#include <sched.h> int sched_setscheduler(pid_t pid, int policy,
const struct sched_param *param); int sched_getscheduler(pid_t pid);
The scheduling parameters are specified in the param argument, which is a pointer to a structure of the following form:
struct sched_param {
...
int sched_priority;
...
};
In the current implementation, the structure contains only one field, sched_priority. The interpretation of param depends on the selected policy.
Currently, Linux supports the following "normal" (i.e., non-real-time) scheduling policies as values that may be specified in policy:
For each of the above policies, param->sched_priority must be 0.
Various "real-time" policies are also supported, for special time-critical applications that need precise control over the way in which runnable threads are selected for execution. For the rules governing when a process may use these policies, see sched(7). The real-time policies that may be specified in policy are:
For each of the above policies, param->sched_priority specifies a scheduling priority for the thread. This is a number in the range returned by calling sched_get_priority_min(2) and sched_get_priority_max(2) with the specified policy. On Linux, these system calls return, respectively, 1 and 99.
Since Linux 2.6.32, the SCHED_RESET_ON_FORK flag can be ORed in policy when calling sched_setscheduler(). As a result of including this flag, children created by fork(2) do not inherit privileged scheduling policies. See sched(7) for details.
sched_getscheduler() returns the current scheduling policy of the thread identified by pid. If pid equals zero, the policy of the calling thread will be retrieved.
POSIX systems on which sched_setscheduler() and sched_getscheduler() are available define _POSIX_PRIORITY_SCHEDULING in <unistd.h>.
POSIX.1 does not detail the permissions that an unprivileged thread requires in order to call sched_setscheduler(), and details vary across systems. For example, the Solaris 7 manual page says that the real or effective user ID of the caller must match the real user ID or the save set-user-ID of the target.
The scheduling policy and parameters are in fact per-thread attributes on Linux. The value returned from a call to gettid(2) can be passed in the argument pid. Specifying pid as 0 will operate on the attributes of the calling thread, and passing the value returned from a call to getpid(2) will operate on the attributes of the main thread of the thread group. (If you are using the POSIX threads API, then use pthread_setschedparam(3), pthread_getschedparam(3), and pthread_setschedprio(3), instead of the sched_*(2) system calls.)