concurrency - C Semaphore strange precedence behavior -

i'm practicing concurrency in c, , seem encounter problems semaphores. i'm using xcode 6.3.2 in macosx.

here sample program seems act strangely: purpose of example print either abcd or bacd strings

#include <stdio.h> #include <semaphore.h> #include <pthread.h> #include <errno.h>  void *thread1(void*); void *thread2(void*);  sem_t *sem0, *sem1, *sem2;;  int main(int argc, const char * argv[]) {      pthread_t t1, t2;      sem0 = sem_open("sem0", o_creat, 0600, 2);     sem1 = sem_open("sem1", o_creat, 0600, 0);     sem2 = sem_open("sem2", o_creat, 0600, 0);      // quick check     if (sem0 == sem_failed || sem1 == sem_failed || sem2 == sem_failed) {         printf("something went wrong\n");         return 0;     }      pthread_create(&t1, null, thread1, null);     pthread_create(&t2, null, thread2, null);      pthread_join(t1, null);     pthread_join(t2, null);      sem_close(sem0);     sem_close(sem1);     sem_close(sem2);      return 0; }  void *thread1(void* arg) {     int n=0;     while (n<10) {         sem_wait(sem0);         printf("a");         fflush(stdout);         sem_wait(sem1);         printf("c");         fflush(stdout);         sem_post(sem2);         n++;     }     pthread_exit(null); }  void *thread2(void* arg) {     int n=0;     while (n<10) {         sem_wait(sem0);         printf("b");         fflush(stdout);         sem_post(sem1);         sem_wait(sem2);         printf("d\n");         fflush(stdout);         sem_post(sem0);         sem_post(sem0);         n++;     }     pthread_exit(null); } 

if implemented correctly semaphores, result either abcd or bacd, in reality whole variety of strange output.

i include part of output here

abcd bad cabcd bad cbad cbad cbad cbad cbad cbad c 

is able me? in advance

important edit: downloaded ubuntu, , code working smoothly there, no problems @ all. so, resume

• macosx 10.10.3 xcode 6.3.2 --> not working
• ubuntu 15.04 --> working properly

don't know why.

your problem printf output buffered , shared between threads. you'd have ensure empty buffer fflush before posting on semaphores.