使用未声明的标识符 'PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP'

Question

I'm trying to compile using gcc on macOS 10.14 with gcc in the Terminal.

I've included #define _GNU_SOURCE at the top of my C program, and #include <pthread.h>

But when I use the following: gcc input.c -o output -lpthread or -pthread I get the following error. I've also tried with -std=c99 :

input.c:50:33: error: use of undeclared identifier
'PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP'
pthread_mutex_t request_mutex = PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP;

---

Here's a summary of my code:

input.c:

#define _GNU_SOURCE
#include <stdlib.h>
…
#include <pthread.h>

pthread_mutex_t request_mutex = PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP;

int main(int argc, char* argv[])
{
  ...
}

Edit

I was able to get the code working by removing _NP at the end of PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP . Thanks for everyone's suggestions.

Answer 1

PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP is a non-standard extension, and evidently MacOS doesn't support it.

You should instead initialise your mutex at the beginning of main() :

#include <stdio.h>
#include <string.h>
#include <pthread.h>

pthread_mutex_t request_mutex;

int init_recursive_mutex(pthread_mutex_t *mutex)
{
    pthread_mutexattr_t attr;
    int r;

    r = pthread_mutexattr_init(&attr);
    if (r != 0)
        return r;

    r = pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE);

    if (r == 0)
        r = pthread_mutex_init(mutex, &attr);

    pthread_mutexattr_destroy(&attr);

    return r;
}

int main(int argc, char* argv[])
{
    int r;

    r = init_recursive_mutex(&request_mutex);
    if (r != 0)
    {
        fprintf(stderr, "Failed to initialise request_mutex: %s\n", strerror(r));
        return 1;
    }

    /* ... */
}

Answer 2

_GNU_SOURCE will not do anything to reveal library extensions on Mac OS, because Mac OS isn't from the GNU project and doesn't use the GNU C library.

On Mac OS, the "reveal extensions" feature selection macro is _DARWIN_C_SOURCE .

However, this will not necessarily reveal a PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP ; I don't think it exists. Of course, the recursive mutex type PTHREAD_MUTEX_RECURSIVE exists; only there is no initializer to set one up statically. The code will have to be ported to explicitly call pthread_mutex_init to initialize its recursive mutexes.

Another idea is to just write code that doesn't require recursive mutexes. Recursive mutexes are for "scatter brained" concurrent programming. "Well, I don't know whether I already have this lock or not, so I will grab it just in case." The sane alternative is to design the program so that you know!

Recursive mutexes are essentially a band-aid solution for certain situations that arise when when legacy single-threaded code is being converted to multithreading. Recursive mutexes are out of place in a completely new design.

If you avoid recursive mutexes, you can use the standard PTHREAD_MUTEX_INITIALIZER .

Answer 3

PTHREAD_RECURSIVE_MUTEX_INITIALIZER is available on OSX, but only starting from 10.7, so if you have to support earlier versions, you have to either initialize mutex directly before use (like @caf suggested in this answer ) or initialize it lazily using pthread_once :

#if ((__MAC_OS_X_VERSION_MIN_REQUIRED && __MAC_OS_X_VERSION_MIN_REQUIRED >= 1070) || (__IPHONE_OS_VERSION_MIN_REQUIRED && __IPHONE_OS_VERSION_MIN_REQUIRED >= 50000)) || defined(__DRIVERKIT_VERSION_MIN_REQUIRED)

static pthread_mutex_t s_mutex = PTHREAD_RECURSIVE_MUTEX_INITIALIZER;

int lock() {
  return pthread_mutex_lock(&s_mutex);
}

int unlock() {
  return pthread_mutex_unlock(&s_mutex);
}

#else

static pthread_mutex_t s_mutex;
static pthread_once_t s_mutex_init = PTHREAD_ONCE_INIT;

static void init_mutex() {
  init_recursive_mutex(&s_mutex);
} 

int lock() {
  pthread_once(&s_mutex_init, init_mutex);
  return pthread_mutex_lock(&s_mutex);
} 

int unlock() {
  return pthread_mutex_unlock(&s_mutex);
}

#endif