[PATCH] run-command.c: fix build warnings on Ubuntu

Building git on Ubuntu 9.10 warns that the return value of write(2)
isn't checked. These warnings were introduced in commits:

  2b541bf8 ("start_command: detect execvp failures early")
  a5487ddf ("start_command: report child process setup errors to the
parent's stderr")

GCC details:

  $ gcc --version
  gcc (Ubuntu 4.4.1-4ubuntu9) 4.4.1

Silence the warnings by reading (but not making use of) the return value
of write(2).

Signed-off-by: Michael Wookey <redacted>
---
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.

 run-command.c |   10 ++++++----
 1 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/run-command.c b/run-command.c
index 2feb493..3206d61 100644
--- a/run-command.c
+++ b/run-command.c

@@ -67,19 +67,21 @@ static int child_notifier = -1;

 static void notify_parent(void)
 {
-	write(child_notifier, "", 1);
+	ssize_t unused;
+	unused = write(child_notifier, "", 1);
 }

 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);

-	write(child_err, "fatal: ", 7);
-	write(child_err, msg, len);
-	write(child_err, "\n", 1);
+	unused = write(child_err, "fatal: ", 7);
+	unused = write(child_err, msg, len);
+	unused = write(child_err, "\n", 1);
 	exit(128);
 }

-- 
1.7.0.rc0.48.gdace5

Michael Wookey, 2010-01-29:

Building git on Ubuntu 9.10 warns that the return value of write(2)
isn't checked.

GCC details:

  $ gcc --version
  gcc (Ubuntu 4.4.1-4ubuntu9) 4.4.1

Silence the warnings by reading (but not making use of) the return value
of write(2).

Since a few weeks I get several warnings about fwrite(), currently 28
times this:
warning: ignoring return value of ‘fwrite’, declared with attribute warn_unused_result

gcc (Gentoo 4.3.4 p1.0, pie-10.1.5) 4.3.4

Not sure if it should be muted, that are really many places.

Markus

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 <redacted>
---
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