Name

Library

Synopsis

Return values

Detailed description

struct sembuf {
        u_short sem_num;        /* semaphore # */
        short   sem_op;         /* semaphore operation */
        short   sem_flg;        /* operation flags */
};

         

For each element in array, sem_op and sem_flg determine an operation to be performed on semaphore number sem_num in the set. The values SEM_UNDO and IPC_NOWAIT may be OR ’ed into the sem_flg member in order to modify the behavior of the given operation.

The operation performed depends as follows on the value of sem_op:

• When sem_op is positive and the process has alter permission, the semaphore’s value is incremented by sem_op ’s value. If SEM_UNDO is specified, the semaphore’s adjust on exit value is decremented by sem_op ’s value. A positive value for sem_op generally corresponds to a process releasing a resource associated with the semaphore.
• The behavior when sem_op is negative and the process has alter permission, depends on the current value of the semaphore:
  • If the current value of the semaphore is greater than or equal to the absolute value of sem_op, then the value is decremented by the absolute value of sem_op. If SEM_UNDO is specified, the semaphore’s adjust on exit value is incremented by the absolute value of sem_op.
  • If the current value of the semaphore is less than the absolute value of sem_op, one of the following happens:
    • If IPC_NOWAIT was specified, then semop returns immediately with a return value of EAGAIN.
    • Otherwise, the calling process is put to sleep until one of the following conditions is satisfied:
      • Some other process removes the semaphore with the IPC_RMID option of semctl. In this case, semop returns immediately with a return value of EIDRM.
      • The semaphore’s value is greater than or equal to the absolute value of sem_op. When this condition becomes true, the semaphore’s value is decremented by the absolute value of sem_op, the semaphore’s adjust on exit value is incremented by the absolute value of sem_op.

  A negative value for sem_op generally means that a process is waiting for a resource to become available.

• When sem_op is zero and the process has read permission, one of the following will occur:
  • If the current value of the semaphore is equal to zero then semop can return immediately.
  • If IPC_NOWAIT was specified, then semop returns immediately with a return value of EAGAIN.
  • Otherwise, the calling process is put to sleep until one of the following conditions is satisfied:
    • Some other process removes the semaphore with the IPC_RMID option of semctl. In this case, semop returns immediately with a return value of EIDRM.
    • The semaphore’s value becomes zero.

For each semaphore a process has in use, an "adjust on exit" value is maintained, as alluded to earlier. When a process exits, either voluntarily or involuntarily, the adjust on exit value for each semaphore is added to the semaphore’s value. This can be used to insure that a resource is released if a process terminates unexpectedly.

Examples

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

         
void File_Update(char* path, int val)
{
    struct sembuf sem_op;
    FILE* fp;
    /*
     * Wait on the semaphore till the value is non-negative.
     */
    sem_op.sem_num = 0;
    sem_op.sem_op = -1;
    sem_op.sem_flg = 0;
    semop(sem_set_id, &sem_op, 1);
    /*
     * If we are here, then We have locked the semaphore,
     * and are assured exclusive access to file.
     * We can now manipulate the file
     */
    fp = fopen(path, "w");
    if (fp) {
        fprintf(fp, "%d\n", val);
        fclose(fp);
    }
    /*
     * Increase the value of the semaphore by 1 so that others blocked on
     * this semaphore get awakened.
     */
    sem_op.sem_num = 0;
    sem_op.sem_op = 1;
    sem_op.sem_flg = 0;
    semop(sem_set_id, &sem_op, 1);
}

         

Errors

See also

Bugs

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