getpriority, setpriority - get and set the nice value
#include <sys/resource.h> int getpriority(int which, id_t who);
int setpriority(int which, id_t who, int value);
The getpriority() function obtains the nice value of a process, thread, or set of processes. The setpriority() function sets the nice value of a process, thread, or set of processes to value+NZERO, where NZERO is defined to be 20.
Target entities are specified by the values of the which and who arguments. The which argument can be one of the following values: PRIO_PROCESS, PRIO_PGRP, PRIO_USER, PRIO_GROUP, PRIO_SESSION, PRIO_LWP, PRIO_TASK, PRIO_PROJECT, PRIO_ZONE, or PRIO_CONTRACT, indicating that the who argument is to be interpreted as a process ID, a process group ID, an effective user ID, an effective group ID, a session ID, a thread (lwp) ID, a task ID, a project ID, a zone ID, or a process contract ID, respectively. A 0 value for the who argument specifies the current process, process group, or user. A 0 value for the who argument is treated as valid group ID, session ID, thread (lwp) ID, task ID, project ID, zone ID, or process contract ID. A P_MYID value for the who argument can be used to specify the current group, session, thread, task, project, zone, or process contract, respectively.
If a specified process is multi-threaded, the nice value set with setpriority() affects all threads in the process.
If more than one process is specified, getpriority() returns NZERO less than the lowest nice value pertaining to any of the specified entities, and setpriority() sets the nice values of all of the specified processes to value+ NZERO.
The default nice value is NZERO. Lower nice values cause more favorable scheduling. The range of valid nice values is 0 to NZERO*2-1. If value+NZERO is less than the system's lowest supported nice value, setpriority () sets the nice value to the lowest supported value. If value+NZERO is greater than the system's highest supported nice value, setpriority() sets the nice value to the highest supported value.
Only a process with appropriate privileges can lower the nice value.
Any process or thread using SCHED_FIFO or SCHED_RR is unaffected by a call to setpriority(). This is not considered an error. A process or thread that subsequently reverts to SCHED_OTHER will not have its priority affected by such a setpriority() call.
The effect of changing the nice value varies depending on the scheduling policy 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() returns an integer in the range from -NZERO to NZERO-1. Otherwise, −1 is returned and errno is set to indicate the error.
Upon successful completion, setpriority() returns 0. Otherwise, −1 is returned and errno is set to indicate the error.
The getpriority() and setpriority() functions will fail if:
No process or thread could be located using the which and who argument values specified.
The value of the which argument was not recognized, or the value of the who argument is not a valid process ID, process group ID, user ID, group ID, session ID, thread (lwp) ID, task ID, project ID, or zone ID.
In addition, setpriority() may fail if:
A process was located, but neither the real nor effective user ID of the executing process match the effective user ID of the process whose nice value is being changed.
A request was made to change the nice value to a lower numeric value and the current process does not have appropriate privileges.
The following example returns the current scheduling priority for the process ID returned by the call to getpid(2).
#include <sys/resource.h> ... int which = PRIO_PROCESS; id_t pid; int ret; pid = getpid(); ret = getpriority(which, pid);Example 2 Example using setpriority()
The following example sets the nice value for the current process to 0.
#include <sys/resource.h> ... int which = PRIO_PROCESS; id_t pid; int value = -20; int ret; pid = getpid(); ret = setpriority(which, pid, value);
The getpriority() and setpriority() functions work with an offset nice value (value-NZERO). The nice value is in the range 0 to 2*NZERO-1, while the return value for getpriority() and the third parameter for setpriority() are in the range -NZERO to NZERO-1.
See attributes(5) for descriptions of the following attributes: