1GETPRIORITY(2) Linux Programmer's Manual GETPRIORITY(2)
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6 getpriority, setpriority - get/set program scheduling priority
7
9 #include <sys/time.h>
10 #include <sys/resource.h>
11 int getpriority(int which, int who);
13 int setpriority(int which, int who, int prio);
14
16 The scheduling priority of the process, process group, or user, as
17 indicated by which and who is obtained with the getpriority() call and
18 set with the setpriority() call.
19 The value which is one of PRIO_PROCESS, PRIO_PGRP, or PRIO_USER, and
21 who is interpreted relative to which (a process identifier for
22 PRIO_PROCESS, process group identifier for PRIO_PGRP, and a user ID for
23 PRIO_USER). A zero value for who denotes (respectively) the calling
24 process, the process group of the calling process, or the real user ID
25 of the calling process. Prio is a value in the range -20 to 19 (but
26 see the Notes below). The default priority is 0; lower priorities
27 cause more favorable scheduling.
28 The getpriority() call returns the highest priority (lowest numerical
30 value) enjoyed by any of the specified processes. The setpriority()
31 call sets the priorities of all of the specified processes to the spec‐
32 ified value. Only the superuser may lower priorities.
33
35 Since getpriority() can legitimately return the value -1, it is neces‐
36 sary to clear the external variable errno prior to the call, then check
37 it afterwards to determine if -1 is an error or a legitimate value.
38 The setpriority() call returns 0 if there is no error, or -1 if there
39 is.
40
42 EINVAL which was not one of PRIO_PROCESS, PRIO_PGRP, or PRIO_USER.
43 ESRCH No process was located using the which and who values specified.
45 In addition to the errors indicated above, setpriority() may fail if:
47 EACCES The caller attempted to lower a process priority, but did not
49 have the required privilege (on Linux: did not have the
50 CAP_SYS_NICE capability). Since Linux 2.6.12, this error only
51 occurs if the caller attempts to set a process priority outside
52 the range of the RLIMIT_NICE soft resource limit of the target
53 process; see getrlimit(2) for details.
54 EPERM A process was located, but its effective user ID did not match
56 either the effective or the real user ID of the caller, and was
57 not privileged (on Linux: did not have the CAP_SYS_NICE capabil‐
58 ity). But see NOTES below.
59
61 SVr4, 4.4BSD (these function calls first appeared in 4.2BSD),
62 POSIX.1-2001.
63
65 A child created by fork(2) inherits its parent's nice value. The nice
66 value is preserved across execve(2).
67 The degree to which their relative nice value affects the scheduling of
69 processes varies across Unix systems, and, on Linux, across kernel ver‐
70 sions. Starting with kernel 2.6.23, Linux adopted an algorithm that
71 causes relative differences in nice values to have a much stronger
72 effect. This causes very low nice values (+19) to truly provide little
73 CPU to a process whenever there is any other higher priority load on
74 the system, and makes high nice values (-20) deliver most of the CPU to
75 applications that require it (e.g., some audio applications).
76 The details on the condition for EPERM depend on the system. The above
78 description is what POSIX.1-2001 says, and seems to be followed on all
79 System V-like systems. Linux kernels before 2.6.12 required the real
80 or effective user ID of the caller to match the real user of the
81 process who (instead of its effective user ID). Linux 2.6.12 and later
82 require the effective user ID of the caller to match the real or effec‐
83 tive user ID of the process who. All BSD-like systems (SunOS 4.1.3,
84 Ultrix 4.2, 4.3BSD, FreeBSD 4.3, OpenBSD-2.5, ...) behave in the same
85 manner as Linux 2.6.12 and later.
86 The actual priority range varies between kernel versions. Linux before
88 1.3.36 had -infinity..15. Since kernel 1.3.43 Linux has the range
89 -20..19. Within the kernel, nice values are actually represented using
90 the corresponding range 40..1 (since negative numbers are error codes)
91 and these are the values employed by the setpriority() and getprior‐
92 ity() system calls. The glibc wrapper functions for these system calls
93 handle the translations between the user-land and kernel representa‐
94 tions of the nice value according to the formula unice = 20 - knice.
95 On some systems, the range of nice values is -20..20.
97 Including <sys/time.h> is not required these days, but increases porta‐
99 bility. (Indeed, <sys/resource.h> defines the rusage structure with
100 fields of type struct timeval defined in <sys/time.h>.)
101
103 nice(1), fork(2), capabilities(7), renice(8)
104 Documentation/scheduler/sched-nice-design.txt in the kernel source tree
106 (since Linux 2.6.23).
107
109 This page is part of release 3.22 of the Linux man-pages project. A
110 description of the project, and information about reporting bugs, can
111 be found at http://www.kernel.org/doc/man-pages/.
112Linux 2008-05-29 GETPRIORITY(2)