Subject: [PATCH] run-command: prettify -D_FORTIFY_SOURCE workaround

From: Jonathan Nieder <[email-masked]>
Date: Tue, 15 Mar 2011 22:51:52 -0500

Current gcc + glibc with -D_FORTIFY_SOURCE try very aggressively to
protect against a programming style which uses write(...) without
checking the return value for errors.  Even the usual hint of casting
to (void) does not suppress the warning.

Sometimes when there is an output error, especially right before exit,
there really is nothing to be done.  The obvious solution, adopted in
v1.7.0.3~20^2 (run-command.c: fix build warnings on Ubuntu,
2010-01-30), is to save the return value to a dummy variable:

	ssize_t dummy;
	dummy = write(...);

But that (1) is ugly and (2) triggers -Wunused-but-set-variable
warnings with gcc-4.6 -Wall, so we are not much better off than when
we started.

Instead, use an "if" statement with an empty body to make the intent
clear.

	if (write(...))
		; /* yes, yes, there was an error. */

Signed-off-by: Jonathan Nieder <[email-masked]>
---
Hi,

Michael Wookey wrote:

> Although this will fix the build warnings, I am unsure if there is a
> better way to achieve the same result. Using "(void)write(...)" still
> gives warnings and I am unaware of any annotations that will silence
> gcc.

It's been a long time (and meanwhile the patch has been working;
thanks!).  How about something like this?

 run-command.c |   12 ++++++------
 1 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/run-command.c b/run-command.c
index 3206d61..5b68907 100644
--- a/run-command.c
+++ b/run-command.c
@@ -67,21 +67,21 @@ static int child_notifier = -1;
 
 static void notify_parent(void)
 {
-	ssize_t unused;
-	unused = write(child_notifier, "", 1);
+	if (write(child_notifier, "", 1))
+		; /* ok. */
 }
 
 static NORETURN void die_child(const char *err, va_list params)
 {
 	char msg[4096];
-	ssize_t unused;
 	int len = vsnprintf(msg, sizeof(msg), err, params);
 	if (len > sizeof(msg))
 		len = sizeof(msg);
 
-	unused = write(child_err, "fatal: ", 7);
-	unused = write(child_err, msg, len);
-	unused = write(child_err, "\n", 1);
+	if (write(child_err, "fatal: ", 7) ||
+	    write(child_err, msg, len) ||
+	    write(child_err, "\n", 1))
+		; /* ok. */
 	exit(128);
 }
 
-- 
1.7.4.1