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IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" %%%LICENSE_END .\" .\" @(#)getpriority.2 6.9 (Berkeley) 3/10/91 .\" .\" Modified 1993-07-24 by Rik Faith .\" Modified 1996-07-01 by Andries Brouwer .\" Modified 1996-11-06 by Eric S. Raymond .\" Modified 2001-10-21 by Michael Kerrisk .\" Corrected statement under EPERM to clarify privileges required .\" Modified 2002-06-21 by Michael Kerrisk .\" Clarified meaning of 0 value for 'who' argument .\" Modified 2004-05-27 by Michael Kerrisk .\" .TH GETPRIORITY 2 2016-07-17 "Linux" "Linux Programmer's Manual" .SH NAME getpriority, setpriority \- get/set program scheduling priority .SH SYNOPSIS .B #include .br .B #include .sp .BI "int getpriority(int " which ", id_t " who ); .br .BI "int setpriority(int " which ", id_t " who ", int " prio ); .SH DESCRIPTION The scheduling priority of the process, process group, or user, as indicated by .I which and .I who is obtained with the .BR getpriority () call and set with the .BR setpriority () call. The process attribute dealt with by these system calls is the same attribute (also known as the "nice" value) that is dealt with by .BR nice (2). The value .I which is one of .BR PRIO_PROCESS , .BR PRIO_PGRP , or .BR PRIO_USER , and .I who is interpreted relative to .I which (a process identifier for .BR PRIO_PROCESS , process group identifier for .BR PRIO_PGRP , and a user ID for .BR PRIO_USER ). A zero value for .I who denotes (respectively) the calling process, the process group of the calling process, or the real user ID of the calling process. The .I prio argument is a value in the range \-20 to 19 (but see NOTES below). with \-20 being the highest priority and 19 being the lowest priority. Attempts to set a priority outside this range are silently clamped to the range. The default priority is 0; lower values give a process a higher scheduling priority. The .BR getpriority () call returns the highest priority (lowest numerical value) enjoyed by any of the specified processes. The .BR setpriority () call sets the priorities of all of the specified processes to the specified value. 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 .BR RLIMIT_NICE soft limit; see .BR getrlimit (2) for details. .SH RETURN VALUE Since .BR getpriority () can legitimately return the value \-1, it is necessary to clear the external variable .I errno prior to the call, then check it afterward to determine if \-1 is an error or a legitimate value. The .BR setpriority () call returns 0 if there is no error, or \-1 if there is. .SH ERRORS .TP .B EINVAL .I which was not one of .BR PRIO_PROCESS , .BR PRIO_PGRP , or .BR PRIO_USER . .TP .B ESRCH No process was located using the .I which and .I who values specified. .PP In addition to the errors indicated above, .BR setpriority () may fail if: .TP .B EACCES The caller attempted to set a lower nice value (i.e., a higher process priority), but did not have the required privilege (on Linux: did not have the .B CAP_SYS_NICE capability). .TP .B EPERM A process was located, but its effective user ID did not match either the effective or the real user ID of the caller, and was not privileged (on Linux: did not have the .B CAP_SYS_NICE capability). But see NOTES below. .SH CONFORMING TO POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD (these interfaces first appeared in 4.2BSD). .SH NOTES A child created by .BR fork (2) inherits its parent's nice value. The nice value is preserved across .BR execve (2). The degree to which their relative nice value affects the scheduling of processes varies across UNIX systems, and, on Linux, across kernel versions. Starting with kernel 2.6.23, Linux adopted an algorithm that causes relative differences in nice values to have a much stronger effect. This causes very low nice values (+19) to truly provide little CPU to a process whenever there is any other higher priority load on the system, and makes high nice values (\-20) deliver most of the CPU to applications that require it (e.g., some audio applications). The details on the condition for .B EPERM depend on the system. The above description is what POSIX.1-2001 says, and seems to be followed on all System\ V-like systems. Linux kernels before 2.6.12 required the real or effective user ID of the caller to match the real user of the process \fIwho\fP (instead of its effective user ID). Linux 2.6.12 and later require the effective user ID of the caller to match the real or effective user ID of the process \fIwho\fP. All BSD-like systems (SunOS 4.1.3, Ultrix 4.2, 4.3BSD, FreeBSD 4.3, OpenBSD-2.5, ...) behave in the same manner as Linux 2.6.12 and later. .LP The range of the nice value varies across kernel versions. Linux before 1.3.36 had \-infinity..15. Since kernel 1.3.43, Linux has the range \-20..19. On some other systems, the range of nice values is \-20..20. Including .I is not required these days, but increases portability. (Indeed, .I defines the .I rusage structure with fields of type .I struct timeval defined in .IR .) .\" .SS C library/kernel differences Within the kernel, nice values are actually represented using the range 40..1 (since negative numbers are error codes) and these are the values employed by the .BR setpriority () and .BR getpriority () system calls. The glibc wrapper functions for these system calls handle the translations between the user-land and kernel representations of the nice value according to the formula .IR "unice\ =\ 20\ \-\ knice" . (Thus, the kernel's 40..1 range corresponds to the range \-20..19 as seen by user space.) .SH BUGS According to POSIX, the nice value is a per-process setting. However, under the current Linux/NPTL implementation of POSIX threads, the nice value is a per-thread attribute: different threads in the same process can have different nice values. Portable applications should avoid relying on the Linux behavior, which may be made standards conformant in the future. .SH SEE ALSO .BR nice (1), .BR renice (1), .BR fork (2), .BR capabilities (7), .BR sched (7) .I Documentation/scheduler/sched-nice-design.txt in the Linux kernel source tree (since Linux 2.6.23)