Undefined reference to pthread_create in Linux

Question

I picked up the following demo off the web from https://computing.llnl.gov/tutorials/pthreads/

#include <pthread.h> #include <stdio.h> #define NUM_THREADS     5  void *PrintHello(void *threadid) {    long tid;    tid = (long)threadid;    printf("Hello World! It's me, thread #%ld!\n", tid);    pthread_exit(NULL); }  int main (int argc, char *argv[]) {    pthread_t threads[NUM_THREADS];    int rc;    long t;    for(t=0; t<NUM_THREADS; t++){       printf("In main: creating thread %ld\n", t);       rc = pthread_create(&threads[t], NULL, PrintHello, (void *)t);       if (rc){          printf("ERROR; return code from pthread_create() is %d\n", rc);          exit(-1);       }    }    pthread_exit(NULL); } 

But when I compile it on my machine (running Ubuntu Linux 9.04) I get the following error:

corey@ubuntu:~/demo$ gcc -o term term.c term.c: In function ‘main’: term.c:23: warning: incompatible implicit declaration of built-in function ‘exit’ /tmp/cc8BMzwx.o: In function `main': term.c:(.text+0x82): undefined reference to `pthread_create' collect2: ld returned 1 exit status 

This doesn't make any sense to me, because the header includes pthread.h, which should have the pthread_create function. Any ideas what's going wrong?

Answer 1

For Linux the correct command is:

gcc -pthread -o term term.c 

In general, libraries should follow sources and objects on command line, and -lpthread is not an "option", it's a library specification. On a system with only libpthread.a installed,

gcc -lpthread ... 

will fail to link.