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(libc.info)Processor Resources

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Learn about the processors available
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   The use of threads or processes with shared memory allows an
application to take advantage of all the processing power a system can
provide.  If the task can be parallelized the optimal way to write an
application is to have at any time as many processes running as there
are processors.  To determine the number of processors available to the
system one can run

       sysconf (_SC_NPROCESSORS_CONF)

which returns the number of processors the operating system configured.
But it might be possible for the operating system to disable individual
processors and so the call

       sysconf (_SC_NPROCESSORS_ONLN)

returns the number of processors which are currently inline (i.e.,
available).

   For these two pieces of information the GNU C library also provides
functions to get the information directly.  The functions are declared
in `sys/sysinfo.h'.

 - Function: int get_nprocs_conf (void)
     The `get_nprocs_conf' function returns the number of processors the
     operating system configured.

     This function is a GNU extension.

 - Function: int get_nprocs (void)
     The `get_nprocs' function returns the number of available
     processors.

     This function is a GNU extension.

   Before starting more threads it should be checked whether the
processors are not already overused.  Unix systems calculate something
called the "load average".  This is a number indicating how many
processes were running.  This number is average over different periods
of times (normally 1, 5, and 15 minutes).

 - Function: int getloadavg (double LOADAVG[], int NELEM)
     This function gets the 1, 5 and 15 minute load averages of the
     system. The values are placed in LOADAVG.  `getloadavg' will place
     at most NELEM elements into the array but never more than three
     elements.  The return value is the number of elements written to
     LOADAVG, or -1 on error.

     This function is declared in `stdlib.h'.