nice (2) Linux Manual Page
nice – change process priority
Synopsis
#include <unistd.h>int nice(int inc);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
nice(): _XOPEN_SOURCE
Description
nice() adds inc to the nice value for the calling thread. (A higher nice value means a lower priority.)
The range of the nice value is +19 (low priority) to -20 (high priority). Attempts to set a nice value outside the range are clamped to the range.
Traditionally, only a privileged process could lower the nice value (i.e., set a higher priority). However, since Linux 2.6.12, an unprivileged 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, the new nice value is returned (but see NOTES below). On error, -1 is returned, and errno is set appropriately. A successful call can legitimately return -1. To detect an error, set errno to 0 before the call, and check whether it is nonzero after nice() returns -1.
Errors
- EPERM
- The calling process attempted to increase its priority by supplying a negative inc but has insufficient privileges. Under Linux, the CAP_SYS_NICE capability is required. (But see the discussion of the RLIMIT_NICE resource limit in setrlimit(2).)
Conforming To
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD. However, the raw system call and (g)libc (earlier than glibc 2.2.4) return value is nonstandard, see below.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 circumstances. For details, see sched(7).
C library/kernel differences
POSIX.1 specifies that nice() should return the new nice value. However, the raw Linux system call returns 0 on success. Likewise, the nice() wrapper function provided in glibc 2.2.3 and earlier returns 0 on success. Since glibc 2.2.4, the nice() wrapper function provided by glibc provides conformance to POSIX.1 by calling getpriority(2) to obtain the new nice value, which is then returned to the caller.