Undefined Reference issues using Semaphores

I am playing around with using Semaphores, but I keep encountering Undefined Reference warnings, thus causing my code not to work. I pulled example code from a text, but was having issues with some of their syntax, so I went to POSIX's semaphore tutorial and changed things around to their syntax and as a result am now getting these reference errors.

I may simply be overlooking something, but I cannot find it.

Errors:

Producers_Consumers.c:52: warning: return type of ‘main’ is not ‘int’ /tmp/cceeOM6F.o: In function `producer': Producers_Consumers.c:(.text+0x1e): undefined reference to `sem_init' Producers_Consumers.c:(.text+0x3a): undefined reference to `sem_init' Producers_Consumers.c:(.text+0x46): undefined reference to `sem_wait' Producers_Consumers.c:(.text+0x52): undefined reference to `sem_wait' Producers_Consumers.c:(.text+0x5e): undefined reference to `sem_post' Producers_Consumers.c:(.text+0x6a): undefined reference to `sem_post' /tmp/cceeOM6F.o: In function `consumer': Producers_Consumers.c:(.text+0x7e): undefined reference to `sem_wait' Producers_Consumers.c:(.text+0x8a): undefined reference to `sem_wait' Producers_Consumers.c:(.text+0x96): undefined reference to `sem_post' Producers_Consumers.c:(.text+0xa2): undefined reference to `sem_post' collect2: ld returned 1 exit status 

What I have (It may look a bit ugly due to the way I commented things out from my old method) I also know my adding method won't work, but I'll get to that when I fix my syntax issues:

#include <stdio.h> #include <semaphore.h> #include <string.h> #include <pthread.h> #include <unistd.h> #include <stdlib.h> #include <sys/types.h> #include <sys/wait.h> #include <errno.h> #define N 10 //Number of slots in buffer typedef int semaphore; //Semaphores ae a special kind of int sem_t mutex; //Controls access to critical region 1 sem_t empty; //Counts empty buffer slots N sem_t full; //Counts full buffer slots 0 int count = 0; //What we're putting in //int buffer[N]; void producer(void) { sem_init(&mutex, 0, 1); //sem_init(&empty, 0, N); sem_init(&full, 0, 0); while(1) { sem_wait(&empty); sem_wait(&mutex); //printf("Empy: %d\n",empty); //printf("Mutex: %d\n",mutex); //printf("Both Downs Ran\n"); //buffer = buffer + 1; sem_post(&mutex); sem_post(&full); //printf("Producer produced: %d\n",buffer); } } void consumer(void) { while(1) { sem_wait(&full); sem_wait(&mutex); //item = buffer; sem_post(&mutex); sem_post(&empty); //printf("Consumer consumed: %d/n",item); } } void main() { } 

2 Answers

If you are on a linux system, you'll need to compile and link with the -pthread flag to link the pthreads library.

gcc -pthread Producers_Consumers.c 

As Paul Griffiths has pointed out, you can also use -lrt, which is more portable, and links the POSIX Realtime Extensions library

gcc Producers_Consumers.c -lrt 

Other notes specific to the code in the question:

• int main(void) not void main()
• typedef int semaphore is wrong, sem_t should be treated as an opaque type, you never use this typedef in your code anyway.
• A problem I foresee is that your consumer code uses the semaphores before they are initialized in producer. You should initialize them in your main

Got same error in ubuntu qt. After adding

LIBS += -lpthread -lrt

to project.pro file all compiled fine.