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NAME
pthread_create — thread creation
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 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
If successful, the pthread_create() function shall return zero; otherwise, an error number shall be
returned to indicate the error.
ERRORS
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
None.
APPLICATION USAGE
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 thread argument of the pthread_create() function, and the newly created thread can obtain its ID by a
call to pthread_self().
RATIONALE
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
None.
SEE ALSO
fork(), pthread_exit(), pthread_join()
The Base Definitions volume of POSIX.1‐2017, Section 4.12, Memory Synchronization, <pthread.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. 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.opengroup.org/unix/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 2017 PTHREAD_CREATE(3POSIX)