them are ready for reading, are ready for writing, or have an
exceptional condition pending, respectively. For more details about
the arguments and return code semantics see poll(2).
ssize_t pth_read(int fd, void *buf, size_t nbytes);
This is a variant of the POSIX read(2) function. It reads up to
nbytes bytes into buf from file descriptor fd. The difference
between read(2) and pth_read(2) is that pth_read(2) suspends
execution of the current thread until the file descriptor is ready
for reading. For more details about the arguments and return code
semantics see read(2).
ssize_t pth_readv(int fd, const struct iovec *iovec, int iovcnt);
This is a variant of the POSIX readv(2) function. It reads data
from file descriptor fd into the first iovcnt rows of the iov
vector. The difference between readv(2) and pth_readv(2) is that
pth_readv(2) suspends execution of the current thread until the
file descriptor is ready for reading. For more details about the
arguments and return code semantics see readv(2).
ssize_t pth_write(int fd, const void *buf, size_t nbytes);
This is a variant of the POSIX write(2) function. It writes nbytes
bytes from buf to file descriptor fd. The difference between
write(2) and pth_write(2) is that pth_write(2) suspends execution
of the current thread until the file descriptor is ready for
writing. For more details about the arguments and return code
semantics see write(2).
ssize_t pth_writev(int fd, const struct iovec *iovec, int iovcnt);
This is a variant of the POSIX writev(2) function. It writes data
to file descriptor fd from the first iovcnt rows of the iov vector.
The difference between writev(2) and pth_writev(2) is that
pth_writev(2) suspends execution of the current thread until the
file descriptor is ready for reading. For more details about the
arguments and return code semantics see writev(2).
ssize_t pth_pread(int fd, void *buf, size_t nbytes, off_t offset);
This is a variant of the POSIX pread(3) function. It performs the
same action as a regular read(2), except that it reads from a given
position in the file without changing the file pointer. The first
three arguments are the same as for pth_read(3) with the addition
of a fourth argument offset for the desired position inside the
file.
ssize_t pth_pwrite(int fd, const void *buf, size_t nbytes, off_t
offset);
This is a variant of the POSIX pwrite(3) function. It performs the
same action as a regular write(2), except that it writes to a given
position in the file without changing the file pointer. The first
three arguments are the same as for pth_write(3) with the addition
of a fourth argument offset for the desired position inside the
file.
ssize_t pth_recv(int fd, void *buf, size_t nbytes, int flags);
This is a variant of the SUSv2 recv(2) function and equal to
``pth_recvfrom(fd, buf, nbytes, flags, NULL, 0)''.
ssize_t pth_recvfrom(int fd, void *buf, size_t nbytes, int flags,
struct sockaddr *from, socklen_t *fromlen);
This is a variant of the SUSv2 recvfrom(2) function. It reads up to
nbytes bytes into buf from file descriptor fd while using flags and
from/fromlen. The difference between recvfrom(2) and
pth_recvfrom(2) is that pth_recvfrom(2) suspends execution of the
current thread until the file descriptor is ready for reading. For
more details about the arguments and return code semantics see
recvfrom(2).
ssize_t pth_send(int fd, const void *buf, size_t nbytes, int flags);
This is a variant of the SUSv2 send(2) function and equal to
``pth_sendto(fd, buf, nbytes, flags, NULL, 0)''.
ssize_t pth_sendto(int fd, const void *buf, size_t nbytes, int flags,
const struct sockaddr *to, socklen_t tolen);
This is a variant of the SUSv2 sendto(2) function. It writes nbytes
bytes from buf to file descriptor fd while using flags and
to/tolen. The difference between sendto(2) and pth_sendto(2) is
that pth_sendto(2) suspends execution of the current thread until
the file descriptor is ready for writing. For more details about
the arguments and return code semantics see sendto(2).
EXAMPLE
The following example is a useless server which does nothing more than
listening on TCP port 12345 and displaying the current time to the
socket when a connection was established. For each incoming connection
a thread is spawned. Additionally, to see more multithreading, a
useless ticker thread runs simultaneously which outputs the current
time to "stderr" every 5 seconds. The example contains no error
checking and is only intended to show you the look and feel of Pth.
