[PATCH 14 of 18] tools/libvchan: fix build errors caused by Werror in io.c

[Olaf Hering <olaf@aepfle.de>]

# HG changeset patch
# User Olaf Hering <olaf@aepfle.de>
# Date 1333397572 -7200
# Node ID 3d15aa971865cf18dac45cb70b53b5380604cc8d
# Parent  1a0b5c53a3da2a47a98ec093b296616c4b22d810
tools/libvchan: fix build errors caused by Werror in io.c

-O2 -Wall -Werror triggers these warnings:

io.c: In function 'do_send':
io.c:196: warning: ignoring return value of 'writev', declared with attribute warn_unused_result
io.c: In function 'do_recv':
io.c:287: warning: ignoring return value of 'writev', declared with attribute warn_unused_result

writev to -1 will always fail, silence the warning.

v2:
 - add comment about writev use case (see comment about strace in the code)

Signed-off-by: Olaf Hering <olaf@aepfle.de>

diff -r 1a0b5c53a3da -r 3d15aa971865 tools/libvchan/io.c
--- a/tools/libvchan/io.c
+++ b/tools/libvchan/io.c
@@ -193,7 +193,8 @@ static int do_send(struct libxenvchan *c
 		iov[0].iov_len = snprintf(metainfo, 32, "vchan@%p wr", ctrl);
 		iov[1].iov_base = (void *)data;
 		iov[1].iov_len = size;
-		writev(-1, iov, 2);
+		/* Silence gcc warning, will always fail */
+		if (writev(-1, iov, 2));
 	}
 	if (avail_contig > size)
 		avail_contig = size;
@@ -284,7 +285,8 @@ static int do_recv(struct libxenvchan *c
 		iov[0].iov_len = snprintf(metainfo, 32, "vchan@%p rd", ctrl);
 		iov[1].iov_base = data;
 		iov[1].iov_len = size;
-		writev(-1, iov, 2);
+		/* Silence gcc warning, will always fail */
+		if (writev(-1, iov, 2));
 	}
 	if (send_notify(ctrl, VCHAN_NOTIFY_READ))
 		return -1;