The semop() function is used to perform atomically an array of semaphore operations on the set of semaphores associated with the semaphore identifier specified by semid. The sops argument is a pointer to the array of semaphore-operation structures. The nsops argument is the number of such structures in the array.

Each sembuf structure contains 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. The permission required for a semaphore operation is given as {token}, where token is the type of permission needed. The types of permission are interpreted as follows:

00400 READ by user 00200 ALTER by user 00040 READ by group 00020 ALTER by group 00004 READ by others 00002 ALTER by others

See the Semaphore Operation Permissions section of intro(2) for more information.

A process maintains a value, semadj, for each semaphore it modifies. This value contains the cumulative effect of operations the process has performed on an individual semaphore with the SEM_UNDO flag set (so that they can be undone if the process terminates unexpectedly). The value of semadj can affect the behavior of calls to sempo(), semtimedop(), exit(), and _exit() (the latter two functions documented on exit(2)), but is otherwise unobservable. See below for details.

The sem_op member specifies one of three semaphore operations:

1. The sem_op member is a negative integer; {ALTER}
   • If semval (see intro(2)) 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(2)) 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:
     • The value of 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(2)). When this occurs, errno is set to EIDRM and -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(3C).
2. The sem_op member is a positive integer; {ALTER}

   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.

3. The sem_op member is 0; {READ}
   • If semval is 0, semop() returns immediately.
   • If semval is not equal to 0 and (sem_flg&IPC_NOWAIT) is true, semop() returns immediately.
   • If semval is not equal to 0 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:
     • The value of semval becomes 0, at which time the value of semzcnt associated with the specified semaphore is set to 0 and all processes waiting on semval to become 0 are awakened.
     • The semid for which the calling process is awaiting action is removed from the system. When this occurs, errno is set to EIDRM and -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(3C).

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

The semtimedop() function behaves as semop() except when it must suspend execution of the calling process to complete its operation. If semtimedop() must suspend the calling process after the time interval specified in timeout expires, or if the timeout expires while the process is suspended, semtimedop() returns with an error. If the timespec structure pointed to by timeout is zero-valued and semtimedop() needs to suspend the calling process to complete the requested operation(s), it returns immediately with an error. If timeout is the NULL pointer, the behavior of semtimedop() is identical to that of semop().

Upon successful completion, 0 is returned. Otherwise, -1 is returned and errno is set to indicate the error.

The semop() and semtimedop() functions will fail if:

E2BIG

The nsops argument is greater than the system-imposed maximum.

EACCES

Operation permission is denied to the calling process (see intro(2)).

EAGAIN

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

EFAULT

The sops argument points to an illegal address.

EFBIG

The value of sem_num is less than 0 or greater than or equal to the number of semaphores in the set associated with semid.

EIDRM

A semid was removed from the system.

EINTR

A signal was received.

EINVAL

The semid argument is not a valid semaphore identifier, or the number of individual semaphores for which the calling process requests a 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 or a semadj value to overflow the system-imposed limit.

The semtimedop() function will fail if:

EAGAIN

The timeout expired before the requested operation could be completed.

The semtimedop() function will fail if one of the following is detected:

EFAULT

The timeout argument points to an illegal address.

EINVAL

The timeout argument specified a tv_sec or tv_nsec value less than 0, or a tv_nsec value greater than or equal to 1000 million.

ipcs(1), intro(2), exec(2), exit(2), fork(2), semctl(2), semget(2)