C: How do you declare a recursive mutex with POSIX threads?

The code from Michael Foukarakis is almost good but he initializes the mutex twice which leads to undefined behavior. It should just be:

pthread_mutex_t Mutex;
pthread_mutexattr_t Attr;

pthread_mutexattr_init(&Attr);
pthread_mutexattr_settype(&Attr, PTHREAD_MUTEX_RECURSIVE);
pthread_mutex_init(&Mutex, &Attr);

I actually use this code in production, and I know it works correctly on Linux, Solaris, HP-UX, AIX, Mac OSX and FreeBSD.

You also need to add proper linker flag to compile this:

AIX, Linux, FreeBSD:
CPLATFORM += -pthread

mingw32:
LDFLAGS += -lpthread

To create a recursive mutex, you can either use:

#include <pthread.h>
/* Don't forget to check the return value! */
int pthread_mutexatttr_settype(pthread_mutexattr_t *attr,
                               int type);

where type is PTHREAD_MUTEX_RECURSIVE, or an initialiser.

For example:

/* ..or PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP */
pthread_mutex_t       mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_mutexattr_t   mta;

or alternatively, initialize at runtime (don't do both, it's undefined behaviour):

pthread_mutexattr_init(&mta);
/* or PTHREAD_MUTEX_RECURSIVE_NP */
pthread_mutexattr_settype(&mta, PTHREAD_MUTEX_RECURSIVE);

pthread_mutex_init(&mutex, &mta);