semop -- semaphore operations


cc . . . -lc

#include  <sys/types.h>
#include  <sys/ipc.h>
#include  <sys/sem.h>

int semop (semid, sops, nsops) int semid; struct sembuf *sops; unsigned nsops;


The semop system call is used to automatically perform an array of semaphore operations on the set of semaphores associated with the semaphore identifier specified by semid. sops is a pointer to the array of semaphore-operation structures. nsops is the number of such structures in the array. The contents of each structure includes the following members:

   short   sem_num;        /* semaphore number */
   short   sem_op;         /* semaphore operation */
   short   sem_flg;        /* operation flags */

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

semop specifies one of three semaphore operations as follows:

  1. If semop is a negative integer, one of the following occurs: {ALTER}

  2. If semop is a positive integer, the value of semop is added to semval and, if (sem_flg & SEM_UNDO) is ``true'', the value of semop is subtracted from the calling process's semadj value for the specified semaphore. {ALTER}

  3. If semop is zero, one of the following occurs: {READ}

The semop system call fails if one or more of the following is true for any of the semaphore operations specified by sops:

nsops is greater than the system-imposed maximum.

Operation permission is denied to the calling process.

The operation would result in suspension of the calling process but (sem_flg & IPC_NOWAIT) is ``true''.

sops points to an illegal address.

sem_num is less than zero or greater than or equal to the number of semaphores in the set associated with semid.

semid is not a valid semaphore identifier.

The number of individual semaphores for which the calling process requests an SEM_UNDO would exceed the limit.

The limit on the number of individual processes requesting an SEM_UNDO would be exceeded.

An operation would cause a semval to overflow the system-imposed limit.

An operation would cause a semadj value to overflow the system-imposed limit.

Upon successful completion, the value of sempid for each semaphore specified in the array pointed to by sops is set equal to the process ID of the calling process.


If semop returns due to the receipt of a signal, a value of -1 is returned to the calling process and errno is set to EINTR. If it returns due to the removal of a semid from the system, a value of -1 is returned and errno is set to EIDRM.

Upon successful completion, a value of zero is returned. Otherwise, a value of -1 is returned and errno is set to indicate the error.

See also

exec(S-osr5), exit(S-osr5), fork(S-osr5), semctl(S-osr5), semget(S-osr5)

Standards conformance

semop is conformant with:

X/Open Portability Guide, Issue 3, 1989 .

© 2005 The SCO Group, Inc. All rights reserved.
SCO OpenServer Release 6.0.0 -- 02 June 2005