The Linux Programming Interface

978 Chapter 47

This rather complex solution to the race problem is not required in all applica-

tions. We don’t need it if one process is guaranteed to be able to create and initialize

the semaphore before any other processes attempt to use it. This would be the

case, for example, if a parent creates and initializes the semaphore before creating

child processes with which it shares the semaphore. In such cases, it is sufficient for

the first process to follow its semget() call by a semctl() SETVAL or SETALL operation.

Figure 47-2: Two processes racing to initialize the same semaphore

47.6 Semaphore Operations

The semop() system call performs one or more operations on the semaphores in the

semaphore set identified by semid.

First semget() succeeds

(because set didn’t exist)

Process A Process B

time slice

expires

time slice

begins

time slice

begins

time slice

ends

First semget() fails (so this

process knows set exists)

Second semget() succeeds

Executes semop()

semctl() initializes semaphore

Executes semop() Executing

on CPU

Waiting

for CPU

Key

#include <sys/types.h> /* For portability */

#include <sys/sem.h>

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

Returns 0 on success, or –1 on error