[PATCH v3 1/3] gdb/nat: Use procfs(5) indexes in linux_common_core_of_thread

[Thiago Jung Bauermann <thiago.bauermann@linaro.org>]

The code and comment reference stat fields by made-up indexes.  The
procfs(5) man page, which describes the /proc/PID/stat file, has a numbered
list of these fields so it's more convenient to use those numbers instead.

This is currently an implementation detail inside the function so it's
not really relevant with the code as-is, but a future patch will do some
refactoring which will make the index more prominent.

Therefore, make this change in a separate patch so that it's simpler to
review.

Reviewed-By: Luis Machado <luis.machado@arm.com>
---
 gdb/nat/linux-osdata.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

Changes in v3:
- Use macro names in comment, and clarify why we use those fields
  (suggested by Pedro).

Changes in v2:
- Added macros for field indexes in /proc/PID/stat (Suggested by Luis).

diff --git a/gdb/nat/linux-osdata.c b/gdb/nat/linux-osdata.c
index 6ffabe90aa7d..4812bc735e86 100644
--- a/gdb/nat/linux-osdata.c
+++ b/gdb/nat/linux-osdata.c
@@ -52,6 +52,10 @@ typedef long long TIME_T;
 
 #define MAX_PID_T_STRLEN  (sizeof ("-9223372036854775808") - 1)
 
+/* Index of fields of interest in /proc/PID/stat, from procfs(5) man page.  */
+#define LINUX_PROC_STAT_STATE 3
+#define LINUX_PROC_STAT_PROCESSOR 39
+
 /* Returns the CPU core that thread PTID is currently running on.  */
 
 /* Compute and return the processor core of a given thread.  */
@@ -74,10 +78,9 @@ linux_common_core_of_thread (ptid_t ptid)
   if (pos == std::string::npos)
     return -1;
 
-  /* If the first field after program name has index 0, then core number is
-     the field with index 36 (so, the 37th).  There's no constant for that
-     anywhere.  */
-  for (int i = 0; i < 37; ++i)
+  /* The first field after program name is LINUX_PROC_STAT_STATE, and we are
+     interested in field LINUX_PROC_STAT_PROCESSOR.  */
+  for (int i = LINUX_PROC_STAT_STATE; i <= LINUX_PROC_STAT_PROCESSOR; ++i)
     {
       /* Find separator.  */
       pos = content->find_first_of (' ', pos);