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NAME

       pthread_create - create a new thread

SYNOPSIS

       #include <pthread.h>

       int pthread_create(pthread_t *thread, const pthread_attr_t *attr,
                          void *(*start_routine) (void *), void *arg);

       Compile and link with -pthread.

DESCRIPTION

       The  pthread_create()  function  starts  a  new  thread  in  the  calling process.  The new thread starts
       execution by invoking start_routine(); arg is passed as the sole argument of start_routine().

       The new thread terminates in one of the following ways:

       * It calls pthread_exit(3), specifying an exit status value that is available to another  thread  in  the
         same process that calls pthread_join(3).

       * It returns from start_routine().  This is equivalent to calling pthread_exit(3) with the value supplied
         in the return statement.

       * It is canceled (see pthread_cancel(3)).

       * Any  of  the  threads  in  the process calls exit(3), or the main thread performs a return from main().
         This causes the termination of all threads in the process.

       The attr argument points to a pthread_attr_t structure whose contents are used at thread creation time to
       determine attributes for the new thread; this structure is  initialized  using  pthread_attr_init(3)  and
       related functions.  If attr is NULL, then the thread is created with default attributes.

       Before  returning,  a  successful  call to pthread_create() stores the ID of the new thread in the buffer
       pointed to by thread; this identifier is used to refer  to  the  thread  in  subsequent  calls  to  other
       pthreads functions.

       The  new  thread  inherits  a copy of the creating thread's signal mask (pthread_sigmask(3)).  The set of
       pending signals for the new thread is empty  (sigpending(2)).   The  new  thread  does  not  inherit  the
       creating thread's alternate signal stack (sigaltstack(2)).

       The new thread inherits the calling thread's floating-point environment (fenv(3)).

       The initial value of the new thread's CPU-time clock is 0 (see pthread_getcpuclockid(3)).

   Linux-specific details
       The  new  thread  inherits  copies  of the calling thread's capability sets (see capabilities(7)) and CPU
       affinity mask (see sched_setaffinity(2)).

RETURN VALUE

       On success, pthread_create() returns 0; on error, it returns an error number, and the contents of *thread
       are undefined.

ERRORS

       EAGAIN Insufficient resources to create another thread.

       EAGAIN A system-imposed limit on the number of threads was encountered.  There are  a  number  of  limits
              that  may  trigger  this error: the RLIMIT_NPROC soft resource limit (set via setrlimit(2)), which
              limits the number of processes and threads for a real user ID, was reached; the  kernel's  system-
              wide  limit on the number of processes and threads, /proc/sys/kernel/threads-max, was reached (see
              proc(5)); or the maximum number of PIDs, /proc/sys/kernel/pid_max, was reached (see proc(5)).

       EINVAL Invalid settings in attr.

       EPERM  No permission to set the scheduling policy and parameters specified in attr.

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).
       ┌──────────────────┬───────────────┬─────────┐
       │ InterfaceAttributeValue   │
       ├──────────────────┼───────────────┼─────────┤
       │ pthread_create() │ Thread safety │ MT-Safe │
       └──────────────────┴───────────────┴─────────┘

CONFORMING TO

       POSIX.1-2001, POSIX.1-2008.

NOTES

       See pthread_self(3) for further information on the thread ID returned  in  *thread  by  pthread_create().
       Unless  real-time  scheduling  policies  are  being  employed,  after  a  call to pthread_create(), it is
       indeterminate which thread—the caller or the new thread—will next execute.

       A thread may either be joinable or detached.  If a thread is  joinable,  then  another  thread  can  call
       pthread_join(3)  to  wait  for the thread to terminate and fetch its exit status.  Only when a terminated
       joinable thread has been joined are the last of its resources  released  back  to  the  system.   When  a
       detached  thread  terminates,  its  resources  are  automatically  released back to the system: it is not
       possible to join with the thread in order to obtain its exit status.  Making a thread detached is  useful
       for  some  types  of  daemon  threads  whose exit status the application does not need to care about.  By
       default, a new thread is created in a joinable state, unless attr was set  to  create  the  thread  in  a
       detached state (using pthread_attr_setdetachstate(3)).

       Under  the NPTL threading implementation, if the RLIMIT_STACK soft resource limit at the time the program
       started has any value other than "unlimited", then it determines the default stack size of  new  threads.
       Using  pthread_attr_setstacksize(3),  the stack size attribute can be explicitly set in the attr argument
       used to create a thread, in order to obtain a stack size other than the  default.   If  the  RLIMIT_STACK
       resource  limit  is set to "unlimited", a per-architecture value is used for the stack size.  Here is the
       value for a few architectures:
              ┌──────────────┬────────────────────┐
              │ ArchitectureDefault stack size │
              ├──────────────┼────────────────────┤
              │ i386         │               2 MB │
              ├──────────────┼────────────────────┤
              │ IA-64        │              32 MB │
              ├──────────────┼────────────────────┤
              │ PowerPC      │               4 MB │
              ├──────────────┼────────────────────┤
              │ S/390        │               2 MB │
              ├──────────────┼────────────────────┤
              │ Sparc-32     │               2 MB │
              ├──────────────┼────────────────────┤
              │ Sparc-64     │               4 MB │
              ├──────────────┼────────────────────┤
              │ x86_64       │               2 MB │
              └──────────────┴────────────────────┘

BUGS

       In the obsolete LinuxThreads implementation, each of the threads in a process has a different process ID.
       This is in violation of the POSIX threads specification, and is the source of many other  nonconformances
       to the standard; see pthreads(7).

