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Pthread Library
HP-UX 11i Version 3: February 2007

Technical documentation

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pthread_getconcurrency(), pthread_setconcurrency() — get or set the concurrency level of unbound threads


#include <pthread.h>

int pthread_setconcurrency( int new_level );

int pthread_getconcurrency(void);



New concurrency level for the unbound threads in the calling process.


The 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 system as a result of this function call is unspecified.

If new_level is zero, it will cause the implementation to maintain the concurrency level at its discretion as if pthread_setconcurrency() were never called.

The pthread_getconcurrency() function returns the value set by a previous call to pthread_setconcurrency(). If the pthread_setconcurrency() function was not previously called, this function returns zero to indicate that the system is maintaining the concurrency level.

Note: When an application calls pthread_setconcurrency(), it is informing the implementation of its desired concurrency level. The implementation will use this as a hint, not a requirement. A call to pthread_getconcurrency() immediately after a call to pthread_setconcurrency() may return a different concurrency level specified in pthread_setconcurrency().

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


If successful, pthread_setconcurrency() returns zero. Otherwise, an error number is returned to indicate the error (the errno variable is not set).

The pthread_getconcurrency() function always returns 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.


If any of the following occur, the pthread_setconcurrency() function shall return the corresponding error number.


The value specified by new_level is invalid.


The value specified by new_level would cause a system resource to be exceeded.


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


pthread_getconcurrency() and pthread_setconcurrency() were developed by X/Open.


pthread_num_processors_np(3T), pthread_processor_bind_np(3T), pthread_processor_id_np(3T).


pthread_getconcurrency(): X/Open. pthread_setconcurrency(): X/Open.

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