Go to main content

man pages section 3: Basic Library Functions

Exit Print View

Updated: Wednesday, July 27, 2022



thr_create - create a thread


#include <thread.h> 

int thr_create(void *stack_base, size_t stack_size,
     void *(*start_func) (void*), void *arg, long flags,
     thread_t *new_thread_ID);


Thread creation adds a new thread of control to the current process. The procedure main() is a single thread of control. Each thread executes concurrently with all other threads within the calling process and with other threads from other active processes.

Although a newly created thread shares all of the calling process's global data with the other threads in the process, it has its own set of attributes and private execution stack. The new thread inherits the calling thread's signal mask and scheduling priority. Pending signals for a new thread are not inherited and will be empty.

The call to create a thread takes the address of a user-defined function, specified by start_func, as one of its arguments. This function is the complete execution routine for the new thread.

The lifetime of a thread begins with the successful return from thr_create(), which calls start_func( ) and ends with one of the following:

  • the normal completion of start_func( ),

  • an explicit call to thr_exit(3C), or

  • the conclusion of the calling process (see exit(2)).

The new thread performs by calling the function defined by start_func with only one argument, arg. If more than one argument needs to be passed to start_func, the arguments can be packed into a structure, the address of which can be passed to arg.

If start_func returns, the thread terminates with the exit status set to the start_func return value (see thr_exit(3C)).

When the thread from which main() originated returns, the effect is the same as if an implicit call to exit() were made using the return value of main() as the exit status. This behavior differs from a start_func return. If main() calls thr_exit(3C), only the main thread exits, not the entire process.

If the thread creation fails, a new thread is not created and the contents of the location referenced by the pointer to the new thread are undefined.

The flags argument specifies which attributes are modifiable for the created thread. The value in flags is determined by the bitwise inclusive-OR of the following:


This flag is obsolete and is maintained for compatibility.


This flag affects the detachstate attribute of the thread. The new thread is created detached. The exit status of a detached thread is not accessible to other threads. Its thread ID and other resources may be re-used as soon as the thread terminates. thr_join(3C) will not wait for a detached thread.


This flag is obsolete and is maintained for compatibility.


This flag affects the suspended attribute of the thread. The new thread is created suspended and will not execute start_func until it is started by thr_continue().


This flag affects the daemon attribute of the thread. In addition to being created detached (THR_DAEMON implies THR_DETACHED), the thread is marked as a daemon. Daemon threads do not interfere with the exit conditions for a process. A process will terminate when the last non-daemon thread exits or the process calls exit(2). Also, a thread that is waiting in thr_join(3C) for any thread to terminate will return EDEADLK when all remaining threads in the process are either daemon threads or other threads waiting in thr_join(). Daemon threads are most useful in libraries that want to use threads.

Default thread creation:

thread_t tid;
void *start_func(void *), *arg;
thr_create(NULL, 0, start_func, arg, 0, &tid);

Create a detached thread whose thread ID we do not care about:

thr_create(NULL, 0, start_func, arg, THR_DETACHED, NULL); 

If stack_base is not NULL, the new thread uses the stack beginning at the address specified by stack_base and continuing for stack_size bytes, where stack_size must be greater than or equal to THR_MIN_STACK. If stack_base is NULL, thr_create() allocates a stack for the new thread with at least stack_size bytes. If stack_size is 0, a default size is used. If stack_size is not 0, it must be greater than or equal to THR_MIN_STACK. See NOTES.

When new_thread_ID is not NULL, it points to a location where the ID of the new thread is stored if thr_create() is successful. The ID is only valid within the calling process.

Return Values

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



A resource control limit on the total number of threads in a process, task, project, or zone has been exceeded or some system resource has been exceeded.


The stack_base argument is not NULL and stack_size is less than THR_MIN_STACK, or the stack_base argument is NULL and stack_size is not 0 and is less than THR_MIN_STACK.


The system cannot allocate stack for the thread.

The thr_create() function may use mmap() to allocate thread stacks from MAP_PRIVATE, MAP_NORESERVE, and MAP_ANON memory mappings if stack_base is NULL, and consequently may return upon failure the relevant error values returned by mmap(). See the mmap(2) manual page for these error values.


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)


/usr/bin/time a.out 1 (for Solaris threading)

Example 1 An example of concurrency with multithreading.
#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 */

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);

    case '1':  /* Solaris */
        for ( i = 0; i < NUM_THREADS; i++)
            thr_create(NULL, 0, sleeping, (void *)SLEEP_TIME, 0,
        while (thr_join(0, NULL, NULL) == 0)
    }  /* 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( ),
    printf("\nthread %d awakening\n", thr_self( ));
    return (NULL);

Had main() 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)).

Example 2 Creating a default thread with a new signal mask.

The following example demonstrates how to create a default thread with a new signal mask. The new_mask argument is assumed to have a value different from the creator's signal mask (orig_mask). The new_mask argument is set to block all signals except for SIGINT. The creator's signal mask is changed so that the new thread inherits a different mask, and is restored to its original value after thr_create() returns.

This example assumes that SIGINT is also unmasked in the creator. If it is masked by the creator, then unmasking the signal opens the creator to this signal. The other alternative is to have the new thread set its own signal mask in its start routine.

thread_t tid;
sigset_t new_mask, orig_mask;
int error;

(void)sigdelset(&new_mask, SIGINT);
(void)thr_sigsetmask(SIG_SETMASK, &new_mask, &orig_mask);
error = thr_create(NULL, 0, do_func, NULL, 0, &tid);
(void)thr_sigsetmask(SIG_SETMASK, &orig_mask, NULL);


See attributes(7) for descriptions of the following attributes:


See Also

exit(2), getrlimit(2), mmap(2), exit(3C), sleep(3C), thr_exit(3C), thr_join(3C), thr_min_stack(3C), thr_setconcurrency(3C), thr_suspend(3C), attributes(7), standards(7), threads(7), sxadm(8)


Since multithreaded-application threads execute independently of each other, their relative behavior is unpredictable. It is therefore possible for the thread executing main() to finish before all other user-application threads.

Using thr_join(3C) in the following syntax,

while (thr_join(0, NULL, NULL) == 0)

will cause the invoking thread (which may be main()) to wait for the termination of all non-daemon threads, excluding threads that are themselves waiting in thr_join(); however, the second and third arguments to thr_join() need not necessarily be NULL.

A thread has not terminated until thr_exit() has finished. The only way to determine this is by thr_join(). When thr_join() returns a departed thread, it means that this thread has terminated and its resources are reclaimable. For instance, if a user specified a stack to thr_create(), this stack can only be reclaimed after thr_join() has reported this thread as a departed thread. It is not possible to determine when a detached thread has terminated. A detached thread disappears without leaving a trace.

Typically, thread stacks allocated by thr_create() begin on page boundaries and any specified (a red-zone) size is rounded up to the next page boundary. A page with no access permission is appended to the top of the stack so that most stack overflows will result in a SIGSEGV signal being sent to the offending thread. Thread stacks allocated by the caller are used as is.

Using a default stack size for the new thread, instead of passing a user-specified stack size, results in much better thr_create() performance. The default stack size for a user-thread is 1 megabyte in a 32-bit process and 2 megabyte in a 64-bit process.

A user-specified stack size must be greater than or equal to THR_MIN_STACK. 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. THR_MIN_STACK specifies how much stack storage is required to execute a trivial 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 (like 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.

If the ADISTACK security extension is enabled for the process, thread stacks allocated by thr_create() are mapped with ADI enabled. ADI must be enabled on any stack specified by the user in order to extend coverage to the stack. See mmap(2) and sxadm(8).