POSIX

PThreads

General Information

Links:

• POSIX Threads Programming, excellent introduction to pthreads, by Blaise Barney, at Lawrence Livermore National Laboratory.
• Advanced Linux Programming Ch 4 — Threads
• POSIX thread (pthread) libraries
• POSIX Threads (wikipedia)

Manual page:

• pthreads (7)

Portability

Joinable / detachable state

For portability, always create joinable or detachable threads by setting explicitly the thread attribute (using pthread_attr_getdetachstate). This provides portability as not all implementations may create threads as joinable by default.

Linux vs. POSIX manual pages

Most manual pages on pthreads are available as either standard linux manual pages or as extract from the POSIX programmer manual.

For instance:

• pthread_cond_signal(3posix)
• pthread_cond_signal(3)

Both versions are worth reading.

Create threads

See pthread_create(3).

Conditional variables

See

• pthread_cond_init
• pthread_cond_destroy
• pthread_cond_signal
• pthread_cond_broadcast
• pthread_cond_wait
• pthread_cond_timedwait

Note that these functions are NOT signal-async safe (see [1], [2]). They CANNOT be called safely from a async signal handler (like SIGIO signal handler).

Thread-Local Data

• Using thread-specific data area foreseen by POSIX, which requires first creating a key that is later used to set or get thread-specific data attached to that key (see Advanced Linux Programming - Ch 4)
• Using keyword __thread, see http://www.akkadia.org/drepper/tls.pdf (not available in all compiler, not standard C99 [3]).

// Compile this code with : gcc local.c -o local -lpthread

#include <stdio.h>
#include <unistd.h>
#include <pthread.h>

static __thread int myint;
static int globalint = 0;

void * handler(void *arg)
{
    myint = ++globalint;                              // thread unsafe - should use a mutex
    while(1) {
        printf("myint is %d\n", myint);
        sleep(1);
    }
}

int main (int argc, char **argv)
{
    pthread_t p1,p2;

    pthread_create(&p1, NULL, handler, NULL);
    pthread_create(&p2, NULL, handler, NULL);

    pause();
    return 0;
}

Debugging

See Debugging page.