pthread_create - create a new thread

#include <pthread.h>
int pthread_create(pthread_t *restrict thread,
 const pthread_attr_t *restrict attr,
 void *(*start_routine)(void *),
 void *restrict arg);

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)).

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

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);
}

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)