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <signal.h>
#include <netdb.h>
#include <unistd.h>
#include "pth.h"
#define PORT 12345
/* the socket connection handler thread */
static void *handler(void *_arg)
{
int fd = (int)_arg;
time_t now;
char *ct;
now = time(NULL);
ct = ctime(&now);
pth_write(fd, ct, strlen(ct));
close(fd);
return NULL;
}
/* the stderr time ticker thread */
static void *ticker(void *_arg)
{
time_t now;
char *ct;
float load;
for (;;) {
pth_sleep(5);
now = time(NULL);
ct = ctime(&now);
ct[strlen(ct)-1] = '\0';
pth_ctrl(PTH_CTRL_GETAVLOAD, &load);
printf("ticker: time: %s, average load: %.2f\n", ct, load);
}
}
/* the main thread/procedure */
int main(int argc, char *argv[])
{
pth_attr_t attr;
struct sockaddr_in sar;
struct protoent *pe;
struct sockaddr_in peer_addr;
int peer_len;
int sa, sw;
int port;
pth_init();
signal(SIGPIPE, SIG_IGN);
attr = pth_attr_new();
pth_attr_set(attr, PTH_ATTR_NAME, "ticker");
pth_attr_set(attr, PTH_ATTR_STACK_SIZE, 64*1024);
pth_attr_set(attr, PTH_ATTR_JOINABLE, FALSE);
pth_spawn(attr, ticker, NULL);
pe = getprotobyname("tcp");
sa = socket(AF_INET, SOCK_STREAM, pe->p_proto);
sar.sin_family = AF_INET;
sar.sin_addr.s_addr = INADDR_ANY;
sar.sin_port = htons(PORT);
bind(sa, (struct sockaddr *)&sar, sizeof(struct sockaddr_in));
listen(sa, 10);
pth_attr_set(attr, PTH_ATTR_NAME, "handler");
for (;;) {
peer_len = sizeof(peer_addr);
sw = pth_accept(sa, (struct sockaddr *)&peer_addr, &peer_len);
pth_spawn(attr, handler, (void *)sw);
}
}
BUILD ENVIRONMENTS
In this section we will discuss the canonical ways to establish the
build environment for a Pth based program. The possibilities supported
by Pth range from very simple environments to rather complex ones.
Manual Build Environment (Novice)
As a first example, assume we have the above test program staying in
the source file "foo.c". Then we can create a very simple build
environment by just adding the following "Makefile":
$ vi Makefile
| CC = cc
| CFLAGS = `pth-config --cflags`
| LDFLAGS = `pth-config --ldflags`
| LIBS = `pth-config --libs`
|
| all: foo
| foo: foo.o
| $(CC) $(LDFLAGS) -o foo foo.o $(LIBS)
| foo.o: foo.c
| $(CC) $(CFLAGS) -c foo.c
| clean:
| rm -f foo foo.o
This imports the necessary compiler and linker flags on-the-fly from
the Pth installation via its "pth-config" program. This approach is
straight-forward and works fine for small projects.
Autoconf Build Environment (Advanced)
The previous approach is simple but inflexible. First, to speed up
building, it would be nice to not expand the compiler and linker flags
every time the compiler is started. Second, it would be useful to also
be able to build against uninstalled Pth, that is, against a Pth source
tree which was just configured and built, but not installed. Third, it
would be also useful to allow checking of the Pth version to make sure
it is at least a minimum required version. And finally, it would be
also great to make sure Pth works correctly by first performing some
sanity compile and run-time checks. All this can be done if we use GNU
autoconf and the "AC_CHECK_PTH" macro provided by Pth. For this, we
establish the following three files:
First we again need the "Makefile", but this time it contains autoconf
placeholders and additional cleanup targets. And we create it under the
name "Makefile.in", because it is now an input file for autoconf:
$ vi Makefile.in
| CC = @CC@
| CFLAGS = @CFLAGS@
| LDFLAGS = @LDFLAGS@
| LIBS = @LIBS@
|
| all: foo
| foo: foo.o
| $(CC) $(LDFLAGS) -o foo foo.o $(LIBS)
| foo.o: foo.c
| $(CC) $(CFLAGS) -c foo.c
| clean:
| rm -f foo foo.o
| distclean:
| rm -f foo foo.o
| rm -f config.log config.status config.cache
| rm -f Makefile
Because autoconf generates additional files, we added a canonical
"distclean" target which cleans this up. Secondly, we wrote
"configure.ac", a (minimal) autoconf script specification:
$ vi configure.ac
| AC_INIT(Makefile.in)
| AC_CHECK_PTH(1.3.0)
| AC_OUTPUT(Makefile)
Then we let autoconf's "aclocal" program generate for us an
