#include <sys/resource.h>
int getpriority(int which, id_t who);
int setpriority(int which, id_t who, int
value);
The getpriority() function shall obtain the nice value of a process, process group, or user. The setpriority() function shall set the nice value of a process, process group, or user to value+ {NZERO}.
Target processes are specified by the values of the which and who arguments. The which argument may be one of the following values: PRIO_PROCESS, PRIO_PGRP, or PRIO_USER, indicating that the who argument is to be interpreted as a process ID, a process group ID, or an effective user ID, respectively. A 0 value for the who argument specifies the current process, process group, or user.
The nice value set with setpriority() shall be applied to the process. If the process is multi-threaded, the nice value shall affect all system scope threads in the process.
If more than one process is specified, getpriority() shall return value {NZERO} less than the lowest nice value pertaining to any of the specified processes, and setpriority() shall set the nice values of all of the specified processes to value+ {NZERO}.
The default nice value is {NZERO}; lower nice values shall cause more favorable scheduling. While the range of valid nice values is [0,{NZERO}*2-1], implementations may enforce more restrictive limits. If value+ {NZERO} is less than the system's lowest supported nice value, setpriority() shall set the nice value to the lowest supported value; if value+ {NZERO} is greater than the system's highest supported nice value, setpriority() shall set the nice value to the highest supported value.
Only a process with appropriate privileges can lower its nice value.
Any processes or threads using SCHED_FIFO or SCHED_RR shall be unaffected by a call to setpriority(). This is not considered an error. A process which subsequently reverts to SCHED_OTHER need not have its priority affected by such a setpriority() call.
The effect of changing the nice value may vary depending on the process-scheduling algorithm in effect.
Since getpriority() can return the value -1 on successful completion, it is necessary to set errno to 0 prior to a call to getpriority(). If getpriority() returns the value -1, then errno can be checked to see if an error occurred or if the value is a legitimate nice value.
Upon successful completion, getpriority() shall return an integer in the range -{NZERO} to {NZERO}-1. Otherwise, -1 shall be returned and errno set to indicate the error.
Upon successful completion, setpriority() shall return 0; otherwise, -1 shall be returned and errno set to indicate the error.
The getpriority() and setpriority() functions shall fail if:
In addition, setpriority() may fail if:
The following sections are informative.
The following example returns the current scheduling priority for the process ID returned by the call to getpid().
#include <sys/resource.h> ... int which = PRIO_PROCESS; id_t pid; int ret; pid = getpid(); ret = getpriority(which, pid);
The following example sets the priority for the current process ID to -20.
#include <sys/resource.h> ... int which = PRIO_PROCESS; id_t pid; int priority = -20; int ret; pid = getpid(); ret = setpriority(which, pid, priority);
The getpriority() and setpriority() functions work with an offset nice value (nice value -{NZERO}). The nice value is in the range [0,2*{NZERO} -1], while the return value for getpriority() and the third parameter for setpriority() are in the range [-{NZERO},{NZERO} -1].
nice() , sched_get_priority_max() , sched_setscheduler() , the Base Definitions volume of IEEE Std 1003.1-2001, <sys/resource.h>