pthread_attr_setinheritsched函数的具体作用详解——基于POSIX线程优先级调度的调试疑问

Let's break down exactly what that line does, why you're seeing those confusing behaviors, and how to fix them.

What does pthread_attr_setinheritsched(&tattr1, PTHREAD_EXPLICIT_SCHED); do? #

This function controls how a new thread inherits scheduling attributes from its parent (the thread that creates it):

• PTHREAD_EXPLICIT_SCHED: Tells the pthread library to use the explicit scheduling settings you've configured in the attribute object (in your case, SCHED_RR policy and the priority you set with param1.sched_priority). It ignores the parent thread's scheduling properties entirely.
• The default behavior (if you don't call this function) is PTHREAD_INHERIT_SCHED, where the new thread copies all scheduling attributes from the creating thread (your main thread, in this case).

Why your two test cases behave the way they do #

1. Deleting the line: Threads run but ignore priorities #

When you omit this line, the default PTHREAD_INHERIT_SCHED takes effect. Your main thread is using the default scheduling policy: SCHED_OTHER (the standard time-sharing policy for non-real-time processes).

SCHED_OTHER doesn't support explicit priority values—any priority settings you tried to configure in tattr1/tattr2 get ignored. All threads inherit the main thread's SCHED_OTHER policy, so the OS schedules them in a round-robin fashion without respecting your priority rules.

2. Keeping the line: Threads fail to run entirely #

This is almost certainly a permission issue, combined with a possible invalid priority value:

• Real-time scheduling policies like SCHED_RR require elevated privileges. On Linux, your process needs either:
  • Root access (run with sudo), or
  • The CAP_SYS_NICE capability assigned to the executable (via sudo setcap cap_sys_nice+ep ./your_program).
• If you don't have these permissions, pthread_create will silently fail (return an error code like EPERM), but your code doesn't check the return value of pthread_create—so it looks like the threads never start.
• Additionally, your priority value for tattr2 is 0, which is below the minimum valid priority for SCHED_RR (on Linux, the range is 1 to 99). Setting an invalid priority will also cause pthread_create to fail.

Fixes to get your priority scheduling working #

1. Check for pthread_create errors (always do this!):

   #include <string.h>
   #include <stdlib.h>
   #include <stdio.h>
   
   // ...
   
   int ret;
   ret = pthread_create(&tid_1, &tattr1, taskOne, NULL);
   if (ret != 0) {
       fprintf(stderr, "Failed to create taskOne: %s\n", strerror(ret));
       exit(EXIT_FAILURE);
   }
   // Repeat this check for all pthread_create calls
   

   This will tell you exactly why threads aren't starting (permission denied, invalid priority, etc.).

2. Fix priority values to be within the valid range:
   Use these calls to get the valid priority bounds for SCHED_RR on your system:

   int min_prio = sched_get_priority_min(SCHED_RR);
   int max_prio = sched_get_priority_max(SCHED_RR);
   printf("SCHED_RR priority range: %d to %d\n", min_prio, max_prio);
   

   Adjust your priorities to fit this range (e.g., set tattr1 to min_prio and tattr2 to max_prio if you want task3/task4 to run first).

3. Run with sufficient privileges:
   Either execute your program with sudo, or assign the CAP_SYS_NICE capability to avoid running as root every time.

4. Double-check your priority assignment:
   Wait a second—you said you want task3 and task4 to run before task1 and task2, but your code assigns task1/task2 to tattr1 (priority 10) and task3/task4 to tattr2 (priority 0). That's backwards! You need to set tattr2 to a higher priority than tattr1 to get the desired behavior.

内容的提问来源于stack exchange，提问作者Typhaon