util: Add xa{v}sprintf_append functions

Add variadic and va_list functions, xa{v}sprintf, which appends a
formatted string to an existing string and re-allocate the string buffer
if necessary. xasprintf becomes just a special case of xasprintf_append
with a NULL starting string.

Rather than looping to get a big enough buffer, simply the implementation
by assuming we have a C99 compliant vsnprintf implementation to return the
necessary size. A side effect is glibc 2.0 support is dropped which seems
unnecessary.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

util.c

@@ -46,36 +46,54 @@ char *xstrdup(const char *s)
 	return d;
 }
 
-/* based in part from (3) vsnprintf */
-int xasprintf(char **strp, const char *fmt, ...)
+int xavsprintf_append(char **strp, const char *fmt, va_list ap)
 {
-	int n, size = 128;	/* start with 128 bytes */
+	int n, size = 0;	/* start with 128 bytes */
 	char *p;
-	va_list ap;
+	va_list ap_copy;
 
-	/* initial pointer is NULL making the fist realloc to be malloc */
-	p = NULL;
-	while (1) {
-		p = xrealloc(p, size);
+	p = *strp;
+	if (p)
+		size = strlen(p);
 
-		/* Try to print in the allocated space. */
-		va_start(ap, fmt);
-		n = vsnprintf(p, size, fmt, ap);
-		va_end(ap);
+	va_copy(ap_copy, ap);
+	n = vsnprintf(NULL, 0, fmt, ap_copy) + 1;
+	va_end(ap_copy);
+
+	p = xrealloc(p, size + n);
+
+	n = vsnprintf(p + size, n, fmt, ap);
 
-		/* If that worked, return the string. */
-		if (n > -1 && n < size)
-			break;
-		/* Else try again with more space. */
-		if (n > -1)	/* glibc 2.1 */
-			size = n + 1; /* precisely what is needed */
-		else		/* glibc 2.0 */
-			size *= 2; /* twice the old size */
-	}
 	*strp = p;
 	return strlen(p);
 }
 
+int xasprintf_append(char **strp, const char *fmt, ...)
+{
+	int n;
+	va_list ap;
+
+	va_start(ap, fmt);
+	n = xavsprintf_append(strp, fmt, ap);
+	va_end(ap);
+
+	return n;
+}
+
+int xasprintf(char **strp, const char *fmt, ...)
+{
+	int n;
+	va_list ap;
+
+	*strp = NULL;
+
+	va_start(ap, fmt);
+	n = xavsprintf_append(strp, fmt, ap);
+	va_end(ap);
+
+	return n;
+}
+
 char *join_path(const char *path, const char *name)
 {
 	int lenp = strlen(path);

util.h

@@ -72,6 +72,8 @@ static inline void *xrealloc(void *p, size_t len)
 extern char *xstrdup(const char *s);
 
 extern int PRINTF(2, 3) xasprintf(char **strp, const char *fmt, ...);
+extern int PRINTF(2, 3) xasprintf_append(char **strp, const char *fmt, ...);
+extern int xavsprintf_append(char **strp, const char *fmt, va_list ap);
 extern char *join_path(const char *path, const char *name);
 
 /**