PTHREAD_CREATE(3)

Linux Programmer's Manual

PTHREAD_CREATE(3)

NAME

pthread_create - create a new thread

SYNOPSIS

#include
<pthread.h>

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

Compile and link with -pthread.

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, or a system-imposed limit on the number of threads was encountered. The latter case may occur in two ways: the RLIMIT_NPROC soft resource limit (set via 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, /proc/sys/kernel/threads-max, was reached.

EINVAL: Invalid settings in attr.

EPERM: No permission to set the scheduling policy and parameters specified in attr.

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.

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).

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:

$ 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 1MB (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 <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 = 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);
}

COLOPHON

This page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.

2012-08-03

Linux

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