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.\" Copyright (c) 2008 Linux Foundation, written by Michael Kerrisk
.\" <mtk.manpages@gmail.com>
.\"
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date. The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein. The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\"
.TH PTHREAD_CREATE 3 2012-03-15 "Linux" "Linux Programmer's Manual"
.SH NAME
pthread_create \- create a new thread
.SH SYNOPSIS
.nf
.B #include <pthread.h>

.BI "int pthread_create(pthread_t *" thread ", const pthread_attr_t *" attr ,
.BI " void *(*" start_routine ") (void *), void *" arg );
.fi
.sp
Compile and link with \fI\-pthread\fP.
.SH DESCRIPTION
The
.BR pthread_create ()
function starts a new thread in the calling process.
The new thread starts execution by invoking
.IR start_routine ();
.IR arg
is passed as the sole argument of
.IR start_routine ().

The new thread terminates in one of the following ways:
.IP * 2
It calls
.BR pthread_exit (3),
specifying an exit status value that is available to another thread
in the same process that calls
.BR pthread_join (3).
.IP *
It returns from
.IR start_routine ().
This is equivalent to calling
.BR pthread_exit (3)
with the value supplied in the
.I return
statement.
.IP *
It is canceled (see
.BR pthread_cancel (3)).
.IP *
Any of the threads in the process calls
.BR exit (3),
or the main thread performs a return from
.IR main ().
This causes the termination of all threads in the process.
.PP
The
.I attr
argument points to a
.I pthread_attr_t
structure whose contents are used at thread creation time to
determine attributes for the new thread;
this structure is initialized using
.BR pthread_attr_init (3)
and related functions.
If
.I attr
is NULL,
then the thread is created with default attributes.

Before returning, a successful call to
.BR pthread_create ()
stores the ID of the new thread in the buffer pointed to by
.IR 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
.RB ( pthread_sigmask (3)).
The set of pending signals for the new thread is empty
.RB ( sigpending (2)).
The new thread does not inherit the creating thread's
alternate signal stack
.RB ( sigaltstack (2)).

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

The initial value of the new thread's CPU-time clock is 0
(see
.BR pthread_getcpuclockid (3)).
.\" CLOCK_THREAD_CPUTIME_ID in clock_gettime(2)
.SS Linux-specific details
The new thread inherits copies of the calling thread's capability sets
(see
.BR capabilities (7))
and CPU affinity mask (see
.BR sched_setaffinity (2)).
.SH RETURN VALUE
On success,
.BR pthread_create ()
returns 0;
on error, it returns an error number, and the contents of
.IR *thread
are undefined.
.SH ERRORS
.TP
.B EAGAIN
Insufficient resources to create another thread,
or a system-imposed limit on the number of threads was encountered.
The latter case may occur in two ways:
the
.BR RLIMIT_NPROC
soft resource limit (set via
.BR setrlimit (2)),
which limits the number of process for a real user ID,
was reached;
or the kernel's system-wide limit on the number of threads,
.IR /proc/sys/kernel/threads-max ,
was reached.
.TP
.B EINVAL
Invalid settings in
.IR attr .
.TP
.\" FIXME . Test the following
.B EPERM
No permission to set the scheduling policy and parameters specified in
.IR attr .
.SH CONFORMING TO
POSIX.1-2001.
.SH NOTES
See
.BR pthread_self (3)
for further information on the thread ID returned in
.IR *thread
by
.BR pthread_create ().
Unless real-time scheduling policies are being employed,
after a call to
.BR pthread_create (),
it is indeterminate which thread\(emthe caller or the new thread\(emwill
next execute.

A thread may either be
.I joinable
or
.IR detached .
If a thread is joinable, then another thread can call
.BR 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
.I attr
was set to create the thread in a detached state (using
.BR pthread_attr_setdetachstate (3)).

.\" FIXME . Perhaps some of the following detail should be in
.\" a future pthread_attr_setstacksize(3) page.
On Linux/x86-32, the default stack size for a new thread is 2 megabytes.
Under the NPTL threading implementation, if the
.BR RLIMIT_STACK
soft resource limit
.IR "at the time the program started"
has any value other than "unlimited",
then it determines the default stack size of new threads.
Using
.BR pthread_attr_setstacksize (3),
the stack size attribute can be explicitly set in the
.I attr
argument used to create a thread,
in order to obtain a stack size other than the default.
.SH 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
.BR pthreads (7).
.SH EXAMPLE
The program below demonstrates the use of
.BR 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:

.in +4n
.nf
.RB "$" " ulimit \-s"
8192 # The stack size limit is 8 MB (0x80000 bytes)
.RB "$" " ./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
.fi
.in

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

.in +4n
.nf
.RB "$" " ./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
.fi
.in
.SS Program source
\&
.nf
#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 = (struct thread_info *) arg;
    char *uargv, *p;

    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 (p = uargv; *p != \(aq\\0\(aq; p++)
        *p = toupper(*p);

    return uargv;
}

int
main(int argc, char *argv[])
{
    int s, tnum, opt, num_threads;
    struct thread_info *tinfo;
    pthread_attr_t attr;
    int 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 \(aqs\(aq:
            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(struct thread_info));
    if (tinfo == NULL)
        handle_error("calloc");

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

    for (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 (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);
}
.fi
.SH SEE ALSO
.BR getrlimit (2),
.BR pthread_attr_init (3),
.BR pthread_cancel (3),
.BR pthread_detach (3),
.BR pthread_equal (3),
.BR pthread_exit (3),
.BR pthread_getattr_np (3),
.BR pthread_join (3),
.BR pthread_self (3),
.BR pthreads (7)
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