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| Threads Library Functions | pthread_create(3THR) |
| | pthread_create - create a thread |
SYNOPSIS
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cc -mt [ flag... ] file... -lpthread [ -lrt library... ]
#include <pthread.h> int pthread_create(pthread_t *thread, const pthread_attr_t *attr, void *(*start_routine, void*),void *arg); |
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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(3THR)). 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:
- The signal mask is inherited from the creating thread.
- The set of signals pending for the new thread is empty.
Default thread creation:
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pthread_t tid;
void *start_func(void *), *arg;
pthread_create(&tid, NULL, start_func, arg);
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This would have the same effect as:
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pthread_attr_t attr;
pthread_attr_init(&attr); /* initialize attr with default attributes */
pthread_create(&tid, &attr, start_func, arg);
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User-defined thread creation: To create a thread that is scheduled on a system-wide basis, use:
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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);
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To customize the attributes for POSIX threads, see pthread_attr_init(3THR).
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(3THR)). 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.
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If successful, the pthread_create() function returns 0. Otherwise, an error number is returned to indicate the error.
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The pthread_create() function will fail if:
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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.
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EINVAL
- The value specified by attr is invalid.
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EPERM
- The caller does not have appropriate permission to set the required scheduling parameters or scheduling policy.
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| | Example 1. Example of concurrency with multithreading
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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:
/usr/bin/time a.out 0 (for POSIX threading)
or
/usr/bin/time a.out 1 (for Solaris threading)
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/* 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);
}
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Example 2. If main had not waited for the completion of the other threads (using pthread_join3THR or thr_join3THR), it would have continued to process concurrently until it reached the end of its routine and the entire process would have exited prematurely (see exit2).
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See attributes(5) for descriptions of the following attributes:
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
| MT-Level | MT-Safe |
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MT application threads execute independently of each other, thus their relative behavior is unpredictable. Therefore, it is possible for the thread executing main() to finish before all other user application threads.
pthread_join(3THR), 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.
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