PTHREAD_ATTR_INIT(3)      (2020-11-01)       PTHREAD_ATTR_INIT(3)

     NAME
          pthread_attr_init, pthread_attr_destroy - initialize and
          destroy thread attributes object

     SYNOPSIS
          #include <pthread.h>

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

          Compile and link with -pthread.

     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() func-
          tion.  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 pos-
          sible error return).

     ATTRIBUTES
          For an explanation of the terms used in this section, see
          attributes(7).  allbox; lbw22 lb lb l l l.
          Interface Attribute Value T{ pthread_attr_init(),
          pthread_attr_destroy() T}   Thread safety  MT-Safe

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     CONFORMING TO
          POSIX.1-2001, POSIX.1-2008.

     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 ini-
          tialize 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 imple-
          mentation, 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 pro-
          gram 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

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                      Stack address       = 0x40197000
                      Stack size          = 0x3000000 bytes

        Program source

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

          #define handle_error_en(en, msg) \
                  do { errno = en; perror(msg); exit(EXIT_FAILURE); } while (0)

          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)
                  handle_error_en(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)
                  handle_error_en(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)
                  handle_error_en(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)
                  handle_error_en(s, "pthread_attr_getschedpolicy");
              printf("%sScheduling policy   = %s\n", prefix,
                      (i == SCHED_OTHER) ? "SCHED_OTHER" :

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                      (i == SCHED_FIFO)  ? "SCHED_FIFO" :
                      (i == SCHED_RR)    ? "SCHED_RR" :
                      "???");

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

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

              s = pthread_attr_getstack(attr, &stkaddr, &v);
              if (s != 0)
                  handle_error_en(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)
                  handle_error_en(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

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                 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)
                      handle_error_en(s, "pthread_attr_init");

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

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

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

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

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

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

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

              if (attrp != NULL) {
                  s = pthread_attr_destroy(attrp);
                  if (s != 0)
                      handle_error_en(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),

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          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)

     COLOPHON
          This page is part of release 5.10 of the Linux man-pages
          project.  A description of the project, information about
          reporting bugs, and the latest version of this page, can be
          found at https://www.kernel.org/doc/man-pages/.

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