util: Add xasprintf portable asprintf variant

Include a portable asprintf variant that works on any C99
conforming platform.

Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

util.c

@@ -46,6 +46,36 @@ char *xstrdup(const char *s)
 	return d;
 }
 
+/* based in part from (3) vsnprintf */
+int xasprintf(char **strp, const char *fmt, ...)
+{
+	int n, size = 128;	/* start with 128 bytes */
+	char *p;
+	va_list ap;
+
+	/* initial pointer is NULL making the fist realloc to be malloc */
+	p = NULL;
+	while (1) {
+		p = xrealloc(p, size);
+
+		/* Try to print in the allocated space. */
+		va_start(ap, fmt);
+		n = vsnprintf(p, size, fmt, ap);
+		va_end(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);
+}
+
 char *join_path(const char *path, const char *name)
 {
 	int lenp = strlen(path);

util.h

@@ -59,6 +59,7 @@ static inline void *xrealloc(void *p, size_t len)
 }
 
 extern char *xstrdup(const char *s);
+extern int xasprintf(char **strp, const char *fmt, ...);
 extern char *join_path(const char *path, const char *name);
 
 /**