#### POSIX Threads for Windows – REFERENCE - pthreads-win32

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## Name

pthread_getconcurrency, pthread_setconcurrency - get and set the level of concurrency

## Synopsis

#include <pthread.h>

int pthread_getconcurrency(void);
int pthread_setconcurrency(int new_level);

## Description

Unbound threads in a process may or may not be required to be simultaneously active. By default, the threads implementation ensures that a sufficient number of threads are active so that the process can continue to make progress. While this conserves system resources, it may not produce the most effective level of concurrency.

The pthread_setconcurrency function allows an application to inform the threads implementation of its desired concurrency level, new_level. The actual level of concurrency provided by the implementation as a result of this function call is unspecified.

If new_level is zero, it causes the implementation to maintain the concurrency level at its discretion as if pthread_setconcurrency had never been called.

The pthread_getconcurrency function shall return the value set by a previous call to the pthread_setconcurrency function. If the pthread_setconcurrency function was not previously called, this function shall return zero to indicate that the implementation is maintaining the concurrency level.

A call to pthread_setconcurrency shall inform the implementation of its desired concurrency level. The implementation shall use this as a hint, not a requirement.

If an implementation does not support multiplexing of user threads on top of several kernel-scheduled entities, the pthread_setconcurrency and pthread_getconcurrency functions are provided for source code compatibility but they shall have no effect when called. To maintain the function semantics, the new_level parameter is saved when pthread_setconcurrency is called so that a subsequent call to pthread_getconcurrency shall return the same value.

pthreads-win32 provides these routines for source code compatibility only, as described in the previous paragraph.

## Return Value

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

The pthread_getconcurrency function shall always return the concurrency level set by a previous call to pthread_setconcurrency . If the pthread_setconcurrency function has never been called, pthread_getconcurrency shall return zero.

## Errors

The pthread_setconcurrency function shall fail if:

EINVAL
  The value specified by new_level is negative.
EAGAIN
  The value specific by new_level would cause a system resource to be exceeded.

These functions shall not return an error code of [EINTR].

The following sections are informative.

## Examples

None.

## Application Usage

Use of these functions changes the state of the underlying concurrency upon which the application depends. Library developers are advised to not use the pthread_getconcurrency and pthread_setconcurrency functions since their use may conflict with an applications use of these functions.

## Rationale

None.

## Future Directions

None.

## See Also

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 .

Modified by Ross Johnson for use with pthreads-win32.

Table of Contents

- Name

- Synopsis

- Description

- Return Value

- Errors

- Examples

- Application Usage

- Rationale

- Future Directions

- See Also

- Copyright