Introduce die_errno() that appends strerror(errno) to die()

There are many calls to die() that do, or should, report
strerror(errno) to indicate how the syscall they guard failed.
Introduce a small helper function for this case.

Note:

- POSIX says vsnprintf can modify errno in some unlikely cases, so we
  have to use errno early.

- We take some care to pass the original format to die_routine(), in
  case someone wants to call die_errno() with custom format
  characters.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/usage.c b/usage.c
index 820d09f..fd936a1 100644
--- a/usage.c
+++ b/usage.c
@@ -60,6 +60,18 @@ void die(const char *err, ...)
 	va_end(params);
 }
 
+void die_errno(const char *fmt, ...)
+{
+	va_list params;
+	char fmt_with_err[1024];
+
+	snprintf(fmt_with_err, sizeof(fmt_with_err), "%s: %s", fmt, strerror(errno));
+
+	va_start(params, fmt);
+	die_routine(fmt_with_err, params);
+	va_end(params);
+}
+
 int error(const char *err, ...)
 {
 	va_list params;