pthread_create(3)

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 *_Nullable restrict attr,
                   typeof(void *(void *_Nullable)) *start_routine,
                   void *_Nullable 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
 | 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 <err.h>
    #include <pthread.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #include <sys/types.h>
    #include <unistd.h>

    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)
            err(EXIT_FAILURE, "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)
            errc(EXIT_FAILURE, s, "pthread_attr_init");

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

        /* Allocate memory for pthread_create() arguments.  */

        tinfo = calloc(num_threads, sizeof(*tinfo));
        if (tinfo == NULL)
            err(EXIT_FAILURE, "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)
                errc(EXIT_FAILURE, s, "pthread_create");
        }

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

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