##### 22.3.4.2 Functions For Traditional Scheduling

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)

> 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)

> 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)

> 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;
>               }