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NAME

       pthread_getattr_np - get attributes of created thread

SYNOPSIS

       #define _GNU_SOURCE             /* See feature_test_macros(7) */
       #include <pthread.h>

       int pthread_getattr_np(pthread_t thread, pthread_attr_t *attr);

       Compile and link with -pthread.

DESCRIPTION

       The  pthread_getattr_np()  function  initializes  the thread attributes
       object referred to by attr so that it contains actual attribute  values
       describing the running thread thread.

       The  returned  attribute  values  may  differ  from  the  corresponding
       attribute values passed in the attr object that was used to create  the
       thread   using   pthread_create(3).    In   particular,  the  following
       attributes may differ:

       * the detach state, since a joinable thread may  have  detached  itself
         after creation;

       * the  stack  size,  which  the  implementation may align to a suitable
         boundary.

       * and the guard size, which the implementation may round  upward  to  a
         multiple  of  the  page  size,  or  ignore (i.e., treat as 0), if the
         application is allocating its own stack.

       Furthermore, if the stack address attribute was not set in  the  thread
       attributes  object  used to create the thread, then the returned thread
       attributes object  will  report  the  actual  stack  address  that  the
       implementation selected for the thread.

       When  the  thread attributes object returned by pthread_getattr_np() is
       no    longer    required,    it    should    be     destroyed     using
       pthread_attr_destroy(3).

RETURN VALUE

       On  success,  this  function  returns 0; on error, it returns a nonzero
       error number.

ERRORS

       ENOMEM Insufficient memory.

       In  addition,   if   thread   refers   to   the   main   thread,   then
       pthread_getattr_np() can fail because of errors from various underlying
       calls: fopen(3), if /proc/self/maps can't be opened; and  getrlimit(2),
       if the RLIMIT_STACK resource limit is not supported.

VERSIONS

       This function is available in glibc since version 2.2.3.

CONFORMING TO

       This  function  is  a nonstandard GNU extension; hence the suffix "_np"
       (nonportable) in the name.

EXAMPLE

       The program below demonstrates the use  of  pthread_getattr_np().   The
       program  creates  a  thread  that  then  uses  pthread_getattr_np()  to
       retrieve and display its guard size,  stack  address,  and  stack  size
       attributes.  Command-line arguments can be used to set these attributes
       to values other than the default when creating the thread.   The  shell
       sessions below demonstrate the use of the program.

       In  the  first  run,  on  an  x86-32  system, a thread is created using
       default attributes:

           $ ulimit -s      # No stack limit ==> default stack size is 2MB
           unlimited
           $ ./a.out
           Attributes of created thread:
                   Guard size          = 4096 bytes
                   Stack address       = 0x40196000 (EOS = 0x40397000)
                   Stack size          = 0x201000 (2101248) bytes

       In the following run, we see that if a guard size is specified,  it  is
       rounded  up to the next multiple of the system page size (4096 bytes on
       x86-32):

           $ ./a.out -g 4097
           Thread attributes object after initializations:
                   Guard size          = 4097 bytes
                   Stack address       = (nil)
                   Stack size          = 0x0 (0) bytes

           Attributes of created thread:
                   Guard size          = 8192 bytes
                   Stack address       = 0x40196000 (EOS = 0x40397000)
                   Stack size          = 0x201000 (2101248) bytes

       In the last run, the program manually allocates a stack for the thread.
       In this case, the guard size attribute is ignored.

           $ ./a.out -g 4096 -s 0x8000 -a
           Allocated thread stack at 0x804d000

           Thread attributes object after initializations:
                   Guard size          = 4096 bytes
                   Stack address       = 0x804d000 (EOS = 0x8055000)
                   Stack size          = 0x8000 (32768) bytes

           Attributes of created thread:
                   Guard size          = 0 bytes
                   Stack address       = 0x804d000 (EOS = 0x8055000)
                   Stack size          = 0x8000 (32768) 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_stack_related_attributes(pthread_attr_t *attr, char *prefix)
       {
           int s;
           size_t stack_size, guard_size;
           void *stack_addr;