"aclocal.m4" file containing Pth's "AC_CHECK_PTH" macro. Then we
generate the final "configure" script out of this "aclocal.m4" file and
the "configure.ac" file:
$ aclocal --acdir=`pth-config --acdir`
$ autoconf
After these steps, the working directory should look similar to this:
$ ls -l
-rw-r--r-- 1 rse users 176 Nov 3 11:11 Makefile.in
-rw-r--r-- 1 rse users 15314 Nov 3 11:16 aclocal.m4
-rwxr-xr-x 1 rse users 52045 Nov 3 11:16 configure
-rw-r--r-- 1 rse users 63 Nov 3 11:11 configure.ac
-rw-r--r-- 1 rse users 4227 Nov 3 11:11 foo.c
If we now run "configure" we get a correct "Makefile" which immediately
can be used to build "foo" (assuming that Pth is already installed
somewhere, so that "pth-config" is in $PATH):
$ ./configure
creating cache ./config.cache
checking for gcc... gcc
checking whether the C compiler (gcc ) works... yes
checking whether the C compiler (gcc ) is a cross-compiler... no
checking whether we are using GNU C... yes
checking whether gcc accepts -g... yes
checking how to run the C preprocessor... gcc -E
checking for GNU Pth... version 1.3.0, installed under /usr/local
updating cache ./config.cache
creating ./config.status
creating Makefile
rse@en1:/e/gnu/pth/ac
$ make
gcc -g -O2 -I/usr/local/include -c foo.c
gcc -L/usr/local/lib -o foo foo.o -lpth
If Pth is installed in non-standard locations or "pth-config" is not in
$PATH, one just has to drop the "configure" script a note about the
location by running "configure" with the option "--with-pth="dir (where
dir is the argument which was used with the "--prefix" option when Pth
was installed).
Autoconf Build Environment with Local Copy of Pth (Expert)
Finally let us assume the "foo" program stays under either a GPL or
LGPL distribution license and we want to make it a stand-alone package
for easier distribution and installation. That is, we don't want to
oblige the end-user to install Pth just to allow our "foo" package to
compile. For this, it is a convenient practice to include the required
libraries (here Pth) into the source tree of the package (here "foo").
Pth ships with all necessary support to allow us to easily achieve this
approach. Say, we want Pth in a subdirectory named "pth/" and this
directory should be seamlessly integrated into the configuration and
build process of "foo".
First we again start with the "Makefile.in", but this time it is a more
advanced version which supports subdirectory movement:
$ vi Makefile.in
| CC = @CC@
| CFLAGS = @CFLAGS@
| LDFLAGS = @LDFLAGS@
| LIBS = @LIBS@
|
| SUBDIRS = pth
|
| all: subdirs_all foo
|
| subdirs_all:
| @$(MAKE) $(MFLAGS) subdirs TARGET=all
| subdirs_clean:
| @$(MAKE) $(MFLAGS) subdirs TARGET=clean
| subdirs_distclean:
| @$(MAKE) $(MFLAGS) subdirs TARGET=distclean
| subdirs:
| @for subdir in $(SUBDIRS); do \
| echo "===> $$subdir ($(TARGET))"; \
| (cd $$subdir; $(MAKE) $(MFLAGS) $(TARGET) || exit 1) || exit 1; \
| echo "<=== $$subdir"; \
| done
|
| foo: foo.o
| $(CC) $(LDFLAGS) -o foo foo.o $(LIBS)
| foo.o: foo.c
| $(CC) $(CFLAGS) -c foo.c
|
| clean: subdirs_clean
| rm -f foo foo.o
| distclean: subdirs_distclean
| rm -f foo foo.o
| rm -f config.log config.status config.cache
| rm -f Makefile
Then we create a slightly different autoconf script "configure.ac":
$ vi configure.ac
| AC_INIT(Makefile.in)
| AC_CONFIG_AUX_DIR(pth)
| AC_CHECK_PTH(1.3.0, subdir:pth --disable-tests)
| AC_CONFIG_SUBDIRS(pth)
| AC_OUTPUT(Makefile)
Here we provided a default value for "foo"'s "--with-pth" option as the
second argument to "AC_CHECK_PTH" which indicates that Pth can be found
in the subdirectory named "pth/". Additionally we specified that the
"--disable-tests" option of Pth should be passed to the "pth/"
subdirectory, because we need only to build the Pth library itself. And
we added a "AC_CONFIG_SUBDIR" call which indicates to autoconf that it
should configure the "pth/" subdirectory, too. The "AC_CONFIG_AUX_DIR"
directive was added just to make autoconf happy, because it wants to
find a "install.sh" or "shtool" script if "AC_CONFIG_SUBDIRS" is used.
Now we let autoconf's "aclocal" program again generate for us an
"aclocal.m4" file with the contents of Pth's "AC_CHECK_PTH" macro.
Finally we generate the "configure" script out of this "aclocal.m4"
file and the "configure.ac" file.
$ aclocal --acdir=`pth-config --acdir`
$ autoconf
Now we have to create the "pth/" subdirectory itself. For this, we
extract the Pth distribution to the "foo" source tree and just rename
it to "pth/":
$ gunzip <pth-X.Y.Z.tar.gz | tar xvf -
$ mv pth-X.Y.Z pth
SYSTEM CALL WRAPPER FACILITY
Pth per default uses an explicit API, including the system calls. For
instance you've to explicitly use pth_read(3) when you need a thread-
aware read(3) and cannot expect that by just calling read(3) only the
current thread is blocked. Instead with the standard read(3) call the
whole process will be blocked. But because for some applications
(mainly those consisting of lots of third-party stuff) this can be
inconvenient. Here it's required that a call to read(3) `magically'
means pth_read(3). The problem here is that such magic Pth cannot
provide per default because it's not really portable. Nevertheless Pth
provides a two step approach to solve this problem:
Soft System Call Mapping
This variant is available on all platforms and can always be enabled by
building Pth with "--enable-syscall-soft". This then triggers some
"#define"'s in the "pth.h" header which map for instance read(3) to
pth_read(3), etc. Currently the following functions are mapped:
fork(2), nanosleep(3), usleep(3), sleep(3), sigwait(3), waitpid(2),
system(3), select(2), poll(2), connect(2), accept(2), read(2),
write(2), recv(2), send(2), recvfrom(2), sendto(2).
The drawback of this approach is just that really all source files of
the application where these function calls occur have to include
"pth.h", of course. And this also means that existing libraries,
including the vendor's stdio, usually will still block the whole
process if one of its I/O functions block.
Hard System Call Mapping
This variant is available only on those platforms where the syscall(2)
function exists and there it can be enabled by building Pth with
"--enable-syscall-hard". This then builds wrapper functions (for
instances read(3)) into the Pth library which internally call the real
Pth replacement functions (pth_read(3)). Currently the following
functions are mapped: fork(2), nanosleep(3), usleep(3), sleep(3),
waitpid(2), system(3), select(2), poll(2), connect(2), accept(2),
read(2), write(2).
The drawback of this approach is that it depends on syscall(2)
interface and prototype conflicts can occur while building the wrapper
functions due to different function signatures in the vendor C header
files. But the advantage of this mapping variant is that the source
files of the application where these function calls occur have not to
include "pth.h" and that existing libraries, including the vendor's
stdio, magically become thread-aware (and then block only the current
thread).
IMPLEMENTATION NOTES
Pth is very portable because it has only one part which perhaps has to
be ported to new platforms (the machine context initialization). But it
is written in a way which works on mostly all Unix platforms which
support makecontext(2) or at least sigstack(2) or sigaltstack(2) [see
"pth_mctx.c" for details]. Any other Pth code is POSIX and ANSI C based
only.
The context switching is done via either SUSv2 makecontext(2) or POSIX
make[sig]setjmp(3) and [sig]longjmp(3). Here all CPU registers, the
program counter and the stack pointer are switched. Additionally the
Pth dispatcher switches also the global Unix "errno" variable [see
"pth_mctx.c" for details] and the signal mask (either implicitly via
sigsetjmp(3) or in an emulated way via explicit setprocmask(2) calls).
The Pth event manager is mainly select(2) and gettimeofday(2) based,
i.e., the current time is fetched via gettimeofday(2) once per context
switch for time calculations and all I/O events are implemented via a
single central select(2) call [see "pth_sched.c" for details].
The thread control block management is done via virtual priority queues
without any additional data structure overhead. For this, the queue
linkage attributes are part of the thread control blocks and the queues
are actually implemented as rings with a selected element as the entry
point [see "pth_tcb.h" and "pth_pqueue.c" for details].
Most time critical code sections (especially the dispatcher and event
manager) are speeded up by inline functions (implemented as ANSI C pre-
processor macros). Additionally any debugging code is completely
removed from the source when not built with "-DPTH_DEBUG" (see Autoconf
"--enable-debug" option), i.e., not only stub functions remain [see
"pth_debug.c" for details].
