Undefined reference to pthread

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 %ldn", t);
      rc = pthread_create(&threads[t], NULL, PrintHello, (void *)t);
      if (rc){
         printf("ERROR; return code from pthread_create() is %dn", rc);
         exit(-1);
      }
   }
   pthread_exit(NULL);
}

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

[email protected]:~/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?

14 Answers

Both answers to this question so far are incorrect.
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.

in eclipse

properties->c/c++Build->setting->GCC C++ linker->libraries in top part add “pthread”

For Linux the correct command is:

gcc -o term term.c -lpthread

you have to put -lpthread just after the compile command,this command will tell to the compiler to execute program with pthread.h library.
gcc -l links with a library file.Link -l with library name without the lib prefix.

Running from the Linux terminal, what worked for me was compiling using the following command (suppose the c file I want to compile is called test.c):

gcc -o test test.c -pthread

Hope it helps somebody!

Acutally, it gives several examples of compile commands used for pthreads codes are listed in the table below, if you continue reading the following tutorial:

https://computing.llnl.gov/tutorials/pthreads/#Compiling

enter image description here

If you are using cmake, you can use:

add_compile_options(-pthread)

SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pthread")

I believe the proper way of adding pthread in CMake is with the following

find_package (Threads REQUIRED)

target_link_libraries(helloworld
    ${CMAKE_THREAD_LIBS_INIT}
)

Compile it like this : gcc demo.c -o demo -pthread

In Visual Studio 2019 specify -pthread in the property pages for the project under:

Linker -> Command Line -> Additional Options

Type in -pthread in the textbox.

You need to use the option -lpthread with gcc.

you need only Add “pthread” in proprieties=>C/C++ build=>GCC C++ Linker=>Libraries=> top part “Libraries(-l)”. thats it

check man page and you will get.

Compile and link with -pthread.

SYNOPSIS
       #include <pthread.h>

       int pthread_create(pthread_t *thread, const pthread_attr_t *attr,
                          void *(*start_routine) (void *), void *arg);


       Compile and link with -pthread.
       ....

In Anjuta, go to the Build menu, then Configure Project. In the Configure Options box, add:

LDFLAGS='-lpthread'

Hope it’ll help somebody too…

Sometimes, if you use multiple library, check the library dependency. (e.g. -lpthread -lSDL… <==> … -lSDL -lpthread)

Tips

Undefined reference to pthread_create in Linux

14 Answers

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