add skip_prefix_mem helper

peff · gitster · commit ae989a61dad9 · 2016-06-23T11:32:51.000-07:00
The skip_prefix function has been very useful for
simplifying pointer arithmetic and avoiding repeated magic
numbers, but we have no equivalent for length-limited
buffers. So we're stuck with:

  if (3 &lt;= len &amp;&amp; skip_prefix(buf, "foo", &amp;buf))
	  len -= 3;

That's not that complicated, but it needs to use magic
numbers for the length of the prefix (or else write out
strlen("foo"), repeating the string). By using a helper, we
can get the string length behind the scenes (and often at
compile time for string literals).

Signed-off-by: Jeff King &lt;peff@peff.net&gt;
Signed-off-by: Junio C Hamano &lt;gitster@pobox.com&gt;
diff --git a/git-compat-util.h b/git-compat-util.h
@@ -463,6 +463,23 @@ static inline int skip_prefix(const char *str, const char *prefix,
 	return 0;
 }
 
+/*
+ * Like skip_prefix, but promises never to read past "len" bytes of the input
+ * buffer, and returns the remaining number of bytes in "out" via "outlen".
+ */
+static inline int skip_prefix_mem(const char *buf, size_t len,
+				  const char *prefix,
+				  const char **out, size_t *outlen)
+{
+	size_t prefix_len = strlen(prefix);
+	if (prefix_len <= len && !memcmp(buf, prefix, prefix_len)) {
+		*out = buf + prefix_len;
+		*outlen = len - prefix_len;
+		return 1;
+	}
+	return 0;
+}
+
 /*
  * If buf ends with suffix, return 1 and subtract the length of the suffix
  * from *len. Otherwise, return 0 and leave *len untouched.
