Programming POSIX threads on Linux

Part II: Mutexes, condition variables

Here, a short overview over some POSIX means to synchronize access to common resources via mutexes and condition variables is given.

The source code

As example, I will use the source code from part I of this article, which will be refined by using mutexes and condition variables to ensure that pthread_join() can immediately return the return value of each thread. The waiting is done before by counting how many threads have completed so far. If this number is less than the number of started threads, we just wait again until both numbers are equal. Then, each call to pthread_join() immediately returns. The source code is againg written in C and can be downloaded here.

Compiling and linking

You can compile and link the program with the following Makefile, which again is not the smallest possible version. With this Makefile in your working directory, you can simply type make to create or re-create the executable thread.

Usage

The program is started by a command line:

thread 5

Programmer's quick reference

Each program using POSIX mutexes and condition variables has to include the following header file:

#include <pthread.h>

-D_REENTRANT	% compiler switch
-lpthreads	% linker lib

Here are the most important POSIX functions concerning the basics of synchronization programming. For a quick reference, they are given as function prototypes. The usage is shown in the example source code.

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;


int pthread_mutex_lock(pthread_mutex_t* mutex);

  return value: 0 if OK, positive error code otherwise


int pthread_mutex_unlock(pthread_mutex_t* mutex);

  return value: 0 if OK, positive error code otherwise


int pthread_mutex_trylock(pthread_mutex_t* mutex);

  return value: EBUSY if the mutex is locked


int pthread_cond_wait(pthread_cont_t* cond, pthread_mutex_t* mutex);

  return value: 0 if OK, positive error code otherwise


int pthread_cond_signal(pthread_cond_t* cond);

  return value: 0 if OK, positive error code otherwise

References

Literature

• R. W. Stevens, Unix Network Programming, vol. II: Interprocess Communications, Prentice-Hall. A very good book in two volumes which can be seen as classic text about every aspect of Unix network programming, including sockets, threads, IPC and RPC.