Advanced Programming in the UNIX® Environment

ptg10805159

436 Thread Control Chapter 12

main

func1(x)

func2(x)

func1

pthread_mutex_lock(x->lock)

func2_locked(x)

pthread_mutex_unlock(x->lock)

func2

pthread_mutex_lock(x->lock)

func2_locked(x)

pthread_mutex_unlock(x->lock)

func2_locked

Figure 12.7 Avoiding a recursive locking opportunity

Example

The program in Figure12.8 illustrates another situation in which a recursive mutex is

necessary.Here, we have a ‘‘timeout’’function that allows us to schedule another

function to be run at some time in the future. Assuming that threads are an inexpensive

resource, we can create a thread for each pending timeout. The thread waits until the

time has been reached, and then it calls the function we’ve requested.

The problem arises when we can’t create a thread or when the scheduled time to

runthe function has already passed. In these cases, we simply call the requested

function now,fromthe current context. Since the function acquires the same lock that

we currently hold, a deadlock will occur unless the lock is recursive.

#include "apue.h"

#include <pthread.h>

#include <time.h>

#include <sys/time.h>

extern int makethread(void *(*)(void *), void *);

struct to_info {

void (*to_fn)(void *); /* function */