Get/set program scheduling priority
int getpriority(int which, id_t who);
int setpriority(int which, id_t who, int prio);
The scheduling priority of the process, process group, or user, as indicated by which and who is obtained with the getpriority() call and set with the setpriority() call.
The value which is one of PRIO_PROCESS, PRIO_PGRP, or PRIO_USER, and who is interpreted relative to which (a process identifier for PRIO_PROCESS, process group identifier for PRIO_PGRP, and a user ID for PRIO_USER). A zero value for who denotes (respectively) the calling process, the process group of the calling process, or the real user ID of the calling process. Prio is a value in the range -20 to 19 (but see the Notes below). The default priority is 0; lower priorities cause more favorable scheduling.
The getpriority() call returns the highest priority (lowest numerical value) enjoyed by any of the specified processes. The setpriority() call sets the priorities of all of the specified processes to the specified value. Only the superuser may lower priorities.
Since getpriority() can legitimately return the value -1, it is necessary to clear the external variable errno prior to the call, then check it afterward to determine if -1 is an error or a legitimate value. The setpriority() call returns 0 if there is no error, or -1 if there is.
which was not one of PRIO_PROCESS, PRIO_PGRP, or PRIO_USER.
No process was located using the which and who values specified.
In addition to the errors indicated above, setpriority() may fail if:
The caller attempted to lower a process priority, but did not have the required privilege (on Linux: did not have the CAP_SYS_NICE capability). Since Linux 2.6.12, this error occurs only if the caller attempts to set a process priority outside the range of the RLIMIT_NICE soft resource limit of the target process; see getrlimit(2) for details.
A process was located, but its effective user ID did not match either the effective or the real user ID of the caller, and was not privileged (on Linux: did not have the CAP_SYS_NICE capability). But see NOTES below.
SVr4, 4.4BSD (these function calls first appeared in 4.2BSD), POSIX.1-2001.
A child created by fork(2) inherits its parent's nice value. The nice value is preserved across execve(2).
The degree to which their relative nice value affects the scheduling of processes varies across UNIX systems, and, on Linux, across kernel versions. Starting with kernel 2.6.23, Linux adopted an algorithm that causes relative differences in nice values to have a much stronger effect. This causes very low nice values (+19) to truly provide little CPU to a process whenever there is any other higher priority load on the system, and makes high nice values (-20) deliver most of the CPU to applications that require it (e.g., some audio applications).
The details on the condition for EPERM depend on the system. The above description is what POSIX.1-2001 says, and seems to be followed on all System V-like systems. Linux kernels before 2.6.12 required the real or effective user ID of the caller to match the real user of the process who (instead of its effective user ID). Linux 2.6.12 and later require the effective user ID of the caller to match the real or effective user ID of the process who. All BSD-like systems (SunOS 4.1.3, Ultrix 4.2, 4.3BSD, FreeBSD 4.3, OpenBSD-2.5, ...) behave in the same manner as Linux 2.6.12 and later.
The actual priority range varies between kernel versions. Linux before 1.3.36 had -infinity..15. Since kernel 1.3.43, Linux has the range -20..19. On some other systems, the range of nice values is -20..20.
Including <sys/time.h> is not required these days, but increases portability. (Indeed, <sys/resource.h> defines the rusage structure with fields of type struct timeval defined in <sys/time.h>.)
Within the kernel, nice values are actually represented using the range 40..1 (since negative numbers are error codes) and these are the values employed by the setpriority() and getpriority() system calls. The glibc wrapper functions for these system calls handle the translations between the user-land and kernel representations of the nice value according to the formula unice = 20 - knice. (Thus, the kernels 40..1 range corresponds to the range -20..19 as seen by user space.)
According to POSIX, the nice value is a per-process setting. However, under the current Linux/NPTL implementation of POSIX threads, the nice value is a per-thread attribute: different threads in the same process can have different nice values. Portable applications should avoid relying on the Linux behavior, which may be made standards conformant in the future.
nice(1), renice(1), fork(2), capabilities(7), sched(7)
Documentation/scheduler/sched-nice-design.txt in the Linux kernel source tree (since Linux 2.6.23)
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