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PROLOG

This manual page is part of the POSIX Programmer’s Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

NAME

semop — 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 Definitions volume of POSIX.1-2008, Section 4.16, Semaphore). It is unspecified whether this function interoperates with the realtime interprocess communication facilities defined in Section 2.8, Realtime.

The semop() function shall perform atomically a user-defined array of semaphore operations in array order on the set of semaphores associated with the semaphore 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 extensions.

The argument nsops is the number of such structures in the array.

Each structure, sembuf, includes the following members:

Each semaphore operation specified by sem_op is performed on the corresponding semaphore specified by semid and sem_num.

The variable sem_op specifies one of three semaphore operations:

*

--

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 subtracted from the semadj value of the calling process for the specified semaphore.

*

--

Upon successful completion, the value of sempid for each semaphore specified in the array pointed to by sops shall be set to the process ID of the calling process. Also, the sem_otime timestamp shall be set to the current time, as described in Section 2.7.1, IPC General Description.

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:

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;

/* Code to initialize semid. */
...

/* 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 calls to semctl() and 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.

Processes that obtain semid without creating it check that sem_otime is non-zero, to ensure that the creating process has completed the semop() initialization.

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 concurrently.

#include <stdio.h>
#include <sys/sem.h>
#include <sys/stat.h>
#include <errno.h>
#include <stdlib.h>
...
key_t semkey;
int semid;
struct sembuf sbuf;
union semun {
int val;
struct semid_ds *buf;
unsigned short *array;
} arg;
struct semid_ds ds;
...
/* 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. */
arg.val = 0;
sbuf.sem_num = 0;
sbuf.sem_op = 2; /* This is the number of runs without queuing. */
sbuf.sem_flg = 0;
if (semctl(semid, 0, SETVAL, arg) == -1
|| 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);
}
goto check_init;
}
else {
perror("IPC error 2: semget"); exit(1);
}
}
else
{
/* Check that semid has completed initialization. */
/* An application can use a retry loop at this point rather than
exiting. */
check_init:
arg.buf = &ds;
if (semctl(semid, 0, IPC_STAT, arg) < 0) {
perror("IPC error 3: semctl"); exit(1);
}
if (ds.sem_otime == 0) {
perror("IPC error 4: semctl"); 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 interprocess communication. Application developers who need to use IPC should design their applications so that modules using the IPC routines described in Section 2.7, XSI Interprocess Communication can be easily modified to use the alternative interfaces.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

Section 2.7, XSI Interprocess Communication, Section 2.8, Realtime, exec, exit(), fork(), semctl(), semget(), sem_close(), sem_destroy(), sem_getvalue(), sem_init(), sem_open(), sem_post(), sem_trywait(), sem_unlink()

The Base Definitions volume of POSIX.1-2008, Section 4.16, Semaphore, <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, 2013 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .