Bug 974685

This bug also applies to the RHEL6 man-pages project.

Version in RHEL6.4 is man-pages-3.22-20.el6.noarch.

+++ This bug was initially created as a clone of Bug #974679 +++

Description of problem:
The man-pages package contains a patch to the sched_getaffinity manual page that includes an example program that incorrectly relies on EINVAL being returned by sched_getaffinity.

Version-Release number of selected component (if applicable):
man-pages-3.35-4.fc17.noarch

How reproducible:
`man 2 sched_getaffinity`

Actual results:
Man page has the example program:
~~~
EXAMPLE
          #define _GNU_SOURCE

          #include <sched.h>
          #include <stdio.h>
          #include <errno.h>

          int main(void)
          {
               cpu_set_t *mask;
               size_t size;
               int i;
               int nrcpus = 1024;

       realloc:
               mask = CPU_ALLOC(nrcpus);
               size = CPU_ALLOC_SIZE(nrcpus);
               CPU_ZERO_S(size, mask);
               if ( sched_getaffinity(0, size, mask) == -1 ) {
                       CPU_FREE(mask);
                       if (errno == EINVAL &&
                           nrcpus < (1024 << 8)) {
                              nrcpus = nrcpus << 2;
                              goto realloc;
                       }
                       perror("sched_getaffinity");
                       return -1;
               }

               for ( i = 0; i < nrcpus; i++ ) {
                       if ( CPU_ISSET_S(i, size, mask) ) {
                               printf("CPU %d is set\n", (i+1));
                       }
               }

               CPU_FREE(mask);

               return 0;
          }
~~~

Expected results:
Should have the example:
~~~
          #define _GNU_SOURCE

          #include <sched.h>
          #include <stdio.h>
          #include <errno.h>
          #include <unistd.h>

          int main(void)
          {
               cpu_set_t *mask;
               size_t size;
               int i;
               int nrcpus = sysconf(_SC_NPROCESSORS_ONLN);

               if (nrcpus == -1) {
                       perror("sysconf");
                       return -1;
               }

               mask = CPU_ALLOC(nrcpus);
               size = CPU_ALLOC_SIZE(nrcpus);
               CPU_ZERO_S(size, mask);
               if ( sched_getaffinity(0, size, mask) == -1 ) {
                       perror("sched_getaffinity");
                       return -1;
               }

               for ( i = 0; i < nrcpus; i++ ) {
                       if ( CPU_ISSET_S(i, size, mask) ) {
                               printf("CPU %d is set\n", (i+1));
                       }
               }

               CPU_FREE(mask);

               return 0;
          }
~~~

Additional info:
The only easy way to determine the online cpus is to use sysconf (_SC_PROCESSORS_ONLN);

The GNU C Library does not guarantee that sched_getaffinity will return EINVAL if the mask is not the right size. Future versions of glibc will never return EINVAL, and will always copy only the cpus that were requested by the user (which has always been the intent of the library).

If you want the affinity for all of the cpus then you must use sysconf to determine how many there might be and then request that amount.

The value returned by sysconf is constant for the lifetime of the process.

The Linux kernel online cpu mask is also constant.

Notes:

This has been fixed in F18, F19, and rawhide.

The example could go upstream, but should not use the EINVAL method to determine kernel's cpu mask size.