gdb/nat: Factor linux_proc_get_stat_field out of linux_common_core_of_thread

The new function will be used in a subsequent patch to read a different
stat field.

The new code is believed to be equivalent to the old code, so there
should be no change in GDB behaviour.  The only material change was to
use std::string and string_printf rather than a fixed char array to
build the path to the stat file.

Also, take the opportunity to move the function's documentation comment to
the header file, to conform with GDB practice.

Reviewed-By: Luis Machado <luis.machado@arm.com>
Approved-By: Pedro Alves <pedro@palves.net>

gdb/nat/linux-osdata.c

@@ -36,6 +36,7 @@
 #include <sys/stat.h>
 #include "gdbsupport/filestuff.h"
 #include <algorithm>
+#include "linux-procfs.h"
 
 #define NAMELEN(dirent) strlen ((dirent)->d_name)
 
@@ -52,50 +53,18 @@ 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.  */
+/* See linux-osdata.h.  */
 
 int
 linux_common_core_of_thread (ptid_t ptid)
 {
-  char filename[sizeof ("/proc//task//stat") + 2 * MAX_PID_T_STRLEN];
+  std::optional<std::string> field
+    = linux_proc_get_stat_field (ptid, LINUX_PROC_STAT_PROCESSOR);
   int core;
 
-  sprintf (filename, "/proc/%lld/task/%lld/stat",
-	   (PID_T) ptid.pid (), (PID_T) ptid.lwp ());
-
-  std::optional<std::string> content = read_text_file_to_string (filename);
-  if (!content.has_value ())
+  if (!field.has_value () || sscanf (field->c_str (), "%d", &core) == 0)
     return -1;
 
-  /* ps command also relies on no trailing fields ever contain ')'.  */
-  std::string::size_type pos = content->find_last_of (')');
-  if (pos == std::string::npos)
-    return -1;
-
-  /* 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);
-      if (pos == std::string::npos)
-	return {};
-
-      /* Find beginning of field.  */
-      pos = content->find_first_not_of (' ', pos);
-      if (pos == std::string::npos)
-	return {};
-    }
-
-  if (sscanf (&(*content)[pos], "%d", &core) == 0)
-    core = -1;
-
   return core;
 }
 

gdb/nat/linux-osdata.h

@@ -20,7 +20,10 @@
 #ifndef NAT_LINUX_OSDATA_H
 #define NAT_LINUX_OSDATA_H
 
+/* Returns the CPU core that thread PTID is currently running on.  */
+
 extern int linux_common_core_of_thread (ptid_t ptid);
+
 extern LONGEST linux_common_xfer_osdata (const char *annex, gdb_byte *readbuf,
 					 ULONGEST offset, ULONGEST len);
 

gdb/nat/linux-procfs.c

@@ -230,6 +230,50 @@ linux_proc_pid_is_zombie (pid_t pid)
 
 /* See linux-procfs.h.  */
 
+std::optional<std::string>
+linux_proc_get_stat_field (ptid_t ptid, int field)
+{
+  /* We never need to read PID from the stat file, and there's
+     command_from_pid to read the comm field.  */
+  gdb_assert (field >= LINUX_PROC_STAT_STATE);
+
+  std::string filename = string_printf ("/proc/%ld/task/%ld/stat",
+					(long) ptid.pid (), (long) ptid.lwp ());
+
+  std::optional<std::string> content
+    = read_text_file_to_string (filename.c_str ());
+  if (!content.has_value ())
+    return {};
+
+  /* ps command also relies on no trailing fields ever containing ')'.  */
+  std::string::size_type pos = content->find_last_of (')');
+  if (pos == std::string::npos)
+    return {};
+
+  /* The first field after program name is LINUX_PROC_STAT_STATE.  */
+  for (int i = LINUX_PROC_STAT_STATE; i <= field; ++i)
+    {
+      /* Find separator.  */
+      pos = content->find_first_of (' ', pos);
+      if (pos == std::string::npos)
+	return {};
+
+      /* Find beginning of field.  */
+      pos = content->find_first_not_of (' ', pos);
+      if (pos == std::string::npos)
+	return {};
+    }
+
+  /* Find end of field.  */
+  std::string::size_type end_pos = content->find_first_of (' ', pos);
+  if (end_pos == std::string::npos)
+    return content->substr (pos);
+  else
+    return content->substr (pos, end_pos - pos);
+}
+
+/* See linux-procfs.h.  */
+
 const char *
 linux_proc_tid_get_name (ptid_t ptid)
 {

gdb/nat/linux-procfs.h

@@ -54,6 +54,16 @@ extern int linux_proc_pid_is_zombie_nowarn (pid_t pid);
 
 extern int linux_proc_pid_is_gone (pid_t pid);
 
+/* 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 FIELD (as numbered in procfs(5) man page) of
+   /proc/PID/task/LWP/stat file.  */
+
+extern std::optional<std::string> linux_proc_get_stat_field (ptid_t ptid,
+							     int field);
+
 /* Return a string giving the thread's name or NULL if the
    information is unavailable.  The returned value points to a statically
    allocated buffer.  The value therefore becomes invalid at the next