#include <sched.h> int sched_setscheduler(pid_t pid, int policy, const struct sched_param *param);
The sched_setscheduler() function sets the scheduling policy and scheduling parameters of the process specified by pid to policy and the parameters specified in the sched_param structure pointed to by param, respectively. The value of the sched_priority member in the sched_param structure is any integer within the inclusive priority range for the scheduling policy specified by policy. The sched_setscheduler () function ignores the other members of the sched_param structure. If the value of pid is negative, the behavior of the sched_setscheduler() function is unspecified.
The possible values for the policy parameter are defined in the header <sched.h> (see sched.h(3HEAD)):
If a process specified by pid exists and if the calling process has permission, the scheduling policy and scheduling parameters are set for the process whose process ID is equal to pid. The real or effective user ID of the calling process must match the real or saved (from exec(2)) user ID of the target process unless the effective user ID of the calling process is 0. See Intro(2).
If pid is 0, the scheduling policy and scheduling parameters are set for the calling process.
To change the policy of any process to either of the real time policies SCHED_FIFO or SCHED_RR, the calling process must either have the SCHED_FIFO or SCHED_RR policy or have an effective user ID of 0.
The sched_setscheduler() function is considered successful if it succeeds in setting the scheduling policy and scheduling parameters of the process specified by pid to the values specified by policy and the structure pointed to by param, respectively.
Upon successful completion, the function returns the former scheduling policy of the specified process. If the sched_setscheduler() function fails to complete successfully, the policy and scheduling paramenters remain unchanged, and the function returns −1 and sets errno to indicate the error.
The sched_setscheduler() function will fail if:
The value of policy is invalid, or one or more of the parameters contained in param is outside the valid range for the specified scheduling policy.
The requesting process does not have permission to set either or both of the scheduling parameters or the scheduling policy of the specified process.
No process can be found corresponding to that specified by pid.
See attributes(5) for descriptions of the following attributes: