Undefined Reference to sem_wait and pthread_create when compiling with g++ - c

Question

I am fairly new to C and do not understand why I am getting these errors when I try to compile my program using g++ ./main.c. I have tried looking at other resources and I am unable to find the answers I need. If there is a solution that you already know of, please post it here as well.
/tmp/ccSGRAcp.o: In function `producer(void*)':
main.c:(.text+0x12): undefined reference to `sem_wait'
main.c:(.text+0x1c): undefined reference to `sem_wait'
main.c:(.text+0xa0): undefined reference to `sem_post'
main.c:(.text+0xaa): undefined reference to `sem_post'
/tmp/ccSGRAcp.o: In function `consumer(void*)':
main.c:(.text+0xc5): undefined reference to `sem_wait'
main.c:(.text+0xcf): undefined reference to `sem_wait'
main.c:(.text+0x153): undefined reference to `sem_post'
main.c:(.text+0x15d): undefined reference to `sem_post'
/tmp/ccSGRAcp.o: In function `main':
main.c:(.text+0x17e): undefined reference to `sem_init'
main.c:(.text+0x192): undefined reference to `sem_init'
main.c:(.text+0x1a6): undefined reference to `sem_init'
main.c:(.text+0x1c1): undefined reference to `pthread_create'
main.c:(.text+0x1dc): undefined reference to `pthread_create'
main.c:(.text+0x1f7): undefined reference to `pthread_create'
main.c:(.text+0x212): undefined reference to `pthread_create'
main.c:(.text+0x22d): undefined reference to `pthread_create'
/tmp/ccSGRAcp.o:main.c:(.text+0x248): more undefined references to `pthread_create' follow
collect2: error: ld returned 1 exit status
The code I am trying to compile is
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <semaphore.h>
#include <pthread.h>
#define N 10000000
typedef struct
{
char const* buf[N];
char in;
char out;
sem_t mutex;
sem_t full;
sem_t empty;
} bufferItems;
bufferItems sharedBuffer;
void *producer(void *arg) {
while(1) {
sem_wait(&sharedBuffer.empty);
sem_wait(&sharedBuffer.mutex);
sharedBuffer.buf[sharedBuffer.in] = "X";
sharedBuffer.in = (sharedBuffer.in+1)%N;
printf("Producer ");
printf("%c", sharedBuffer.in);
printf("\n");
sem_post(&sharedBuffer.mutex);
sem_post(&sharedBuffer.full);
}
}
void *consumer(void *arg){
while(1){
sem_wait(&sharedBuffer.full);
sem_wait(&sharedBuffer.mutex);
sharedBuffer.buf[sharedBuffer.out] = NULL;
sharedBuffer.out = (sharedBuffer.out+1)%N;
printf("Consumer ");
printf("%c", sharedBuffer.out);
printf("\n");
sem_post(&sharedBuffer.mutex);
sem_post(&sharedBuffer.empty);
}
}
int main(void) {
sem_init(&sharedBuffer.mutex, 0, 0);
sem_init(&sharedBuffer.full, 0, 0);
sem_init(&sharedBuffer.empty, 0, N);
pthread_t p1;
pthread_t p2;
pthread_t p3;
pthread_t p4;
pthread_t c1;
pthread_t c2;
pthread_t c3;
pthread_t c4;
// create four producer threads
pthread_create(&p1,NULL,producer,NULL);
pthread_create(&p2,NULL,producer,NULL);
pthread_create(&p3,NULL,producer,NULL);
pthread_create(&p4,NULL,producer,NULL);
// create four consumer threads
pthread_create(&c1,NULL,consumer,NULL);
pthread_create(&c2,NULL,consumer,NULL);
pthread_create(&c3,NULL,consumer,NULL);
pthread_create(&c4,NULL,consumer,NULL);
}

Answer 1

Add the -pthread param to pull in all the threading stuff for linking
g++ ./main.c -pthread