chrt: support passing `-p 0` to operate on self

Specifying a PID of 0 for the -p option of chrt would previously result
in a "number 0... not in range" error. Now, it means instead that the
calling process (i.e. chrt itself) should be operated on; this is to be
consistent with the behavior of util-linux's version of chrt.

function                                             old     new   delta
chrt_main                                            462     474     +12

Signed-off-by: Zuo An <zuoan.penguin@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 10 insertions, 3 deletions

diff --git a/util-linux/chrt.c b/util-linux/chrt.c
index 51d08584e..f64fa6aa6 100644
--- a/util-linux/chrt.c
+++ b/util-linux/chrt.c
@@ -17,9 +17,9 @@
 //kbuild:lib-$(CONFIG_CHRT) += chrt.o
 
 //usage:#define chrt_trivial_usage
-//usage:       "-m | -p [PRIO] PID | [-rfobi] PRIO PROG ARGS"
+//usage:       "-m | [-rfobi] { -p [PRIO] PID | PRIO PROG ARGS }"
 //usage:#define chrt_full_usage "\n\n"
-//usage:       "Change scheduling priority and class for a process\n"
+//usage:       "Change scheduling priority and class (default RR) for a process\n"
 //usage:     "\n        -m      Show min/max priorities"
 //usage:     "\n        -p      Operate on PID"
 //usage:     "\n        -r      Set SCHED_RR class"
@@ -133,7 +133,14 @@ int chrt_main(int argc UNUSED_PARAM, char **argv)
                         pid_str = *argv;
                 }
                 /* else "-p PID", and *argv == NULL */
-                pid = xatoul_range(pid_str, 1, ((unsigned)(pid_t)ULONG_MAX) >> 1);
+                pid = xatoul_range(pid_str, 0, ((unsigned)(pid_t)ULONG_MAX) >> 1);
+
+                /* sched_{get,set}scheduler accept PID 0 to mean the calling process,
+                 * but this is needed to display the actual PID like util-linux's chrt
+                 */
+                if (pid == 0) {
+                        pid = getpid();
+                }
         } else {
                 priority = *argv++;
                 if (!*argv)