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

           s = pthread_attr_getstack(attr, &stack_addr, &stack_size);
           if (s != 0)
               handle_error_en(s, "pthread_attr_getstack");
           printf("%sStack address       = %p", prefix, stack_addr);
           if (stack_size > 0)
               printf(" (EOS = %p)", (char *) stack_addr + stack_size);
           printf("\n");
           printf("%sStack size          = 0x%x (%d) bytes\n",
                   prefix, stack_size, stack_size);
       }

       static void
       display_thread_attributes(pthread_t thread, char *prefix)
       {
           int s;
           pthread_attr_t attr;

           s = pthread_getattr_np(thread, &attr);
           if (s != 0)
               handle_error_en(s, "pthread_getattr_np");

           display_stack_related_attributes(&attr, prefix);

           s = pthread_attr_destroy(&attr);
           if (s != 0)
               handle_error_en(s, "pthread_attr_destroy");
       }

       static void *           /* Start function for thread we create */
       thread_start(void *arg)
       {
           printf("Attributes of created thread:\n");
           display_thread_attributes(pthread_self(), "\t");

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

       static void
       usage(char *pname, char *msg)
       {
           if (msg != NULL)
               fputs(msg, stderr);
           fprintf(stderr, "Usage: %s [-s stack-size [-a]]"
                   " [-g guard-size]\n", pname);
           fprintf(stderr, "\t\t-a means program should allocate stack\n");
           exit(EXIT_FAILURE);
       }

       static pthread_attr_t *   /* Get thread attributes from command line */
       get_thread_attributes_from_cl(int argc, char *argv[],
                                     pthread_attr_t *attrp)
       {
           int s, opt, allocate_stack;
           long stack_size, guard_size;
                   void *stack_addr;
           pthread_attr_t *ret_attrp = NULL;   /* Set to attrp if we initialize
                                                  a thread attributes object */
           allocate_stack = 0;
           stack_size = -1;
           guard_size = -1;

           while ((opt = getopt(argc, argv, "ag:s:")) != -1) {
               switch (opt) {
               case 'a':   allocate_stack = 1;                     break;
               case 'g':   guard_size = strtoul(optarg, NULL, 0);  break;
               case 's':   stack_size = strtoul(optarg, NULL, 0);  break;
               default:    usage(argv[0], NULL);
               }
           }

           if (allocate_stack && stack_size == -1)
               usage(argv[0], "Specifying -a without -s makes no sense\n");

           if (argc > optind)
               usage(argv[0], "Extraneous command-line arguments\n");

           if (stack_size >= 0 || guard_size > 0) {
               ret_attrp = attrp;

               s = pthread_attr_init(attrp);
               if (s != 0)
                   handle_error_en(s, "pthread_attr_init");
           }

           if (stack_size >= 0) {
               if (!allocate_stack) {
                   s = pthread_attr_setstacksize(attrp, stack_size);
                   if (s != 0)
                       handle_error_en(s, "pthread_attr_setstacksize");
               } else {
                   s = posix_memalign(&stack_addr, sysconf(_SC_PAGESIZE),
                                      stack_size);
                   if (s != 0)
                       handle_error_en(s, "posix_memalign");
                   printf("Allocated thread stack at %p\n\n", stack_addr);

                   s = pthread_attr_setstack(attrp, stack_addr, stack_size);
                   if (s != 0)
                       handle_error_en(s, "pthread_attr_setstacksize");
               }
           }

           if (guard_size >= 0) {
               s = pthread_attr_setguardsize(attrp, guard_size);
               if (s != 0)
                   handle_error_en(s, "pthread_attr_setstacksize");
           }

           return ret_attrp;
       }

       int
       main(int argc, char *argv[])
       {
           int s;
           pthread_t thr;
           pthread_attr_t attr;
           pthread_attr_t *attrp = NULL;    /* Set to &attr if we initialize
                                               a thread attributes object */

           attrp = get_thread_attributes_from_cl(argc, argv, &attr);

           if (attrp != NULL) {
               printf("Thread attributes object after initializations:\n");
               display_stack_related_attributes(attrp, "\t");
               printf("\n");
           }

           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_getaffinity_np(3),         pthread_attr_getdetachstate(3),
       pthread_attr_getguardsize(3),          pthread_attr_getinheritsched(3),
       pthread_attr_getschedparam(3),          pthread_attr_getschedpolicy(3),
       pthread_attr_getscope(3),                     pthread_attr_getstack(3),
       pthread_attr_getstackaddr(3),             pthread_attr_getstacksize(3),
       pthread_attr_init(3), pthread_create(3), pthreads(7)

COLOPHON

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       be found at http://man7.org/linux/man-pages/.