Custom priority value: is a pthread high sched prio positive or negative?

I’m trying to reimplement in a backward-compatible way the RobotC API from C (although some details would better fit or would be easier with C++), and trying to reimplement their multithreading API in some portable way, so I try to use sched_get_priority_max(2) and sched_get_priority_min(2). But on one hand nice(1) and sched(7) say priority is between -20 (highest priority) and 19 (lower priority), on another hand, the former man pages stated that: > […] numerically higher priority values are scheduled before > processes with numerically lower priority values. Thus, the value > returned by sched_get_priority_max() will be greater than the value > returned by sched_get_priority_min() which would mean the opposite: positive value being of higher priority and negative of lower priority (it also gives the example of real-time policies where priority is between 1 and 99). What is going to be returned by sched_get_priority_max and _min? what should I use with pthread_attr_setschedparam(3)? I have three values defining my custom priority range: the low priority (kLowPriority, set to 0), the high one (kHighPriority, set to 255) and the default one (kDefaultPriority, set to 7). Ideally I suppose the default being 0 for the scheduling policies having a default, kDefaultPriority/7 should become 0, kHighPriority/255 the highest priority (19? 99? whatever sched_get_priority_max(3) returns) or maybe the highest priority that can be assigned if unpriviledged? and then kLowPriority either 0 ± kDefaultPriority, either the lowest priority …what would be the more reasonable? Currently I think I do it this way: pthread_attr_t attr; pthread_t thread; struct sched_param param; const int policy = sched_getscheduler(0), sched_high_prio = sched_get_priority_max(policy), // 19, sched_low_prio = sched_get_priority_min(policy), // -20, sched_range_prio = sched_high_prio - sched_low_prio; pthread_attr_init (&attr); pthread_attr_getinheritsched(&attr, PTHREAD_INHERIT_SCHED); pthread_attr_getschedparam (&attr, ¶m); param.sched_priority = -(((nTaskPriority - kLowPriority) * sched_range_prio / kHighPriority) + sched_low_prio - kDefaultTaskPriority); PS: I’m not sure if the best place to ask this question about POSIX API is here or stackoverflow , so I’m doing both, please delete my post or ask me to delete if you think it’s at the wrong place. EDIT: SO post deleted.