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:

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)).

RETURN VALUE

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

ERRORS

CONFORMING TO

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)).

On Linux/x86-32, the default stack size for a new thread is 2 megabytes. 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.

EXAMPLE

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:

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

#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 != '\0'; 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 '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(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);
}

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).

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), pthreads(7)