c - Unnamed semaphore POSIX IPC -

i allocated shared memory segment of size of integer.

the expected outcome on stdout should be:

p: 1 c: 2 

but instead is:

c: 1 p: 2 

why child process not being blocked until parent done , has unlocked shared memory segment?

#include <stdio.h> #include <stdlib.h> #include <sys/types.h> #include <sys/ipc.h> #include <sys/shm.h> #include <sys/wait.h> #include <unistd.h> #include <semaphore.h>  #define shmsegsize sizeof(int)  int main(void){     pid_t pid;    int shmid;    int *shared_mem;     /* initializing shared memory */    shmid  = shmget(ipc_private, shmsegsize, ipc_creat | 0644);    shared_mem  = (int *)shmat(shmid, 0, 0);    *shared_mem = 0;     /* initializing semaphore */    sem_t sem;    int pshared = 1; // !=0 processes, =0 threads    int value = 1; // number of processes @ time    sem_init(&sem, pshared, value); // initialize semaphore     pid = fork();     if(pid>(pid_t)0){ // parent        sem_wait(&sem);        sleep(6);        *shared_mem += 1;        printf("p: %d\n", *shared_mem);        sem_post(&sem);        exit(exit_success);    } // parent     if(pid==(pid_t)0){ // child        sleep(3);        sem_wait(&sem);        *shared_mem += 1;        sem_post(&sem);        printf("c: %d\n", *shared_mem);        exit(exit_success);    } // child     /* fork() failed */    printf("failed fork().");    exit(exit_failure); } 

compiled with:

gcc -o executable sem.c -pthread 

the sem_t must in shared memory if "pshared".

struct my_shared_mem {   sem_t sem;   int   value; };  ... later ...   struct my_shared_mem *shared;   shmid  = shmget(ipc_private, sizeof(*shared), ...);  shared = shmat(shmid, ...);  shared->value = 0;  sem_init(&shared->sem, 1, 1);  ...