NAME

semop, semtimedop — semaphore operations

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <sys/sem.h>

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

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

DESCRIPTION

semop() provides a number of atomic operations on a set of semaphores. The semaphore set is specified by semid, sops is an array of semaphore operations, and nsops is the number of operations in this array. The sembuf structures in the array contain the following members:

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

Each operation (specified in sem_op) is applied to semaphore number sem_num in the set of semaphores specified by semid. The value of sem_op determines the action taken in the following way:

• sem_op is less than 0. The current process is blocked until the value of the semaphore is greater than or equal to the absolute value of sem_op. The absolute value of sem_op is then subtracted from the value of the semaphore, and the calling process continues. Negative values of sem_op are thus used to enter critical regions.
• sem_op is greater than 0. Its value is added to the value of the specified semaphore. This is used to leave critical regions.
• sem_op is equal to 0. The calling process is blocked until the value of the specified semaphore reaches 0.

The behaviour of each operation is influenced by the flags set in sem_flg in the following way:

IPC_NOWAIT

In the case where the calling process would normally block, waiting for a semaphore to reach a certain value, IPC_NOWAIT makes the call return immediately, returning a value of -1 and setting errno to EAGAIN.

SEM_UNDO

Keep track of the changes that this call makes to the value of a semaphore, so that they can be undone when the calling process terminates. This is useful to prevent other processes waiting on a semaphore to block forever, should the process that has the semaphore locked terminate in a critical section.

semtimedop() is similar to semop(), but it also allows specifying a timeout. When the semaphore is not available, the thread typically sleeps until the semaphore is available. semtimedop() allows specifying a maximum amount of time in timeout argument that a thread should sleep while waiting for the semaphore to be available. If the specified time limit has been reached, semtimedop() fails with EAGAIN (and none of the operations in sops are performed). If timeout is NULL, semtimedop() behaves exactly like semop().

RETURN VALUES

Upon successful completion both semop() and semtimedop() return a value of 0. Otherwise, -1 is returned and the global variable errno is set to indicate the error.

ERRORS

semop() will fail if:

[EINVAL]

There is no semaphore associated with semid.

[EIDRM]

The semaphore set was removed while the process was waiting for one of its semaphores to reach a certain value.

[EACCES]

The calling process has no permission to access the specified semaphore set.

[E2BIG]

The value of nsops is too big. The maximum is defined as MAX_SOPS in <sys/sem.h>.

[EFBIG]

sem_num in one of the sem_buf structures is less than 0, or greater than the actual number of semaphores in the set specified by semid.

[ENOSPC]

SEM_UNDO was requested, and there is not enough space left in the kernel to store the undo information.

[EAGAIN]

The requested operation can not immediately be performed, and IPC_NOWAIT was set in sem_flg.

[EFAULT]

sops points to an illegal address.

[EINTR]

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

In addition, semtimedop() will fail if:

[EAGAIN]

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

[EFAULT]

An address specified in the timeout argument isn't accessible.

EXAMPLES

The following example shows how to perform a semaphore operation with a timeout:

/* Performs a semaphore operation with a 5 sec timeout*/

struct sembuf sops[1];        /* Semaphore operation structure */
struct timespec timeout;      /* Timeout structure */

/* Create semaphore set with 1 semaphore */
int semid = semget(key, 1, 0666 | IPC_CREAT);

/* Initialize semaphore to 0 */
if (semctl(semid, 0, SETVAL, 0) == -1) {
    warn("semctl SETVAL");
    exit(EXIT_FAILURE);
}

sops[0].sem_num = 0;          /* Operation on semaphore 0 */
sops[0].sem_op = -1;          /* Decrement semaphore by 1 */
sops[0].sem_flg = 0;          /* No flags */

timeout.tv_sec = 5;           /* 5 seconds */
timeout.tv_nsec = 0;          /* 0 nanoseconds */

if (semtimedop(semid, sops, 1, &timeout) == -1) {
    warn("semtimedop");       /* Print error message */
}

SEE ALSO

semctl(2), semget(2)

STANDARDS

The semop, semtimedop system call conforms to X/Open System Interfaces and Headers Issue 5 (“XSH5”).

HISTORY

Semaphores appeared in the first release of AT&T System V UNIX.