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pthread_create

NAME
LIBRARY
SYNOPSIS
DESCRIPTION
Linux-specific details
RETURN VALUE
ERRORS
ATTRIBUTES
STANDARDS
HISTORY
NOTES
BUGS
EXAMPLES
Program source
SEE ALSO

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 *restrict attr ,
void *(* start_routine )(void *),
void *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).

Image grohtml-211638-1.png

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 <errno.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.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);
}