getpriority(2)

1getpriority(2)                System Calls Manual               getpriority(2)
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NAME

6       getpriority, setpriority - get/set program scheduling priority
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LIBRARY

9       Standard C library (libc, -lc)
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SYNOPSIS

12       #include <sys/resource.h>
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14       int getpriority(int which, id_t who);
15       int setpriority(int which, id_t who, int prio);
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DESCRIPTION

18       The  scheduling priority of the process, process group, or user, as in‐
19       dicated by which and who is obtained with the  getpriority()  call  and
20       set  with  the setpriority() call.  The process attribute dealt with by
21       these system calls is the same attribute  (also  known  as  the  "nice"
22       value) that is dealt with by nice(2).
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24       The  value  which  is one of PRIO_PROCESS, PRIO_PGRP, or PRIO_USER, and
25       who  is  interpreted  relative  to  which  (a  process  identifier  for
26       PRIO_PROCESS, process group identifier for PRIO_PGRP, and a user ID for
27       PRIO_USER).  A zero value for who denotes  (respectively)  the  calling
28       process,  the process group of the calling process, or the real user ID
29       of the calling process.
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31       The prio argument is a value in the range -20 to 19 (but see NOTES  be‐
32       low),  with -20 being the highest priority and 19 being the lowest pri‐
33       ority.  Attempts to set a priority  outside  this  range  are  silently
34       clamped  to  the range.  The default priority is 0; lower values give a
35       process a higher scheduling priority.
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37       The getpriority() call returns the highest priority  (lowest  numerical
38       value)  enjoyed  by  any of the specified processes.  The setpriority()
39       call sets the priorities of all of the specified processes to the spec‐
40       ified value.
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42       Traditionally,  only  a  privileged  process could lower the nice value
43       (i.e., set a higher priority).  However, since Linux 2.6.12, an unpriv‐
44       ileged process can decrease the nice value of a target process that has
45       a suitable RLIMIT_NICE soft limit; see getrlimit(2) for details.
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RETURN VALUE

48       On success, getpriority() returns  the  calling  thread's  nice  value,
49       which may be a negative number.  On error, it returns -1 and sets errno
50       to indicate the error.
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52       Since a successful call to getpriority() can  legitimately  return  the
53       value  -1, it is necessary to clear errno prior to the call, then check
54       errno afterward to determine if -1 is an error or a legitimate value.
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56       setpriority() returns 0 on success.  On failure, it returns -1 and sets
57       errno to indicate the error.
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ERRORS

60       EACCES The  caller  attempted to set a lower nice value (i.e., a higher
61              process priority), but did not have the required  privilege  (on
62              Linux: did not have the CAP_SYS_NICE capability).
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64       EINVAL which was not one of PRIO_PROCESS, PRIO_PGRP, or PRIO_USER.
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66       EPERM  A  process  was located, but its effective user ID did not match
67              either the effective or the real user ID of the caller, and  was
68              not privileged (on Linux: did not have the CAP_SYS_NICE capabil‐
69              ity).  But see NOTES below.
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71       ESRCH  No process was located using the which and who values specified.
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STANDARDS

74       POSIX.1-2008.
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HISTORY

77       POSIX.1-2001, SVr4, 4.4BSD (these interfaces first appeared in 4.2BSD).
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NOTES

80       For further details on the nice value, see sched(7).
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82       Note: the addition of the "autogroup" feature  in  Linux  2.6.38  means
83       that  the  nice value no longer has its traditional effect in many cir‐
84       cumstances.  For details, see sched(7).
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86       A child created by fork(2) inherits its parent's nice value.  The  nice
87       value is preserved across execve(2).
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89       The details on the condition for EPERM depend on the system.  The above
90       description is what POSIX.1-2001 says, and seems to be followed on  all
91       System V-like  systems.  Linux kernels before Linux 2.6.12 required the
92       real or effective user ID of the caller to match the real user  of  the
93       process who (instead of its effective user ID).  Linux 2.6.12 and later
94       require the effective user ID of the caller to match the real or effec‐
95       tive  user  ID  of the process who.  All BSD-like systems (SunOS 4.1.3,
96       Ultrix 4.2, 4.3BSD, FreeBSD 4.3, OpenBSD-2.5, ...) behave in  the  same
97       manner as Linux 2.6.12 and later.
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99   C library/kernel differences
100       The getpriority system call returns nice values translated to the range
101       40..1, since a negative return value would be interpreted as an  error.
102       The  glibc wrapper function for getpriority() translates the value back
103       according to the formula unice = 20 - knice (thus, the 40..1 range  re‐
104       turned  by  the kernel corresponds to the range -20..19 as seen by user
105       space).
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BUGS

108       According to POSIX, the nice value is a per-process setting.   However,
109       under  the current Linux/NPTL implementation of POSIX threads, the nice
110       value is a per-thread attribute: different threads in the same  process
111       can have different nice values.  Portable applications should avoid re‐
112       lying on the Linux behavior, which may be made standards conformant  in
113       the future.
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