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NAME

pthread_create - thread creation

SYNOPSIS

#include <pthread.h>

int pthread_create(pthread_t *restrictthread,
       const pthread_attr_t *restrictattr,
       void *(*start_routine)(void*), void *restrictarg);


DESCRIPTION

Thepthread_create() function shall create a new thread, with attributes specified byattr, within a process. Ifattr is NULL, the default attributes shall be used. If the attributes specified byattr are modified later, thethread's attributes shall not be affected. Upon successful completion,pthread_create() shall store the ID of the createdthread in the location referenced bythread.

The thread is created executingstart_routine witharg as its sole argument. If thestart_routine returns,the effect shall be as if there was an implicit call topthread_exit() usingthe return value ofstart_routine as the exit status. Note that the thread in whichmain() was originally invokeddiffers from this. When it returns frommain(), the effect shall be as if there was an implicit call toexit() using the return value ofmain() 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^[XSI]  and the alternate stack shall not be inherited.

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

Ifpthread_create() fails, no new thread is created and the contents of the location referenced bythread areundefined.

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

The behavior is undefined if the value specified by theattr argument topthread_create() does not refer to aninitialized thread attributes object.

RETURN VALUE

If successful, thepthread_create() function shall return zero; otherwise, an error number shall be returned to indicatethe error.

ERRORS

Thepthread_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 threadsin 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.

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

EXAMPLES

None.

APPLICATION USAGE

There is no requirement on the implementation that the ID of the created thread be available before the newly created threadstarts executing. The calling thread can obtain the ID of the created thread through thethread argument of thepthread_create() function, and the newly created thread can obtain its ID by a call topthread_self().

RATIONALE

A suggested alternative topthread_create() would be to define two separate operations: create and start. Someapplications 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 onapplications that do not require the additional synchronization. The second call, however, could be avoided by the additionalcomplication 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 thethread 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 thatare currently provided. Thestart_routine() can synchronize by waiting on a condition variable that is signaled by the startoperation.

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, thestart_routine() needs towait on a condition variable to receive the order to begin "activation". The second approach requires no such condition variableor extra synchronization. In either approach, a separate Ada task control block would need to be created when the task object iscreated 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. Thiswould 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 offunctions 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 upthe threads would be dramatically increased. Use of a thread attributes object permits the application to make one set of attributesetting 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 otherimplementation 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 semanticswould provide more flexibility and the "create" function can be implemented simply by doing a thread fork followed immediately bya 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 thereis 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 needsthe copied context, since it merely calls the desired start routine.

If an implementation detects that the value specified by theattr argument topthread_create() does not refer toan 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

XBDMemory Synchronization,<pthread.h>

CHANGE HISTORY

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

Issue 6

Thepthread_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 [EPERM] mandatory error condition is added.

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

Therestrict keyword is added to thepthread_create() prototype for alignment with the ISO/IEC 9899:1999standard.

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 notinherited.

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

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

Issue 7

Thepthread_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.