lib: allow memparse() to accept a NULL and ignorable second parm

Extend memparse() to allow the caller to use a NULL second parameter, which
would represent no interest in returning the address of the end of the parsed
string.

In numerous cases, callers invoke memparse() to parse a possibly-suffixed
string (such as "64K" or "2G" or whatever) and define a character pointer to
accept the end pointer being returned by memparse() even though they have no
interest in it and promptly throw it away.

This (backward-compatible) enhancement allows callers to use NULL in the cases
where they just don't care about getting back that end pointer.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: NRobert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

lib/cmdline.c

@@ -116,7 +116,7 @@ char *get_options(const char *str, int nints, int *ints)
 /**
  *	memparse - parse a string with mem suffixes into a number
  *	@ptr: Where parse begins
- *	@retptr: (output) Pointer to next char after parse completes
+ *	@retptr: (output) Optional pointer to next char after parse completes
  *
  *	Parses a string into a number.  The number stored at @ptr is
  *	potentially suffixed with %K (for kilobytes, or 1024 bytes),
@@ -126,11 +126,13 @@ char *get_options(const char *str, int nints, int *ints)
  *	megabyte, or one gigabyte, respectively.
  */
 
-unsigned long long memparse (char *ptr, char **retptr)
+unsigned long long memparse(char *ptr, char **retptr)
 {
-	unsigned long long ret = simple_strtoull (ptr, retptr, 0);
+	char *endptr;	/* local pointer to end of parsed string */
 
-	switch (**retptr) {
+	unsigned long long ret = simple_strtoull(ptr, &endptr, 0);
+
+	switch (*endptr) {
 	case 'G':
 	case 'g':
 		ret <<= 10;
@@ -140,10 +142,14 @@ unsigned long long memparse (char *ptr, char **retptr)
 	case 'K':
 	case 'k':
 		ret <<= 10;
-		(*retptr)++;
+		endptr++;
 	default:
 		break;
 	}
+
+	if (retptr)
+		*retptr = endptr;
+
 	return ret;
 }