[PATCH 1/2] strbuf: add xstrdup_fmt helper

[Jeff King <peff@peff.net>]

You can use a strbuf to build up a string from parts, and
then detach it. In the general case, you might use multiple
strbuf_add* functions to do the building. However, in many
cases, a single strbuf_addf is sufficient, and we end up
with:

  struct strbuf buf = STRBUF_INIT;
  ...
  strbuf_addf(&buf, fmt, some, args);
  str = strbuf_detach(&buf, NULL);

We can make this much more readable (and avoid introducing
an extra variable, which can clutter the code) by
introducing a convenience function:

  str = xstrdup_fmt(fmt, some, args);

Signed-off-by: Jeff King <peff@peff.net>
---
I'm open to suggestions on the name. This really is the same thing
conceptually as the GNU asprintf(), but the interface is different (that
function takes a pointer-to-pointer as an out-parameter, and returns the
number of characters return).

 strbuf.c | 19 +++++++++++++++++++
 strbuf.h |  9 +++++++++
 2 files changed, 28 insertions(+)

diff --git a/strbuf.c b/strbuf.c
index ac62982..6674d74 100644
--- a/strbuf.c
+++ b/strbuf.c
@@ -600,3 +600,22 @@ char *xstrdup_tolower(const char *string)
 	result[i] = '\0';
 	return result;
 }
+
+char *xstrdup_vfmt(const char *fmt, va_list ap)
+{
+	struct strbuf buf = STRBUF_INIT;
+	strbuf_vaddf(&buf, fmt, ap);
+	return strbuf_detach(&buf, NULL);
+}
+
+char *xstrdup_fmt(const char *fmt, ...)
+{
+	va_list ap;
+	char *ret;
+
+	va_start(ap, fmt);
+	ret = xstrdup_vfmt(fmt, ap);
+	va_end(ap);
+
+	return ret;
+}
diff --git a/strbuf.h b/strbuf.h
index e9ad03e..61818f9 100644
--- a/strbuf.h
+++ b/strbuf.h
@@ -187,4 +187,13 @@ extern int fprintf_ln(FILE *fp, const char *fmt, ...);
 
 char *xstrdup_tolower(const char *);
 
+/*
+ * Create a newly allocated string using printf format. You can do this easily
+ * with a strbuf, but this provides a shortcut to save a few lines.
+ */
+__attribute__((format (printf, 1, 0)))
+char *xstrdup_vfmt(const char *fmt, va_list ap);
+__attribute__((format (printf, 1, 2)))
+char *xstrdup_fmt(const char *fmt, ...);
+
 #endif /* STRBUF_H */
-- 
2.0.0.566.gfe3e6b2