When I tried to compile this program, it failed:
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>
void *WriteNumbers(void *threadArg)
{
int start, stop;
start = atoi((char *)threadArg);
stop = start + 10;
while (start < stop)
{
printf("%d\n", start++);
sleep(1);
}
return 0;
}
int main(int argc, char **argv)
{
pthread_t thread1, thread2;
// create the threads and start the printing
pthread_create(&thread1, NULL, WriteNumbers, (void *)argv[1] );
pthread_create(&thread2, NULL, WriteNumbers, (void *)argv[2]);
pthread_join(thread1, NULL);
pthread_join(thread2, NULL);
return 0;
}
It gave me the following errors:
tmp/ccrW21s7.o: In function `main':
pthread.c:(.text+0x83): undefined reference to `pthread_create'
pthread.c:(.text+0xaa): undefined reference to `pthread_create'
pthread.c:(.text+0xbd): undefined reference to `pthread_join'
pthread.c:(.text+0xd0): undefined reference to `pthread_join'
collect2: ld returned 1 exit status
Why does it give me these undefined reference errors even though I had included pthread.h
, which declares these functions?
You probably forgot to link with the Pthreads library (using -lpthread
on the command line).
Others have mentioned that you haven't linked with the pthread library using the -lpthread
flag. Modern GCC (not sure how modern, mine is 4.3.3) allows you to use just -pthread
. From the man page:
-pthread
Adds support for multithreading with the pthreads library. This option sets flags for both the preprocessor and linker.
You need to link pthread
library to your binary, like this:
cc -o myapp myapp.c -lpthread
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