SEMOP(P) POSIX Programmer's Manual SEMOP(P)NAMEsemop - XSI semaphore operations
SYNOPSIS
#include <sys/sem.h>
int semop(int semid, struct sembuf *sops, size_t nsops);
DESCRIPTION
The semop() function operates on XSI semaphores (see the Base Defini‐
tions volume of IEEE Std 1003.1-2001, Section 4.15, Semaphore). It is
unspecified whether this function interoperates with the realtime
interprocess communication facilities defined in Realtime .
The semop() function shall perform atomically a user-defined array of
semaphore operations on the set of semaphores associated with the sema‐
phore identifier specified by the argument semid.
The argument sops is a pointer to a user-defined array of semaphore
operation structures. The implementation shall not modify elements of
this array unless the application uses implementation-defined exten‐
sions.
The argument nsops is the number of such structures in the array.
Each structure, sembuf, includes the following members:
Member Type Member Name Description
short sem_num Semaphore number.
short sem_op Semaphore operation.
short sem_flg Operation flags.
Each semaphore operation specified by sem_op is performed on the corre‐
sponding semaphore specified by semid and sem_num.
The variable sem_op specifies one of three semaphore operations:
1. If sem_op is a negative integer and the calling process has alter
permission, one of the following shall occur:
* If semval(see <sys/sem.h>) is greater than or equal to the abso‐
lute value of sem_op, the absolute value of sem_op is subtracted
from semval. Also, if (sem_flg &SEM_UNDO) is non-zero, the abso‐
lute value of sem_op shall be added to the calling process'
semadj value for the specified semaphore.
* If semval is less than the absolute value of sem_op and (sem_flg
&IPC_NOWAIT) is non-zero, semop() shall return immediately.
* If semval is less than the absolute value of sem_op and (sem_flg
&IPC_NOWAIT) is 0, semop() shall increment the semncnt associ‐
ated with the specified semaphore and suspend execution of the
calling thread until one of the following conditions occurs:
* The value of semval becomes greater than or equal to the
absolute value of sem_op. When this occurs, the value of sem‐
ncnt associated with the specified semaphore shall be decre‐
mented, the absolute value of sem_op shall be subtracted from
semval and, if (sem_flg &SEM_UNDO) is non-zero, the absolute
value of sem_op shall be added to the calling process' semadj
value for the specified semaphore.
* The semid for which the calling thread is awaiting action is
removed from the system. When this occurs, errno shall be set
equal to [EIDRM] and -1 shall be returned.
* The calling thread receives a signal that is to be caught.
When this occurs, the value of semncnt associated with the
specified semaphore shall be decremented, and the calling
thread shall resume execution in the manner prescribed in
sigaction() .
2. If sem_op is a positive integer and the calling process has alter
permission, the value of sem_op shall be added to semval and, if
(sem_flg &SEM_UNDO) is non-zero, the value of sem_op shall be sub‐
tracted from the calling process' semadj value for the specified
semaphore.
3. If sem_op is 0 and the calling process has read permission, one of
the following shall occur:
* If semval is 0, semop() shall return immediately.
* If semval is non-zero and (sem_flg &IPC_NOWAIT) is non-zero,
semop() shall return immediately.
* If semval is non-zero and (sem_flg &IPC_NOWAIT) is 0, semop()
shall increment the semzcnt associated with the specified sema‐
phore and suspend execution of the calling thread until one of
the following occurs:
* The value of semval becomes 0, at which time the value of
semzcnt associated with the specified semaphore shall be
decremented.
* The semid for which the calling thread is awaiting action is
removed from the system. When this occurs, errno shall be set
equal to [EIDRM] and -1 shall be returned.
* The calling thread receives a signal that is to be caught.
When this occurs, the value of semzcnt associated with the
specified semaphore shall be decremented, and the calling
thread shall resume execution in the manner prescribed in
sigaction() .
Upon successful completion, the value of sempid for each semaphore
specified in the array pointed to by sops shall be set equal to the
process ID of the calling process.
RETURN VALUE
Upon successful completion, semop() shall return 0; otherwise, it shall
return -1 and set errno to indicate the error.
ERRORS
The semop() function shall fail if:
E2BIG The value of nsops is greater than the system-imposed maximum.
EACCES Operation permission is denied to the calling process; see XSI
Interprocess Communication .
EAGAIN The operation would result in suspension of the calling process
but (sem_flg &IPC_NOWAIT) is non-zero.
