semop(S)

semop -- semaphore operations

Synopsis

   #include <sys/types.h>
   #include <sys/ipc.h>
   #include <sys/sem.h>
   
   int semop(int semid, struct sembuf sops, size_t nsops);

Description

semop is used to perform atomically 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 sem_op is performed on the corresponding semaphore specified by semid and sem_num.

sem_op specifies one of three semaphore operations as follows, depending on whether its value is negative, positive, or zero:

If sem_op is a negative integer, one of the following occurs: {ALTER}

If semval [see intro(S)] is greater than or equal to the absolute value of sem_op, the absolute value of sem_op is subtracted from semval. Also, if (sem_flg&SEM_UNDO) is true, the absolute value of sem_op 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 sem_op and (sem_flg&IPC_NOWAIT) is true, semop returns immediately.
If semval is less than the absolute value of sem_op and (sem_flg&IPC_NOWAIT) is false, semop increments the semncnt associated with the specified semaphore and suspends execution of the calling process until one of the following conditions occur.
- semval becomes greater than or equal to the absolute value of sem_op. When this occurs, the value of semncnt associated with the specified semaphore is decremented, the absolute value of sem_op is subtracted from semval and, if (sem_flg&SEM_UNDO) is true, the absolute value of sem_op 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 signal(M).

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

If sem_op 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).

Return values

On success, semop returns 0, and 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.

On failure, semop returns -1 and sets errno to identify the error.

Errors

In the following conditions, semop fails and sets errno to:

EINTR: semop returned due to receipt of a signal.
EIDRM: semop returned due to the removal of semid from the system.
EFBIG: sem_num is less than zero or greater than or equal to the number of semaphores in the set associated with semid. The signal SIGXFSZ is always generated by this error condition.
E2BIG: nsops is greater than the system-imposed maximum.
EACCES: Operation permission is denied to the calling process [see intro(S)].
EAGAIN: The operation would result in suspension of the calling process but (sem_flg&IPC_NOWAIT) is true.
ENOSPC: The limit on the number of individual processes requesting an SEM_UNDO would be exceeded.
EINVAL: semid is not a valid semaphore identifier.
EINVAL: The number of individual semaphores for which the calling process requests a SEM_UNDO would exceed the limit.
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.
EFAULT: sops points to an illegal address.

References

exec(S), exit(S), fork(S), intro(S), semctl(S), semget(S)

Notices

Considerations for threads programming

While one thread is blocked, siblings might still be executing.

The Threads Library provides two other semaphore facilities for the synchronization of multithreaded programs. See Intro(PTHREAD) and semaphore(SYNCH) for details. Both facilities can also be used for synchronization between processes.