pthread_attr_init(3)

NAME
pthread_attr_init, pthread_attr_destroy - initialize and destroy thread attributes object

LIBRARY
POSIX threads library (libpthread, -lpthread)

SYNOPSIS
#include <pthread.h>

int pthread_attr_init(pthread_attr_t *attr);
int pthread_attr_destroy(pthread_attr_t *attr);

DESCRIPTION
The pthread_attr_init() function initializes the thread attributes object pointed to by attr with default attribute values. After this call, individual attributes of the object can be set using various related functions (listed under SEE ALSO), and then the object can be used in one or more pthread_create(3) calls that create threads.

Calling pthread_attr_init() on a thread attributes object that has already been initialized results in undefined behavior.

When a thread attributes object is no longer required, it should be destroyed using the pthread_attr_destroy() function. Destroying a thread attributes object has no effect on threads that were created using that object.

Once a thread attributes object has been destroyed, it can be reinitialized using pthread_attr_init(). Any other use of a destroyed thread attributes object has undefined results.

RETURN VALUE
On success, these functions return 0; on error, they return a nonzero error number.

ERRORS
POSIX.1 documents an ENOMEM error for pthread_attr_init(); on Linux these functions always succeed (but portable and future-proof applications should nevertheless handle a possible error return).

ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
Interface | Attribute | Value
 | Thread safety | MT-Safe

STANDARDS
POSIX.1-2008.

HISTORY
POSIX.1-2001.

NOTES
The pthread_attr_t type should be treated as opaque: any access to the object other than via pthreads functions is nonportable and produces undefined results.

EXAMPLES
The program below optionally makes use of pthread_attr_init() and various related functions to initialize a thread attributes object that is used to create a single thread. Once created, the thread uses the pthread_getattr_np(3) function (a nonstandard GNU extension) to retrieve the thread's attributes, and then displays those attributes.

If the program is run with no command-line argument, then it passes NULL as the attr argument of pthread_create(3), so that the thread is created with default attributes. Running the program on Linux/x86-32 with the NPTL threading implementation, we see the following:

    $ ulimit -s       # No stack limit ==> default stack size is 2 MB
    unlimited
    $ ./a.out
    Thread attributes:
            Detach state        = PTHREAD_CREATE_JOINABLE
            Scope               = PTHREAD_SCOPE_SYSTEM
            Inherit scheduler   = PTHREAD_INHERIT_SCHED
            Scheduling policy   = SCHED_OTHER
            Scheduling priority = 0
            Guard size          = 4096 bytes
            Stack address       = 0x40196000
            Stack size          = 0x201000 bytes

When we supply a stack size as a command-line argument, the program initializes a thread attributes object, sets various attributes in that object, and passes a pointer to the object in the call to pthread_create(3). Running the program on Linux/x86-32 with the NPTL threading implementation, we see the following:

    $ ./a.out 0x3000000
    posix_memalign() allocated at 0x40197000
    Thread attributes:
            Detach state        = PTHREAD_CREATE_DETACHED
            Scope               = PTHREAD_SCOPE_SYSTEM
            Inherit scheduler   = PTHREAD_EXPLICIT_SCHED
            Scheduling policy   = SCHED_OTHER
            Scheduling priority = 0
            Guard size          = 0 bytes
            Stack address       = 0x40197000
            Stack size          = 0x3000000 bytes

  Program source
    #define _GNU_SOURCE     /* To get pthread_getattr_np() declaration */
    #include <err.h>
    #include <errno.h>
    #include <pthread.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <unistd.h>

    static void
    display_pthread_attr(pthread_attr_t *attr, char *prefix)
    {
        int s, i;
        size_t v;
        void *stkaddr;
        struct sched_param sp;

        s = pthread_attr_getdetachstate(attr, &i);
        if (s != 0)
            errc(EXIT_FAILURE, s, "pthread_attr_getdetachstate");
        printf("%sDetach state        = %s\n", prefix,
               (i == PTHREAD_CREATE_DETACHED) ? "PTHREAD_CREATE_DETACHED" :
               (i == PTHREAD_CREATE_JOINABLE) ? "PTHREAD_CREATE_JOINABLE" :
               "???");

        s = pthread_attr_getscope(attr, &i);
        if (s != 0)
            errc(EXIT_FAILURE, s, "pthread_attr_getscope");
        printf("%sScope               = %s\n", prefix,
               (i == PTHREAD_SCOPE_SYSTEM)  ? "PTHREAD_SCOPE_SYSTEM" :
               (i == PTHREAD_SCOPE_PROCESS) ? "PTHREAD_SCOPE_PROCESS" :
               "???");

