Question

I am trying to compile the following program with mingw:

#include <pthread.h>
#include <errno.h>
#include <unistd.h>
#include <iostream>
#include <cstdio>

void *hello(void *id) {
  int nid = *static_cast<int*>(id);
  std::printf("Hello from thread %d\n", nid);
  return 0;
}

int main(int argc, char **argv) {
  pthread_t ids[2];
  int *params[2];
  for (int i = 0; i < 2; ++i) {
    params[i] = new int;
    *params[i] = i;
    pthread_create(&ids[i], 0, hello, params[i]);
  }
  for (int i = 0; i < 2; ++i)
    pthread_join(ids[i], 0);
  for (int i = 0; i < 2; ++i)
    delete params[i];
  return 0;
}

using this command:

g++ -lpthread -ohello.exe hello.cc

And I get the following message:

C:\Users\XXXXXX~1\AppData\Local\Temp\cczPlv0w.o:hello.cc:(.text+0xad): undefined 
reference to `_imp__pthread_create'
C:\Users\XXXXXX~1\AppData\Local\Temp\cczPlv0w.o:hello.cc:(.text+0xe9): undefined 
reference to `_imp__pthread_join'
collect2: ld returned 1 exit status

But with an older version of MingGW I had no problems running pthreads programs. (This is just the simple of all the programs that failed, but basically everything that uses pthreads ends up with the same error, C and C++)

Answer 1

Move -lpthread to the end of that command:

g++ -ohello.exe hello.cc -lpthread

The order of the arguments is important. (Using -pthread throughout instead of -lpthread for linking is actually recommended, since it sets flags both for the preprocessor and the linker.)