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home | helpx minix x x minixx GETPRIORITY(2) Linux Programmer's Manual GETPRIORITY(2) NAME getpriority, setpriority - get/set program scheduling priority SYNOPSIS #include <sys/time.h> #include <sys/resource.h> int getpriority(int which, id_t who); int setpriority(int which, id_t who, int prio); DESCRIPTION The scheduling priority of the process, process group, or user, as in- dicated by which and who is obtained with the getpriority() call and set with the setpriority() call. The process attribute dealt with by these system calls is the same attribute (also known as the "nice" value) that is dealt with by nice(2). 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. The prio argument is a value in the range -20 to 19 (but see NOTES be- low). with -20 being the highest priority and 19 being the lowest pri- ority. Attempts to set a priority outside this range are silently clamped to the range. The default priority is 0; lower values give a process a higher scheduling priority. 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 spec- ified value. Traditionally, only a privileged process could lower the nice value (i.e., set a higher priority). However, since Linux 2.6.12, an unpriv- ileged process can decrease the nice value of a target process that has a suitable RLIMIT_NICE soft limit; see getrlimit(2) for details. RETURN VALUE On success, getpriority() returns the calling thread's nice value, which may be a negative number. On error, it returns -1 and sets errno to indicate the cause of the error. Since a successful call to getpri- ority() 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. setpriority() returns 0 on success. On error, it returns -1 and sets errno to indicate the cause of the error. ERRORS EINVAL which was not one of PRIO_PROCESS, PRIO_PGRP, or PRIO_USER. ESRCH No process was located using the which and who values specified. In addition to the errors indicated above, setpriority() may fail if: EACCES The caller attempted to set a lower nice value (i.e., a higher process priority), but did not have the required privilege (on Linux: did not have the CAP_SYS_NICE capability). EPERM 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 capabil- ity). But see NOTES below. CONFORMING TO POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD (these interfaces first ap- peared in 4.2BSD). NOTES For further details on the nice value, see sched(7). Note: the addition of the "autogroup" feature in Linux 2.6.38 means that the nice value no longer has its traditional effect in many cir- cumstances. For details, see sched(7). A child created by fork(2) inherits its parent's nice value. The nice value is preserved across execve(2). 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 effec- tive 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. Including _sys/time.h_ is not required these days, but increases porta- bility. (Indeed, _sys/resource.h_ defines the rusage structure with fields of type struct timeval defined in _sys/time.h_.) C library/kernel differences 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 ac- cording to the formula unice = 20 - knice. (Thus, the kernel's 40..1 range corresponds to the range -20..19 as seen by user space.) BUGS 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 re- lying on the Linux behavior, which may be made standards conformant in the future. SEE ALSO 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) COLOPHON This page is part of release 5.05 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/. Linux 2017-09-15 GETPRIORITY(2)
NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON