semop(S)

Syntax

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

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

Description

   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}

   • If semval is greater than or equal to the absolute value of semop, the absolute value of semop is subtracted from semval. Also, if (sem_flg & SEM_UNDO) is ``true'', the absolute value of semop is added to the calling process's semadj value (see exit(S)) for the specified semaphore.

   • If semval is less than the absolute value of semop and (sem_flg & IPC_NOWAIT) is ``true'', semop returns immediately.
     If semval is less than the absolute value of semop and (sem_flg & IPC_NOWAIT) is ``false'', semop increments the semncnt associated with the specified semaphore and suspend execution of the calling process until one of the following conditions occurs:

     • Semval becomes greater than or equal to the absolute value of semop. When this occurs, the value of semncnt associated with the specified semaphore is decremented, the absolute value of semop is subtracted from semval and, if (sem_flg & SEM_UNDO) is ``true'', the absolute value of semop is added to the calling process's semadj value for the specified semaphore.

     • The semid for which the calling process is awaiting action is removed from the system (see semctl(S)). When this occurs, errno is set equal to EIDRM, and a value of -1 is returned.

     • The calling process receives a signal that is to be caught. When this occurs, the value of semncnt associated with the specified semaphore is decremented, and the calling process resumes execution in the manner prescribed in LR sigaction S .

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}

   • If semval is zero, semop returns immediately.

   • If semval is not equal to zero and (sem_flg & IPC_NOWAIT) is ``true'', semop returns immediately.

   • If semval is not equal to zero and (sem_flg & IPC_NOWAIT) is ``false'', semop increments the semzcnt associated with the specified semaphore and suspends execution of the calling process until one of the following occurs:

     • Semval becomes zero, at which time the value of semzcnt associated with the specified semaphore is decremented.

     • The semid for which the calling process is awaiting action is removed from the system. When this occurs, errno is set equal to EIDRM, and a value of -1 is returned.

     • The calling process receives a signal that is to be caught. When this occurs, the value of semzcnt associated with the specified semaphore is decremented, and the calling process resumes execution in the manner prescribed in signal(S).

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

[E2BIG]

nsops is greater than the system-imposed maximum.

[EACCES]

Operation permission is denied to the calling process.

[EAGAIN]

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

[EFAULT]

sops points to an illegal address.

[EFBIG]

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

[EINVAL]

semid is not a valid semaphore identifier.

[EINVAL]

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

[ENOSPC]

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

[ERANGE]

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

[ERANGE]

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.

Diagnostics

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

Standards conformance

X/Open Portability Guide, Issue 3, 1989 .