Change process priority
int nice(int inc);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
nice(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE
nice() adds inc to the nice value for the calling process. (A higher nice value means a low priority.) Only the superuser may specify a negative increment, or priority increase. The range for nice values is described in getpriority(2).
On success, the new nice value is returned (but see NOTES below). On error, -1 is returned, and errno is set appropriately.
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).)
SVr4, 4.3BSD, POSIX.1-2001. However, the Linux and (g)libc (earlier than glibc 2.2.4) return value is nonstandard, see below. SVr4 documents an additional EINVAL error code.
SUSv2 and POSIX.1-2001 specify that nice() should return the new nice value. However, the Linux syscall and the nice() library function provided in older versions of (g)libc (earlier than glibc 2.2.4) return 0 on success. The new nice value can be found using getpriority(2).
Since glibc 2.2.4, nice() is implemented as a library function that calls getpriority(2) to obtain the new nice value to be returned to the caller. With this implementation, a successful call can legitimately return -1. To reliably detect an error, set errno to 0 before the call, and check its value when nice() returns -1.
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