PTHREAD_CREATE(P) POSIX Programmer's Manual PTHREAD_CREATE(P)NAMEpthread_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 cre‐
ated 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 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.
RETURN VALUE
If successful, the pthread_create() function shall return zero; other‐
wise, 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.
EINVAL The value specified by attr is invalid.
EPERM The caller does not have appropriate permission 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
None.
RATIONALE
A suggested alternative to pthread_create() would be to define two sep‐
arate 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, how‐
ever, 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 cur‐
rently 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.
FUTURE DIRECTIONS
None.
SEE ALSOfork() , pthread_exit() , pthread_join() , the Base Definitions volume
of IEEE Std 1003.1-2001, <pthread.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .
IEEE/The Open Group 2003 PTHREAD_CREATE(P)
