C
Semaphore
Learn semaphores with semget and semop.
Semaphore
A semaphore is a counter used for synchronization between processes. A set is created with semget(key, nsems, flags); IPC_PRIVATE produces a new set. The initial value is set with semctl(semid, 0, SETVAL, val).
The operation is defined by struct sembuf: {sem_num, sem_op, sem_flg}. sem_op = -1 is the wait (P), sem_op = +1 is the signal (V) operation. It is applied with semop(semid, &op, 1). If the counter is zero, the -1 operation waits; the +1 operation wakes a waiter. semctl(semid, 0, IPC_RMID) removes the set.
Parent signal, child wait pattern
You can use a semaphore for event ordering. The parent zeroes it with semctl(semid, 0, SETVAL, 0) before fork. The child waits with struct sembuf op = {0, -1, 0}; semop(semid, &op, 1);; since the counter is zero, it blocks.
When the parent finishes, it signals with struct sembuf op = {0, 1, 0}; semop(semid, &op, 1);. This releases the child's -1 operation; the child wakes up and does printf. This guarantees the child runs after the parent. Cleanup is done with wait(NULL) and semctl(semid, 0, IPC_RMID).
Exercise Order with a semaphore
Set up a set with int semid = semget(IPC_PRIVATE, 1, 0600 | IPC_CREAT);. Zero it with semctl(semid, 0, SETVAL, 0);. Fork a child with pid_t pid = fork();. In the child: struct sembuf op = {0, -1, 0}; semop(semid, &op, 1);, printf("child came\n");, _exit(0);. In the parent: struct sembuf op = {0, 1, 0}; semop(semid, &op, 1);, wait(NULL);, semctl(semid, 0, IPC_RMID);.
Show example solution
#include <stdio.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/wait.h>
#include <unistd.h>
int main(void) {
int semid = semget(IPC_PRIVATE, 1, 0600 | IPC_CREAT);
semctl(semid, 0, SETVAL, 0);
pid_t pid = fork();
if (pid == 0) {
struct sembuf op = {0, -1, 0};
semop(semid, &op, 1);
printf("child came\n");
_exit(0);
}
struct sembuf op = {0, 1, 0};
semop(semid, &op, 1);
wait(NULL);
semctl(semid, 0, IPC_RMID);
return 0;
}
Test yourself
-
Match semaphore functions
- semget
- Creates set
- semop
- Applies operation
- semctl
- Sets value/removes
- sembuf
- Operation definition
-
What does sem_op = -1 do in struct sembuf?
- Remove set
- Signal (V) operation
- Wait (P) operation
- Set value
Show answer
Wait (P) operation — sem_op = -1 is the wait (P) operation; it waits if counter is zero.
-
semop call
struct sembuf op = {0, 1, 0}; semop(semid, &op, ____);
Show answer
struct sembuf op = {0, 1, 0}; semop(semid, &op, 1);
Next lesson: Summary - IPC
