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File: libc.info,  Node: Traditional Scheduling Functions,  Prev: Traditional Scheduling Intro,  Up: Traditional Scheduling
22.3.4.2 Functions For Traditional Scheduling
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This section describes how you can read and set the nice value of a
process.  All these symbols are declared in 'sys/resource.h'.
   The function and macro names are defined by POSIX, and refer to
"priority," but the functions actually have to do with nice values, as
the terms are used both in the manual and POSIX.
   The range of valid nice values depends on the kernel, but typically
it runs from '-20' to '20'.  A lower nice value corresponds to higher
priority for the process.  These constants describe the range of
priority values:
'PRIO_MIN'
     The lowest valid nice value.
'PRIO_MAX'
     The highest valid nice value.
 -- Function: int getpriority (int CLASS, int ID)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.
     Return the nice value of a set of processes; CLASS and ID specify
     which ones (see below).  If the processes specified do not all have
     the same nice value, this returns the lowest value that any of them
     has.
     On success, the return value is '0'.  Otherwise, it is '-1' and
     'ERRNO' is set accordingly.  The 'errno' values specific to this
     function are:
     'ESRCH'
          The combination of CLASS and ID does not match any existing
          process.
     'EINVAL'
          The value of CLASS is not valid.
     If the return value is '-1', it could indicate failure, or it could
     be the nice value.  The only way to make certain is to set 'errno =
     0' before calling 'getpriority', then use 'errno != 0' afterward as
     the criterion for failure.
 -- Function: int setpriority (int CLASS, int ID, int NICEVAL)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.
     Set the nice value of a set of processes to NICEVAL; CLASS and ID
     specify which ones (see below).
     The return value is '0' on success, and '-1' on failure.  The
     following 'errno' error condition are possible for this function:
     'ESRCH'
          The combination of CLASS and ID does not match any existing
          process.
     'EINVAL'
          The value of CLASS is not valid.
     'EPERM'
          The call would set the nice value of a process which is owned
          by a different user than the calling process (i.e., the target
          process' real or effective uid does not match the calling
          process' effective uid) and the calling process does not have
          'CAP_SYS_NICE' permission.
     'EACCES'
          The call would lower the process' nice value and the process
          does not have 'CAP_SYS_NICE' permission.
   The arguments CLASS and ID together specify a set of processes in
which you are interested.  These are the possible values of CLASS:
'PRIO_PROCESS'
     One particular process.  The argument ID is a process ID (pid).
'PRIO_PGRP'
     All the processes in a particular process group.  The argument ID
     is a process group ID (pgid).
'PRIO_USER'
     All the processes owned by a particular user (i.e., whose real uid
     indicates the user).  The argument ID is a user ID (uid).
   If the argument ID is 0, it stands for the calling process, its
process group, or its owner (real uid), according to CLASS.
 -- Function: int nice (int INCREMENT)
     Preliminary: | MT-Unsafe race:setpriority | AS-Unsafe | AC-Safe |
     *Note POSIX Safety Concepts::.
     Increment the nice value of the calling process by INCREMENT.  The
     return value is the new nice value on success, and '-1' on failure.
     In the case of failure, 'errno' will be set to the same values as
     for 'setpriority'.
     Here is an equivalent definition of 'nice':
          int
          nice (int increment)
          {
            int result, old = getpriority (PRIO_PROCESS, 0);
            result = setpriority (PRIO_PROCESS, 0, old + increment);
            if (result != -1)
                return old + increment;
            else
                return -1;
          }