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RedHat 9 (Linux i386) - man page for setpriority (redhat section 2)

GETPRIORITY(2)						     Linux Programmer's Manual						    GETPRIORITY(2)

getpriority, setpriority - get/set program scheduling priority
#include <sys/time.h> #include <sys/resource.h> int getpriority(int which, int who); int setpriority(int which, int who, int prio);
The scheduling priority of the process, process group, or user, as indicated by which and who is obtained with the getpriority call and set with the setpriority call. 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 (respec- tively) the calling process, the process group of the calling process, or the real user ID of the calling process. Prio is a value in the range -20 to 20 (but see the Notes below). The default priority is 0; lower priorities cause more favorable scheduling. 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 specified value. Only the super-user may lower priorities.
Since getpriority can legitimately return the value -1, it is necessary to clear the external variable errno prior to the call, then check it afterwards to determine if a -1 is an error or a legitimate value. The setpriority call returns 0 if there is no error, or -1 if there is.
ESRCH No process was located using the which and who values specified. EINVAL Which was not one of PRIO_PROCESS, PRIO_PGRP, or PRIO_USER. In addition to the errors indicated above, setpriority may fail if: EPERM A process was located, but neither the effective nor the real user ID of the caller matches its effective user ID. EACCES A non super-user attempted to lower a process priority.
The details on the condition for EPERM depend on the system. The above description is what SUSv3 says, and seems to be followed on all SYSV-like systems. Linux requires the real or effective user ID of the caller to match the real user of the process who (instead of its effective user ID). All BSD-like systems (SunOS 4.1.3, Ultrix 4.2, BSD 4.3, FreeBSD 4.3, OpenBSD-2.5, ...) require the effective user ID of the caller to match the real or effective user ID of the process who. The actual priority range varies between kernel versions. Linux before 1.3.36 had -infinity..15. Linux since 1.3.43 has -20..19, and the system call getpriority returns 40..1 for these values (since negative numbers are error codes). The library call converts N into 20-N. Including <sys/time.h> is not required these days, but increases portability. (Indeed, <sys/resource.h> defines the rusage structure with fields of type struct timeval defined in <sys/time.h>.)
SVr4, 4.4BSD (these function calls first appeared in 4.2BSD).
nice(1), fork(2), renice(8) BSD Man Page 2002-06-21 GETPRIORITY(2)