PTHREAD_CREATE(3C) Standard C Library Functions PTHREAD_CREATE(3C)


NAME


pthread_create - create a thread

SYNOPSIS


cc -mt [ flag... ] file... -lpthread [ library... ]
#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 is used to create a new thread, with
attributes specified by attr, within a process. If attr is NULL, the
default attributes are used. (See pthread_attr_init(3C)). If the
attributes specified by attr are modified later, the thread's attributes
are not affected. Upon successful completion, pthread_create() stores the
ID of the created thread in the location referenced by thread.


The thread is created executing start_routine with arg as its sole
argument. If the start_routine returns, the effect is as if there was an
implicit call to pthread_exit() using the return value of start_routine
as the exit status. Note that the thread in which main() was originally
invoked differs from this. When it returns from main(), the effect is as
if there was an implicit call to exit() using the return value of main()
as the exit status.


The signal state of the new thread is initialised as follows:

o The signal mask is inherited from the creating thread.

o The set of signals pending for the new thread is empty.


Default thread creation:

pthread_t tid;
void *start_func(void *), *arg;

pthread_create(&tid, NULL, start_func, arg);


This would have the same effect as:

pthread_attr_t attr;

pthread_attr_init(&attr); /* initialize attr with default */
/* attributes */
pthread_create(&tid, &attr, start_func, arg);


User-defined thread creation: To create a thread that is scheduled on a
system-wide basis, use:

pthread_attr_init(&attr); /* initialize attr with default */
/* attributes */
pthread_attr_setscope(&attr, PTHREAD_SCOPE_SYSTEM);
/* system-wide contention */
pthread_create(&tid, &attr, start_func, arg);


To customize the attributes for POSIX threads, see pthread_attr_init(3C).


A new thread created with pthread_create() uses the stack specified by
the stackaddr attribute, and the stack continues for the number of bytes
specified by the stacksize attribute. By default, the stack size is 1
megabyte for 32-bit processes and 2 megabyte for 64-bit processes (see
pthread_attr_setstacksize(3C)). If the default is used for both the
stackaddr and stacksize attributes, pthread_create() creates a stack for
the new thread with at least 1 megabyte for 32-bit processes and 2
megabyte for 64-bit processes. (For customizing stack sizes, see NOTES).


If pthread_create() fails, no new thread is created and the contents of
the location referenced by thread are undefined.

RETURN VALUES


If successful, the pthread_create() function returns 0. Otherwise, an
error number is returned to indicate the error.

ERRORS


The pthread_create() function will fail if:

EAGAIN
The system lacked the necessary resources to create another
thread, or the system-imposed limit on the total number of
threads in a process PTHREAD_THREADS_MAX would be exceeded.


EINVAL
The value specified by attr is invalid.


EPERM
The caller does not have appropriate permission to set the
required scheduling parameters or scheduling policy.


EXAMPLES


Example 1: Example of concurrency with multithreading




The following is an example of concurrency with multithreading. Since
POSIX threads and Solaris threads are fully compatible even within the
same process, this example uses pthread_create() if you execute a.out 0,
or thr_create() if you execute a.out 1.


Five threads are created that simultaneously perform a time-consuming
function, sleep(10). If the execution of this process is timed, the
results will show that all five individual calls to sleep for ten-seconds
completed in about ten seconds, even on a uniprocessor. If a single-
threaded process calls sleep(10) five times, the execution time will be
about 50-seconds.


The command-line to time this process is:


POSIX threading
/usr/bin/time a.out 0


Solaris threading
/usr/bin/time a.out 1


/* cc thisfile.c -lthread -lpthread */
#define _REENTRANT /* basic 3-lines for threads */
#include <pthread.h>
#include <thread.h>

#define NUM_THREADS 5
#define SLEEP_TIME 10

void *sleeping(void *); /* thread routine */
int i;
thread_t tid[NUM_THREADS]; /* array of thread IDs */

int
main(int argc, char *argv[])
{
if (argc == 1) {
printf("use 0 as arg1 to use pthread_create()\n");
printf("or use 1 as arg1 to use thr_create()\n");
return (1);
}

switch (*argv[1]) {
case '0': /* POSIX */
for ( i = 0; i < NUM_THREADS; i++)
pthread_create(&tid[i], NULL, sleeping,
(void *)SLEEP_TIME);
for ( i = 0; i < NUM_THREADS; i++)
pthread_join(tid[i], NULL);
break;

case '1': /* Solaris */
for ( i = 0; i < NUM_THREADS; i++)
thr_create(NULL, 0, sleeping, (void *)SLEEP_TIME, 0,
&tid[i]);
while (thr_join(0, NULL, NULL) == 0)
;
break;
} /* switch */
printf("main() reporting that all %d threads have
terminated\n", i);
return (0);
} /* main */

void *
sleeping(void *arg)
{
int sleep_time = (int)arg;
printf("thread %d sleeping %d seconds ...\n", thr_self(),
sleep_time);
sleep(sleep_time);
printf("\nthread %d awakening\n", thr_self());
return (NULL);
}


If main() had not waited for the completion of the other threads (using
pthread_join(3C) or thr_join(3C)), it would have continued to process
concurrently until it reached the end of its routine and the entire
process would have exited prematurely. See exit(2).


ATTRIBUTES


See attributes(5) for descriptions of the following attributes:


+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | Standard |
+--------------------+-----------------+
|MT-Level | MT-Safe |
+--------------------+-----------------+

SEE ALSO


fork(2), pthread_attr_init(3C), pthread_cancel(3C), pthread_exit(3C),
pthread_join(3C), sysconf(3C), attributes(5), standards(5)

NOTES


Multithreaded application threads execute independently of each other, so
their relative behavior is unpredictable. Therefore, it is possible for
the thread executing main() to finish before all other user application
threads. The pthread_join(3C)function, on the other hand, must specify
the terminating thread (IDs) for which it will wait.


A user-specified stack size must be greater than the value
PTHREAD_STACK_MIN. A minimum stack size may not accommodate the stack
frame for the user thread function start_func. If a stack size is
specified, it must accommodate start_func requirements and the functions
that it may call in turn, in addition to the minimum requirement.


It is usually very difficult to determine the runtime stack requirements
for a thread. PTHREAD_STACK_MIN specifies how much stack storage is
required to execute a NULL start_func. The total runtime requirements for
stack storage are dependent on the storage required to do runtime
linking, the amount of storage required by library runtimes (as printf())
that your thread calls. Since these storage parameters are not known
before the program runs, it is best to use default stacks. If you know
your runtime requirements or decide to use stacks that are larger than
the default, then it makes sense to specify your own stacks.


March 23, 2005 PTHREAD_CREATE(3C)