The Open Group Base Specifications Issue 7, 2018 edition
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
Copyright © 2001-2018 IEEE and The Open Group


pthread_create - thread creation


#include <pthread.h>

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


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 thread-local current locale [XSI] [Option Start]  and the alternate stack [Option End]  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.

[TCT] [Option Start] 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. [Option End]

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


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


The pthread_create() function shall fail if:

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.
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.




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().


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:

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:

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:

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.




fork, pthread_exit, pthread_join

XBD Memory Synchronization, <pthread.h>


First released in Issue 5. Included for alignment with the POSIX Threads Extension.

Issue 6

The pthread_create() function is marked as part of the Threads option.

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

The thread CPU-time clock semantics are added for alignment with IEEE Std 1003.1d-1999.

The restrict keyword is added to the pthread_create() prototype for alignment with the ISO/IEC 9899:1999 standard.

The DESCRIPTION is updated to make it explicit that the floating-point environment is inherited from the creating thread.

IEEE Std 1003.1-2001/Cor 1-2002, item XSH/TC1/D6/44 is applied, adding text that the alternate stack is not inherited.

IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/93 is applied, updating the ERRORS section to remove the mandatory [EINVAL] error (``The value specified by attr is invalid"), and adding the optional [EINVAL] error (``The attributes specified by attr are invalid").

IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/94 is applied, adding the APPLICATION USAGE section.

Issue 7

The pthread_create() function is moved from the Threads option to the Base.

The [EINVAL] error for an uninitialized thread attributes object is removed; this condition results in undefined behavior.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0458 [302] is applied.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0274 [849] is applied.

End of informative text.


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