EXAMPLES

       The program below demonstrates the use of pthread_create(), as well as a number of other functions in the
       pthreads API.

       In the following run, on a system providing the NPTL threading implementation, the stack size defaults to
       the value given by the "stack size" resource limit:

           $ ulimit -s
           8192            # The stack size limit is 8 MB (0x800000 bytes)
           $ ./a.out hola salut servus
           Thread 1: top of stack near 0xb7dd03b8; argv_string=hola
           Thread 2: top of stack near 0xb75cf3b8; argv_string=salut
           Thread 3: top of stack near 0xb6dce3b8; argv_string=servus
           Joined with thread 1; returned value was HOLA
           Joined with thread 2; returned value was SALUT
           Joined with thread 3; returned value was SERVUS

       In  the  next  run, the program explicitly sets a stack size of 1 MB (using pthread_attr_setstacksize(3))
       for the created threads:

           $ ./a.out -s 0x100000 hola salut servus
           Thread 1: top of stack near 0xb7d723b8; argv_string=hola
           Thread 2: top of stack near 0xb7c713b8; argv_string=salut
           Thread 3: top of stack near 0xb7b703b8; argv_string=servus
           Joined with thread 1; returned value was HOLA
           Joined with thread 2; returned value was SALUT
           Joined with thread 3; returned value was SERVUS

   Program source

       #include <pthread.h>
       #include <string.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>
       #include <errno.h>
       #include <ctype.h>

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

       #define handle_error(msg) \
               do { perror(msg); exit(EXIT_FAILURE); } while (0)

       struct thread_info {    /* Used as argument to thread_start() */
           pthread_t thread_id;        /* ID returned by pthread_create() */
           int       thread_num;       /* Application-defined thread # */
           char     *argv_string;      /* From command-line argument */
       };

       /* Thread start function: display address near top of our stack,
          and return upper-cased copy of argv_string */

       static void *
       thread_start(void *arg)
       {
           struct thread_info *tinfo = arg;
           char *uargv;

           printf("Thread %d: top of stack near %p; argv_string=%s\n",
                   tinfo->thread_num, &p, tinfo->argv_string);

           uargv = strdup(tinfo->argv_string);
           if (uargv == NULL)
               handle_error("strdup");

           for (char *p = uargv; *p != '\0'; p++)
               *p = toupper(*p);

           return uargv;
       }

       int
       main(int argc, char *argv[])
       {
           int s, opt, num_threads;
           pthread_attr_t attr;
           size_t stack_size;
           void *res;

           /* The "-s" option specifies a stack size for our threads */

           stack_size = -1;
           while ((opt = getopt(argc, argv, "s:")) != -1) {
               switch (opt) {
               case 's':
                   stack_size = strtoul(optarg, NULL, 0);
                   break;

               default:
                   fprintf(stderr, "Usage: %s [-s stack-size] arg...\n",
                           argv[0]);
                   exit(EXIT_FAILURE);
               }
           }

           num_threads = argc - optind;

           /* Initialize thread creation attributes */

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

           if (stack_size > 0) {
               s = pthread_attr_setstacksize(&attr, stack_size);
               if (s != 0)
                   handle_error_en(s, "pthread_attr_setstacksize");
           }

           /* Allocate memory for pthread_create() arguments */

           struct thread_info *tinfo = calloc(num_threads, sizeof(*tinfo));
           if (tinfo == NULL)
               handle_error("calloc");

           /* Create one thread for each command-line argument */

           for (int tnum = 0; tnum < num_threads; tnum++) {
               tinfo[tnum].thread_num = tnum + 1;
               tinfo[tnum].argv_string = argv[optind + tnum];

               /* The pthread_create() call stores the thread ID into
                  corresponding element of tinfo[] */

               s = pthread_create(&tinfo[tnum].thread_id, &attr,
                                  &thread_start, &tinfo[tnum]);
               if (s != 0)
                   handle_error_en(s, "pthread_create");
           }

           /* Destroy the thread attributes object, since it is no
              longer needed */

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

           /* Now join with each thread, and display its returned value */

           for (int tnum = 0; tnum < num_threads; tnum++) {
               s = pthread_join(tinfo[tnum].thread_id, &res);
               if (s != 0)
                   handle_error_en(s, "pthread_join");

               printf("Joined with thread %d; returned value was %s\n",
                       tinfo[tnum].thread_num, (char *) res);
               free(res);      /* Free memory allocated by thread */
           }

           free(tinfo);
           exit(EXIT_SUCCESS);
       }

SEE ALSO

       getrlimit(2), pthread_attr_init(3), pthread_cancel(3), pthread_detach(3), pthread_equal(3),
       pthread_exit(3), pthread_getattr_np(3), pthread_join(3), pthread_self(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/.

Linux                                              2020-11-01                                  PTHREAD_CREATE(3)