pthread_create(3) Library Functions Manual pthread_create(3)
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 re-
lated functions. If attr is NULL, then the thread is created with de-
fault 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 func-
tions.
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/ker-
nel/pid_max, was reached (see proc(5)).
EINVAL Invalid settings in attr.
EPERM No permission to set the scheduling policy and parameters speci-
fied in attr.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌────────────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├────────────────────────────────────────────┼───────────────┼─────────┤
│ pthread_create() │ Thread safety │ MT-Safe │
└────────────────────────────────────────────┴───────────────┴─────────┘
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 auto-
matically 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 appli-
cation 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 de-
tached state (using pthread_attr_setdetachstate(3)).
Under the NPTL threading implementation, if the RLIMIT_STACK soft re-
source 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 ex-
plicitly 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 re-
source 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 implemen-
tation, 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);
}
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),
pthread_setattr_default_np(3), pthreads(7)
Linux man-pages 6.9.1 2024-06-15 pthread_create(3)
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