        s = pthread_attr_getinheritsched(attr, &i);
        if (s != 0)
            errc(EXIT_FAILURE, s, "pthread_attr_getinheritsched");
        printf("%sInherit scheduler   = %s\n", prefix,
               (i == PTHREAD_INHERIT_SCHED)  ? "PTHREAD_INHERIT_SCHED" :
               (i == PTHREAD_EXPLICIT_SCHED) ? "PTHREAD_EXPLICIT_SCHED" :
               "???");

        s = pthread_attr_getschedpolicy(attr, &i);
        if (s != 0)
            errc(EXIT_FAILURE, s, "pthread_attr_getschedpolicy");
        printf("%sScheduling policy   = %s\n", prefix,
               (i == SCHED_OTHER) ? "SCHED_OTHER" :
               (i == SCHED_FIFO)  ? "SCHED_FIFO" :
               (i == SCHED_RR)    ? "SCHED_RR" :
               "???");

        s = pthread_attr_getschedparam(attr, &sp);
        if (s != 0)
            errc(EXIT_FAILURE, s, "pthread_attr_getschedparam");
        printf("%sScheduling priority = %d\n", prefix, sp.sched_priority);

        s = pthread_attr_getguardsize(attr, &v);
        if (s != 0)
            errc(EXIT_FAILURE, s, "pthread_attr_getguardsize");
        printf("%sGuard size          = %zu bytes\n", prefix, v);

        s = pthread_attr_getstack(attr, &stkaddr, &v);
        if (s != 0)
            errc(EXIT_FAILURE, s, "pthread_attr_getstack");
        printf("%sStack address       = %p\n", prefix, stkaddr);
        printf("%sStack size          = %#zx bytes\n", prefix, v);
    }

    static void *
    thread_start(void *arg)
    {
        int s;
        pthread_attr_t gattr;

        /* pthread_getattr_np() is a non-standard GNU extension that
           retrieves the attributes of the thread specified in its
           first argument. */

        s = pthread_getattr_np(pthread_self(), &gattr);
        if (s != 0)
            errc(EXIT_FAILURE, s, "pthread_getattr_np");

        printf("Thread attributes:\n");
        display_pthread_attr(&gattr, "\t");

        exit(EXIT_SUCCESS);         /* Terminate all threads */
    }

    int
    main(int argc, char *argv[])
    {
        pthread_t thr;
        pthread_attr_t attr;
        pthread_attr_t *attrp;      /* NULL or &attr */
        int s;

        attrp = NULL;

        /* If a command-line argument was supplied, use it to set the
           stack-size attribute and set a few other thread attributes,
           and set attrp pointing to thread attributes object. */

        if (argc > 1) {
            size_t stack_size;
            void *sp;

            attrp = &attr;

            s = pthread_attr_init(&attr);
            if (s != 0)
                errc(EXIT_FAILURE, s, "pthread_attr_init");

            s = pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
            if (s != 0)
                errc(EXIT_FAILURE, s, "pthread_attr_setdetachstate");

            s = pthread_attr_setinheritsched(&attr, PTHREAD_EXPLICIT_SCHED);
            if (s != 0)
                errc(EXIT_FAILURE, s, "pthread_attr_setinheritsched");

            stack_size = strtoul(argv[1], NULL, 0);

            s = posix_memalign(&sp, sysconf(_SC_PAGESIZE), stack_size);
            if (s != 0)
                errc(EXIT_FAILURE, s, "posix_memalign");

            printf("posix_memalign() allocated at %p\n", sp);

            s = pthread_attr_setstack(&attr, sp, stack_size);
            if (s != 0)
                errc(EXIT_FAILURE, s, "pthread_attr_setstack");
        }

        s = pthread_create(&thr, attrp, &thread_start, NULL);
        if (s != 0)
            errc(EXIT_FAILURE, s, "pthread_create");

        if (attrp != NULL) {
            s = pthread_attr_destroy(attrp);
            if (s != 0)
                errc(EXIT_FAILURE, s, "pthread_attr_destroy");
        }

        pause();    /* Terminates when other thread calls exit() */
    }

SEE ALSO
pthread_attr_setaffinity_np(3), pthread_attr_setdetachstate(3), pthread_attr_setguardsize(3), pthread_attr_setinheritsched(3), pthread_attr_setschedparam(3), pthread_attr_setschedpolicy(3), pthread_attr_setscope(3), pthread_attr_setsigmask_np(3), pthread_attr_setstack(3), pthread_attr_setstackaddr(3), pthread_attr_setstacksize(3), pthread_create(3), pthread_getattr_np(3), pthread_setattr_default_np(3), pthreads(7)