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

Question

I am a bit confused on how to declare a recursive mutex using pthread. What I try to do is have only one thread at a time be able to run a piece of code(including functions) but after scepticism I figured out that the use of mutexes would not work and that instead I should use recursive mutexes. Here is my code:

pthread_mutex_lock(&mutex);                   // LOCK

item = queue_peek(queue);                     // get last item in queue
item_buff=item;                               // save item to a buffer
queue_removelast(queue);                      // remove last item from queue

pthread_mutex_unlock(&mutex);                 // UNLOCK

So what I try to do is just read/remove from the queue serially.

The thing is that there isn't any example out there on how to declare recursive mutexes. Or there maybe a few but they don't compile for me.

Answer 1

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

Answer 2

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);

Answer 3

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!)

Answer 4

You need to add mutex attributes when creating the mutex.

Call pthread_mutexattr_init, then pthread_mutexattr_settype with PTHREAD_MUTEX_RECURSIVE then use these attributes with pthread_mutex_init. Read man pthread_mutexattr_init for more info.