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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);
}
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)
COLOPHON
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information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.