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 += -pthreadmingw32:LDFLAGS += -lpthread

On Linux (but this is non portable to other systems), if the mutex is a global or static variable, you could initialize it like

static pthread_mutex_t recmutex = PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP;

(and by the way, the example is from pthread_mutex_init(3) man pages!)

To create a recursive mutex, use:

#include <pthread.h>int pthread_mutexatttr_settype(pthread_mutexattr_t *attr,                               int type);

where type is PTHREAD_MUTEX_RECURSIVE.

Don't forget to check the return value!

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);