schedgetprioritymax

sched_get_priority_max is a function in the POSIX real-time scheduling interface used to query the maximum allowed scheduling priority for a given scheduling policy. It helps applications determine the upper bound of priorities that can be assigned when manipulating process or thread scheduling.

The function prototype is int sched_get_priority_max(int policy). It returns the maximum priority value for the specified policy, or -1 on error. If -1 is returned, errno is typically set to EINVAL when the policy is invalid or not supported on the executing system.

On many Linux systems, the common real-time policies have a maximum priority of 99. For example, SCHED_FIFO and SCHED_RR often report 99 as their upper limit. For SCHED_OTHER, which corresponds to the normal time-sharing class, the concept of a fixed priority is not applicable in the same way, and the function may return 0. Values for other policies (such as newer or platform-specific ones) are implementation-defined and can vary by kernel and library versions.

Return value guidance: if sched_get_priority_max returns a non-negative integer, that value represents the highest priority you can assign for that policy. It is commonly used in conjunction with sched_get_priority_min to determine the valid range, and with sched_setscheduler or pthread_setschedparam to validate or clamp requested priorities.

Portability: The function is part of POSIX and is supported on systems implementing real-time scheduling, including most Unix-like systems. Availability and exact numeric ranges may differ across platforms, so consulting the local man page (sched_get_priority_max) is recommended.