RESTRICTIONS
Pth (intentionally) provides no replacements for non-thread-safe
functions (like strtok(3) which uses a static internal buffer) or
synchronous system functions (like gethostbyname(3) which doesn't
provide an asynchronous mode where it doesn't block). When you want to
use those functions in your server application together with threads,
you've to either link the application against special third-party
libraries (or for thread-safe/reentrant functions possibly against an
existing "libc_r" of the platform vendor). For an asynchronous DNS
resolver library use the GNU adns package from Ian Jackson ( see
http://www.gnu.org/software/adns/adns.html ).
HISTORY
The Pth library was designed and implemented between February and July
1999 by Ralf S. Engelschall after evaluating numerous (mostly
preemptive) thread libraries and after intensive discussions with Peter
Simons, Martin Kraemer, Lars Eilebrecht and Ralph Babel related to an
experimental (matrix based) non-preemptive C++ scheduler class written
by Peter Simons.
Pth was then implemented in order to combine the non-preemptive
approach of multithreading (which provides better portability and
performance) with an API similar to the popular one found in Pthread
libraries (which provides easy programming).
So the essential idea of the non-preemptive approach was taken over
from Peter Simons scheduler. The priority based scheduling algorithm
was suggested by Martin Kraemer. Some code inspiration also came from
an experimental threading library (rsthreads) written by Robert S. Thau
for an ancient internal test version of the Apache webserver. The
concept and API of message ports was borrowed from AmigaOS' Exec
subsystem. The concept and idea for the flexible event mechanism came
from Paul Vixie's eventlib (which can be found as a part of BIND v8).
BUG REPORTS AND SUPPORT
If you think you have found a bug in Pth, you should send a report as
complete as possible to bug-pth@gnu.org. If you can, please try to fix
the problem and include a patch, made with '"diff -u3"', in your
report. Always, at least, include a reasonable amount of description in
your report to allow the author to deterministically reproduce the bug.
For further support you additionally can subscribe to the
pth-users@gnu.org mailing list by sending an Email to
pth-users-request@gnu.org with `"subscribe pth-users"' (or `"subscribe
pth-users" address' if you want to subscribe from a particular Email
address) in the body. Then you can discuss your issues with other Pth
users by sending messages to pth-users@gnu.org. Currently (as of August
2000) you can reach about 110 Pth users on this mailing list. Old
postings you can find at
http://www.mail-archive.com/pth-users@gnu.org/.
SEE ALSO
Related Web Locations
`comp.programming.threads Newsgroup Archive',
http://www.deja.com/topics_if.xp?
search=topic&group=comp.programming.threads
`comp.programming.threads Frequently Asked Questions (F.A.Q.)',
http://www.lambdacs.com/newsgroup/FAQ.html
`Multithreading - Definitions and Guidelines', Numeric Quest Inc 1998;
http://www.numeric-quest.com/lang/multi-frame.html
`The Single UNIX Specification, Version 2 - Threads', The Open Group
1997; http://www.opengroup.org/onlinepubs /007908799/xsh/threads.html
SMI Thread Resources, Sun Microsystems Inc;
http://www.sun.com/workshop/threads/
Bibliography on threads and multithreading, Torsten Amundsen;
http://liinwww.ira.uka.de/bibliography/Os/threads.html
Related Books
B. Nichols, D. Buttlar, J.P. Farrel: `Pthreads Programming - A POSIX
Standard for Better Multiprocessing', O'Reilly 1996; ISBN 1-56592-115-1
B. Lewis, D. J. Berg: `Multithreaded Programming with Pthreads', Sun
Microsystems Press, Prentice Hall 1998; ISBN 0-13-680729-1
B. Lewis, D. J. Berg: `Threads Primer - A Guide To Multithreaded
Programming', Prentice Hall 1996; ISBN 0-13-443698-9
S. J. Norton, M. D. Dipasquale: `Thread Time - The Multithreaded
Programming Guide', Prentice Hall 1997; ISBN 0-13-190067-6
D. R. Butenhof: `Programming with POSIX Threads', Addison Wesley 1997;
ISBN 0-201-63392-2
Related Manpages
pth-config(1), pthread(3).
getcontext(2), setcontext(2), makecontext(2), swapcontext(2),
sigstack(2), sigaltstack(2), sigaction(2), sigemptyset(2),
sigaddset(2), sigprocmask(2), sigsuspend(2), sigsetjmp(3),
siglongjmp(3), setjmp(3), longjmp(3), select(2), gettimeofday(2).
AUTHOR
Ralf S. Engelschall
rse@engelschall.com
www.engelschall.com
2.0.8pthsem 2.0.8 .::pth(3)