EFBIG The value of sem_num is less than 0 or greater than or equal to
the number of semaphores in the set associated with semid.
EIDRM The semaphore identifier semid is removed from the system.
EINTR The semop() function was interrupted by a signal.
EINVAL The value of semid is not a valid semaphore identifier, or the
number of individual semaphores for which the calling process
requests a SEM_UNDO would exceed the system-imposed limit.
ENOSPC The limit on the number of individual processes requesting a
SEM_UNDO would be exceeded.
ERANGE An operation would cause a semval to overflow the system-imposed
limit, or an operation would cause a semadj value to overflow
the system-imposed limit.
The following sections are informative.
EXAMPLES
Setting Values in Semaphores
The following example sets the values of the two semaphores associated
with the semid identifier to the values contained in the sb array.
#include <sys/sem.h>
...
int semid;
struct sembuf sb[2];
int nsops = 2;
int result;
/* Adjust value of semaphore in the semaphore array semid. */
sb[0].sem_num = 0;
sb[0].sem_op = -1;
sb[0].sem_flg = SEM_UNDO | IPC_NOWAIT;
sb[1].sem_num = 1;
sb[1].sem_op = 1;
sb[1].sem_flg = 0;
result = semop(semid, sb, nsops);
Creating a Semaphore Identifier
The following example gets a unique semaphore key using the ftok()
function, then gets a semaphore ID associated with that key using the
semget() function (the first call also tests to make sure the semaphore
exists). If the semaphore does not exist, the program creates it, as
shown by the second call to semget(). In creating the semaphore for the
queuing process, the program attempts to create one semaphore with
read/write permission for all. It also uses the IPC_EXCL flag, which
forces semget() to fail if the semaphore already exists.
After creating the semaphore, the program uses a call to semop() to
initialize it to the values in the sbuf array. The number of processes
that can execute concurrently without queuing is initially set to 2.
The final call to semget() creates a semaphore identifier that can be
used later in the program.
The final call to semop() acquires the semaphore and waits until it is
free; the SEM_UNDO option releases the semaphore when the process
exits, waiting until there are less than two processes running concur‐
rently.
#include <sys/types.h>
#include <stdio.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/stat.h>
#include <errno.h>
#include <unistd.h>
#include <stdlib.h>
#include <pwd.h>
#include <fcntl.h>
#include <limits.h>
...
key_t semkey;
int semid, pfd, fv;
struct sembuf sbuf;
char *lgn;
char filename[PATH_MAX+1];
struct stat outstat;
struct passwd *pw;
...
/* Get unique key for semaphore. */
if ((semkey = ftok("/tmp", 'a')) == (key_t) -1) {
perror("IPC error: ftok"); exit(1);
}
/* Get semaphore ID associated with this key. */
if ((semid = semget(semkey, 0, 0)) == -1) {
/* Semaphore does not exist - Create. */
if ((semid = semget(semkey, 1, IPC_CREAT | IPC_EXCL | S_IRUSR |
S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH)) != -1)
{
/* Initialize the semaphore. */
sbuf.sem_num = 0;
sbuf.sem_op = 2; /* This is the number of runs without queuing. */
sbuf.sem_flg = 0;
if (semop(semid, &sbuf, 1) == -1) {
perror("IPC error: semop"); exit(1);
}
}
else if (errno == EEXIST) {
if ((semid = semget(semkey, 0, 0)) == -1) {
perror("IPC error 1: semget"); exit(1);
}
}
else {
perror("IPC error 2: semget"); exit(1);
}
}
...
sbuf.sem_num = 0;
sbuf.sem_op = -1;
sbuf.sem_flg = SEM_UNDO;
if (semop(semid, &sbuf, 1) == -1) {
perror("IPC Error: semop"); exit(1);
}
APPLICATION USAGE
The POSIX Realtime Extension defines alternative interfaces for inter‐
process communication. Application developers who need to use IPC
should design their applications so that modules using the IPC routines
described in XSI Interprocess Communication can be easily modified to
use the alternative interfaces.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
XSI Interprocess Communication , Realtime , exec() , exit() , fork() ,
semctl() , semget() , sem_close() , sem_destroy() , sem_getvalue() ,
sem_init() , sem_open() , sem_post() , sem_unlink() , sem_wait() , the
Base Definitions volume of IEEE Std 1003.1-2001, <sys/ipc.h>,
<sys/sem.h>, <sys/types.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 SEMOP(P)
