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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_MINPRIO_MAXReturn 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-1andERRNOis set accordingly. Theerrnovalues 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 seterrno = 0before callinggetpriority, then useerrno != 0afterward as the criterion for failure.
Set the nice value of a set of processes to niceval; class and id specify which ones (see below).
The return value is
0on success, and-1on failure. The followingerrnoerror 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_NICEpermission.EACCES- The call would lower the process' nice value and the process does not have
CAP_SYS_NICEpermission.
The arguments class and id together specify a set of processes in which you are interested. These are the possible values of class:
PRIO_PROCESSPRIO_PGRPPRIO_USERIf the argument id is 0, it stands for the calling process, its process group, or its owner (real uid), according to class.
Increment the nice value of the calling process by increment. The return value is the new nice value on success, and
-1on failure. In the case of failure,errnowill be set to the same values as forsetpriority.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; }