PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

PTHREAD_CREATE(3P)        POSIX Programmer's Manual       PTHREAD_CREATE(3P)

PROLOG         top

       This manual page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the
       corresponding Linux manual page for details of Linux behavior), or
       the interface may not be implemented on Linux.

NAME         top

       pthread_create — thread creation

SYNOPSIS         top

       #include <pthread.h>

       int pthread_create(pthread_t *restrict thread,
           const pthread_attr_t *restrict attr,
           void *(*start_routine)(void*), void *restrict arg);

DESCRIPTION         top

       The pthread_create() function shall create a new thread, with
       attributes specified by attr, within a process. If attr is NULL, the
       default attributes shall be used. If the attributes specified by attr
       are modified later, the thread's attributes shall not be affected.
       Upon successful completion, pthread_create() shall store 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 shall be 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 shall be 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 shall be initialized as follows:

        *  The signal mask shall be inherited from the creating thread.

        *  The set of signals pending for the new thread shall be empty.

       The thread-local current locale and the alternate stack shall not be
       inherited.

       The floating-point environment shall be inherited from the creating
       thread.

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

       If _POSIX_THREAD_CPUTIME is defined, the new thread shall have a CPU-
       time clock accessible, and the initial value of this clock shall be
       set to zero.

       The behavior is undefined if the value specified by the attr argument
       to pthread_create() does not refer to an initialized thread
       attributes object.

RETURN VALUE         top

       If successful, the pthread_create() function shall return zero;
       otherwise, an error number shall be returned to indicate the error.

ERRORS         top

       The pthread_create() function shall 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.

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

       The pthread_create() function shall not return an error code of
       [EINTR].

       The following sections are informative.

EXAMPLES         top

       None.

APPLICATION USAGE         top

       There is no requirement on the implementation that the ID of the
       created thread be available before the newly created thread starts
       executing. The calling thread can obtain the ID of the created thread
       through the return value of the pthread_create() function, and the
       newly created thread can obtain its ID by a call to pthread_self().

RATIONALE         top

       A suggested alternative to pthread_create() would be to define two
       separate operations: create and start. Some applications would find
       such behavior more natural. Ada, in particular, separates the
       ``creation'' of a task from its ``activation''.

       Splitting the operation was rejected by the standard developers for
       many reasons:

        *  The number of calls required to start a thread would increase
           from one to two and thus place an additional burden on
           applications that do not require the additional synchronization.
           The second call, however, could be avoided by the additional
           complication of a start-up state attribute.

        *  An extra state would be introduced: ``created but not started''.
           This would require the standard to specify the behavior of the
           thread operations when the target has not yet started executing.

        *  For those applications that require such behavior, it is possible
           to simulate the two separate steps with the facilities that are
           currently provided. The start_routine() can synchronize by
           waiting on a condition variable that is signaled by the start
           operation.

       An Ada implementor can choose to create the thread at either of two
       points in the Ada program: when the task object is created, or when
       the task is activated (generally at a ``begin''). If the first
       approach is adopted, the start_routine() needs to wait on a condition
       variable to receive the order to begin ``activation''. The second
       approach requires no such condition variable or extra
       synchronization. In either approach, a separate Ada task control
       block would need to be created when the task object is created to
       hold rendezvous queues, and so on.

       An extension of the preceding model would be to allow the state of
       the thread to be modified between the create and start. This would
       allow the thread attributes object to be eliminated. This has been
       rejected because:

        *  All state in the thread attributes object has to be able to be
           set for the thread. This would require the definition of
           functions to modify thread attributes. There would be no
           reduction in the number of function calls required to set up the
           thread. In fact, for an application that creates all threads
           using identical attributes, the number of function calls required
           to set up the threads would be dramatically increased. Use of a
           thread attributes object permits the application to make one set
           of attribute setting function calls.  Otherwise, the set of
           attribute setting function calls needs to be made for each thread
           creation.

        *  Depending on the implementation architecture, functions to set
           thread state would require kernel calls, or for other
           implementation reasons would not be able to be implemented as
           macros, thereby increasing the cost of thread creation.

        *  The ability for applications to segregate threads by class would
           be lost.

       Another suggested alternative uses a model similar to that for
       process creation, such as ``thread fork''. The fork semantics would
       provide more flexibility and the ``create'' function can be
       implemented simply by doing a thread fork followed immediately by a
       call to the desired ``start routine'' for the thread. This
       alternative has these problems:

        *  For many implementations, the entire stack of the calling thread
           would need to be duplicated, since in many architectures there is
           no way to determine the size of the calling frame.

        *  Efficiency is reduced since at least some part of the stack has
           to be copied, even though in most cases the thread never needs
           the copied context, since it merely calls the desired start
           routine.

       If an implementation detects that the value specified by the attr
       argument to pthread_create() does not refer to an initialized thread
       attributes object, it is recommended that the function should fail
       and report an [EINVAL] error.

FUTURE DIRECTIONS         top

       None.

SEE ALSO         top

       fork(3p), pthread_exit(3p), pthread_join(3p)

       The Base Definitions volume of POSIX.1‐2008, Section 4.11, Memory
       Synchronization, pthread.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) In the event of any discrepancy between this version and
       the original IEEE and The Open Group Standard, the original IEEE and
       The Open Group Standard is the referee document. The original
       Standard can be obtained online at http://www.unix.org/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the
       source files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                 2013                  PTHREAD_CREATE(3P)