DLFCN(3) OpenBSD Programmer's Manual DLFCN(3)NAME
dlopen, dlclose, dlsym, dladdr, dlctl, dlerror - dynamic link interface
SYNOPSIS
#include <dlfcn.h>
void *
dlopen(const char *path, int mode);
int
dlclose(void *handle);
void *
dlsym(void *handle, const char *symbol);
int
dladdr(const void *addr, Dl_info *info);
int
dlctl(void *handle, int cmd, void *data);
const char *
dlerror(void);
DESCRIPTION
These functions provide an interface to the run-time linker ld.so(1).
They allow new shared objects to be loaded into a process's address space
under program control.
The dlopen() function takes the name of a shared object as its first
argument. The shared object is mapped into the address space, relocated,
and its external references are resolved in the same way as is done with
the implicitly loaded shared libraries at program startup.
The path parameter can be specified as either an absolute pathname to a
shared library or just the name of the shared library itself. When an
absolute pathname is specified, only the path provided will be searched
for the shared library. When just a shared library is specified, the
same paths will be searched that are used for ``intrinsic'' shared
library searches.
Shared libraries take the following form:
lib<name>.so[.xx[.yy]]
When a shared library is specified without a version or with a partial
version, the same library search rules apply that are used for
``intrinsic'' shared library searches. A null pointer supplied for path
will return a special handle that behaves the same as the RTLD_DEFAULT
special handle.
The mode parameter specifies symbol resolution time and symbol
visibility. One of the following values may be used to specify symbol
resolution time:
RTLD_NOW Symbols are resolved immediately.
RTLD_LAZY Symbols are resolved when they are first referred to.
This is the default value if resolution time is
unspecified.
One of the following values may be used to specify symbol visibility:
RTLD_GLOBAL The object's symbols and the symbols of its
dependencies will be visible to other objects.
RTLD_LOCAL The object's symbols and the symbols of its
dependencies will not be visible to other objects.
This is the default value if visibility is
unspecified.
To specify both resolution time and visibility, bitwise inclusive OR one
of each of the above values together. If an object was opened with
RTLD_LOCAL and later opened with RTLD_GLOBAL, then it is promoted to
RTLD_GLOBAL.
dlopen() returns a handle to be used in calls to dlclose(), dlsym(), and
dlctl(). If the named shared object has already been loaded by a
previous call to dlopen() and not yet unloaded by dlclose(), a handle
referring to the resident copy is returned.
dlclose() unlinks and removes the object referred to by handle from the
process address space. If multiple calls to dlopen() have been done on
this object or the object is a dependency of another object then the
object is removed when its reference count drops to zero. dlclose()
returns 0 on success and non-zero on failure.
dlsym() searches for a definition of symbol in the object designated by
handle and all shared objects that it depends on. The symbol's address
is returned. If the symbol cannot be resolved, NULL is returned.
dlsym() may also be called with special handles. dlsym() respects symbol
visibility as specified by the dlopen() mode parameter. However, the
symbols of an object's dependencies are always visible to it. All shared
objects loaded at program startup are globally visible. Only the symbols
in the main executable that are referenced by a shared object at link
time will be visible unless it has been linked with the --export-dynamic
option where all of its symbols will be visible. The following special
handles may be used with dlsym():
NULL Interpreted as a reference to the executable or
shared object from which the call is being made.
Thus an object can reference its own symbols and
the symbols of its dependencies without calling
dlopen().
RTLD_DEFAULT All the visible shared objects and the executable
will be searched in the order they were loaded.
RTLD_NEXT The search for symbol is limited to the visible
shared objects which were loaded after the one
issuing the call to dlsym(). Thus, if dlsym() is
called from the main program, all the visible
shared libraries are searched. If it is called
from a shared library, all subsequently visible
shared libraries are searched.
RTLD_SELF The search for symbol is limited to the shared
object issuing the call to dlsym() and those shared
objects which were loaded after it that are
visible.
dladdr() queries the dynamic linker for information about the shared
object containing the address addr. The information is returned in the
structure specified by info. The structure contains at least the
following members:
const char *dli_fname The pathname of the shared object containing
the address addr.
void *dli_fbase The base address at which the shared object is
mapped into the address space of the calling
process.
const char *dli_sname The name of the nearest run-time symbol with a
address less than or equal to addr.
If no symbol with a suitable address is found,
both this field and dli_saddr are set to NULL.
void *dli_saddr The address of the symbol returned in
dli_sname.
If a mapped shared object containing addr cannot be found, dladdr()
returns 0. In that case, a message detailing the failure can be
retrieved by calling dlerror(). On success, a non-zero value is
returned. Note: both strings pointed at by dli_fname and dli_sname
reside in memory private to the run-time linker module and should not be
modified by the caller.
In dynamically linked programs, the address of a global function will
point to its program linkage table entry, rather than to the entry point
of the function itself. This causes most global functions to appear to
be defined within the main executable, rather than in the shared
libraries where the actual code resides.
dlctl() provides an interface similar to ioctl(2) to control several
aspects of the run-time linker's operation. This interface is currently
under development.
dlerror() returns a character string representing the most recent error
that has occurred while processing one of the other functions described
here. If no dynamic linking errors have occurred since the last
invocation of dlerror(), dlerror() returns NULL. Thus, invoking
dlerror() a second time, immediately following a prior invocation, will
result in NULL being returned.
SEE ALSOld(1), ld.so(1), link(5)HISTORY
Some of the dl* functions first appeared in SunOS 4.
OpenBSD 4.9 February 20, 2010 OpenBSD 4.9