As title, the key is to set an attribute(PTHREAD_PROCESS_SHARED) to the mutex/condition variable using pthread_mutexattr_setpshared() or pthread_condattr_setpshared(). Without these function calls, the parent in the following code will not get signaled forever.
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/* * gcc mutex.c -o mutex -lrt */ #include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <pthread.h> #include <errno.h> #include <sys/types.h> #include <sys/mman.h> #include <sys/stat.h> #include <fcntl.h> #define MYMUTEX "/mymutex" #define MYCOND "/mycond" int main(int argc, char* argv[]) { pthread_cond_t *cond; pthread_mutex_t *mutex; int cond_id, mutex_id; int mode = S_IRWXU | S_IRWXG; /* mutex */ mutex_id = shm_open(MYMUTEX, O_CREAT | O_RDWR | O_TRUNC, mode); if (mutex_id < 0) { perror("shm_open failed with " MYMUTEX); return -1; } if (ftruncate(mutex_id, sizeof(pthread_mutex_t)) == -1) { perror("ftruncate failed with " MYMUTEX); return -1; } mutex = (pthread_mutex_t *) mmap(NULL, sizeof(pthread_mutex_t), PROT_READ | PROT_WRITE, MAP_SHARED, mutex_id, 0); if (mutex == MAP_FAILED) { perror("mmap failed with " MYMUTEX); return -1; } /* cond */ cond_id = shm_open(MYCOND, O_CREAT | O_RDWR | O_TRUNC, mode); if (cond_id < 0) { perror("shm_open failed with " MYCOND); return -1; } if (ftruncate(cond_id, sizeof(pthread_cond_t)) == -1) { perror("ftruncate failed with " MYCOND); return -1; } cond = (pthread_cond_t *) mmap(NULL, sizeof(pthread_cond_t), PROT_READ | PROT_WRITE, MAP_SHARED, cond_id, 0); if (cond == MAP_FAILED) { perror("ftruncate failed with " MYCOND); return -1; } /* set mutex shared between processes */ pthread_mutexattr_t mattr; pthread_mutexattr_init(&mattr); pthread_mutexattr_setpshared(&mattr, PTHREAD_PROCESS_SHARED); pthread_mutex_init(mutex, &mattr); pthread_mutexattr_destroy(&mattr); /* set condition shared between processes */ pthread_condattr_t cattr; pthread_condattr_init(&cattr); pthread_condattr_setpshared(&cattr, PTHREAD_PROCESS_SHARED); pthread_cond_init(cond, &cattr); pthread_condattr_destroy(&cattr); /*************************************/ pid_t pid; if ((pid = fork()) < 0) { perror("fork failure"); return -1; } else if (pid == 0) { /* child */ sleep(5); pthread_mutex_lock(mutex); pthread_cond_signal(cond); printf("child signaled\n"); pthread_mutex_unlock(mutex); exit(0); } else { /* parent */ printf("parent waiting on condition\n"); pthread_mutex_lock(mutex); pthread_cond_wait(cond, mutex); printf("parent signaled by child, wake up!!!\n"); pthread_mutex_unlock(mutex); pthread_mutex_destroy(mutex); pthread_cond_destroy(cond); shm_unlink(MYCOND); shm_unlink(MYMUTEX); } return 0; } |
Shared memory is used to share the mutex and condition variable.
NOTE: The process-shared mutex attribute isn’t universally supported yet. You should confirm before using them.
Updated Oct 14, 2020: Fixed pthread_mutexattr_t and pthread_condattr_t initialization.