Manual Pages  — SEMOP

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

• 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(2). In this case, semop() returns immediately with a return value of EIDRM.
      • The process receives a signal that is to be caught. In this case, the process will resume execution as defined by sigaction(2).
      • 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(2). In this case, semop() returns immediately with a return value of EIDRM.
    • The process receives a signal that is to be caught. In this case, the process will resume execution as defined by sigaction(2).
    • The semaphore's value becomes zero.

For each semaphore a process has in use, the kernel maintains an "adjust on exit" value, 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 ensure that a resource is released if a process terminates unexpectedly.