Ubuntu Manpage: pthread_create - create a new thread

NAME

       pthread_create - create a new thread

LIBRARY

       POSIX threads library (libpthread, -lpthread)

SYNOPSIS

       #include <pthread.h>

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

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).

       ┌───────────────────────────────────────────────────────────────┬───────────────┬─────────┐
       │Interface                                                      │ Attribute     │ Value   │
       ├───────────────────────────────────────────────────────────────┼───────────────┼─────────┤
       │pthread_create()                                               │ Thread safety │ MT-Safe │
       └───────────────────────────────────────────────────────────────┴───────────────┴─────────┘

STANDARDS

       POSIX.1-2008.

HISTORY

       POSIX.1-2001.

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: 2 MB on most
architectures; 4 MB on POWER and Sparc-64.

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 <ctype.h>
       #include <errno.h>
       #include <pthread.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <string.h>
       #include <unistd.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, (void *) &tinfo, 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;
           void                *res;
           size_t              num_threads;
           ssize_t             stack_size;
           pthread_attr_t      attr;
           struct thread_info  *tinfo;

           /* 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. */

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

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

           for (size_t 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 (size_t 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);
       }