semop(2) -- Linux man page

NAME

SYNOPSIS

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

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

int semtimedop(int semid, struct sembuf *sops, unsigned nsops, struct timespec *timeout);  

DESCRIPTION

unsigned short  semval;   /* semaphore value */
unsigned short  semzcnt;  /* # waiting for zero */
unsigned short  semncnt;  /* # waiting for increase */
pid_t           sempid;   /* process that did last op */

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

The set of operations contained in sops is performed atomically, that is, the operations are performed at the same time, and only if they can all be simultaneously performed. The behaviour of the system call if not all operations can be performed immediately depends on the presence of the IPC_NOWAIT flag in the individual sem_flg fields, as noted below.

Each operation is performed on the sem_num-th semaphore of the semaphore set, where the first semaphore of the set is semaphore 0. There are three types of operation, distinguished by the value of sem_op.

If sem_op is a positive integer, the operation adds this value to the semaphore value (semval). Furthermore, if SEM_UNDO is asserted for this operation, the system updates the process undo count (semadj) for this semaphore. This operation can always proceed - it never forces a process to wait. The calling process must have alter permission on the semaphore set.

If sem_op is zero, the process must have read access permission on the semaphore set. This is a "wait-for-zero" operation: if semval is zero, the operation can immediately proceed. Otherwise, if IPC_NOWAIT is asserted in sem_flg, the system call fails with errno set to EAGAIN (and none of the operations in sops is performed). Otherwise semzcnt (the count of processes waiting until this semaphore's value becomes zero) is incremented by one and the process sleeps until one of the following occurs:

*

semval becomes 0, at which time the value of semzcnt is decremented.

*

The semaphore set is removed: the system call fails, with errno set to EIDRM.

*

The calling process catches a signal: the value of semzcnt is decremented and the system call fails, with errno set to EINTR.

*

The time limit specified by timeout in a semtimedop call expires: the system call fails, with errno set to EAGAIN.

If sem_op is less than zero, the process must have alter permission on the semaphore set. If semval is greater than or equal to the absolute value of sem_op, the operation can proceed immediately: the absolute value of sem_op is subtracted from semval, and, if SEM_UNDO is asserted for this operation, the system updates the process undo count (semadj) for this semaphore. If the absolute value of sem_op is greater than semval, and IPC_NOWAIT is asserted in sem_flg, the system call fails, with errno set to EAGAIN (and none of the operations in sops is performed). Otherwise semncnt (the counter of processes waiting for this semaphore's value to increase) is incremented by one and the process sleeps until one of the following occurs:

*

semval becomes greater than or equal to the absolute value of sem_op, at which time the value of semncnt is decremented, the absolute value of sem_op is subtracted from semval and, if SEM_UNDO is asserted for this operation, the system updates the process undo count (semadj) for this semaphore.

*

The semaphore set is removed from the system: the system call fails with errno set to EIDRM.

*

The calling process catches a signal: the value of semncnt is decremented and the system call fails with errno set to EINTR.

*

The time limit specified by timeout in a semtimedop call expires: the system call fails, with errno set to EAGAIN.

On successful completion, the sempid value for each semaphore specified in the array pointed to by sops is set to the process ID of the calling process. In addition, the sem_otime is set to the current time.

The function semtimedop behaves identically to the function semop except that in those cases were the calling process would sleep, the duration of that sleep is limited by the amount of elapsed time specified by the timespec structure whose address is passed in the timeout parameter. If the specified time limit has been reached, the system call fails with errno set to EAGAIN (and none of the operations in sops is performed). If the timeout parameter is NULL, then semtimedop behaves exactly like semop.  

RETURN VALUE

ERRORS

E2BIG

The argument nsops is greater than SEMOPM, the maximum number of operations allowed per system call.

EACCES

The calling process has no access permissions on the semaphore set as required by one of the specified operations.

EAGAIN

An operation could not proceed immediately and either IPC_NOWAIT was asserted in its sem_flg or the time limit specified in timeout expired.

EFAULT

An address specified in either the sops or timeout parameters isn't accessible.

EFBIG

For some operation the value of sem_num is less than 0 or greater than or equal to the number of semaphores in the set.

EIDRM

The semaphore set was removed.

EINTR

While blocked in this system call, the process caught a signal.

EINVAL

The semaphore set doesn't exist, or semid is less than zero, or nsops has a non-positive value.

ENOMEM

The sem_flg of some operation asserted SEM_UNDO and the system does not have enough memory to allocate the undo structure.

ERANGE

For some operation sem_op+semval is greater than SEMVMX, the implementation dependent maximum value for semval.

NOTES

semop is never automatically restarted after being interrupted by a signal handler, regardless of the setting of the SA_RESTART flags when establishing a signal handler.

semadj is a per-process integer which is simply the (negative) count of all semaphore operations performed specifying the SEM_UNDO flag. When a semaphore's value is directly set using the SETVAL or SETALL request to semctl(2), the corresponding semadj values in all processes are cleared.

The semval, sempid, semzcnt, and semnct values for a semaphore can all be retrieved using appropriate semctl(2) calls.

The followings are limits on semaphore set resources affecting a semop call:

SEMOPM

Maximum number of operations allowed for one semop call (32).

SEMVMX

Maximum allowable value for semval: implementation dependent (32767).

The implementation has no intrinsic limits for the adjust on exit maximum value (SEMAEM), the system wide maximum number of undo structures (SEMMNU) and the per-process maximum number of undo entries system parameters.  

BUGS

CONFORMING TO

SEE ALSO