semop —
semaphore operations
Standard C Library (libc, -lc)
#include <sys/sem.h>
int
  
  semop(int
    semid, struct sembuf
    *sops, size_t
    nsops);
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_NOWAITmakes the call return immediately, returning a value of -1 and setting
      errno toEAGAIN.
- 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.
Upon successful completion, a value of 0 is returned. Otherwise, -1 is returned
  and the global variable errno is set to indicate the
  error.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_SOPSin<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_UNDOwas 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_NOWAITwas set in
      sem_flg.
- [EFAULT]
- sops points to an illegal address.
Thesemop system call conforms to
  X/Open System Interfaces and Headers Issue 5
  (“XSH5”).
Semaphores appeared in the first release of AT&T
  System V UNIX.