Traditional Scheduling Functions

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:

— 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:

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;
          }

22.3.4.2 Functions For Traditional Scheduling