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:

1.

If sem_op is a negative integer and the calling process has alter permission, one of the following shall occur:

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

3.

If sem_op is 0 and the calling process has read permission, one of the following shall occur:

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:

E2BIG

The value of nsops is greater than the system-imposed maximum.

EACCES

Operation permission is denied to the calling process; see Section 2.7, 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 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¶

#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);

#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 .