osmux: fix buffer management mess in snprintf() calls

SNPRINTF_BUFFER_SIZE() looks too complex, previous version maintains two
different variables to account for the remaining space in the buffer,
one of them is always decremented based on what snprintf() returns,
which may result in underflow. These variables are swapped - not used
consistently - all over this code.

Replace this macro by a simplified version, with one single parameter to
account for remaining space. This macro also deals with two corner
cases:

1) snprintf() fails, actually never happens in practise, but
   documentation indicates it may return -1, so let's catch this case
   from here to stick to specs.

2) There is not enough space in the buffer, in that case, keep
   increasing offset, so we know how much would have been printed, just
   like snprintf() does.

Thanks to Pau Espin for reporting, and Holger for clues on this.
I have run osmux_test and, at quick glance, it looks good.

Change-Id: I5b5d6ec57a02f57c23b1ae86dbd894bad28ea797

1 files changed, 4 insertions, 4 deletions

diff --git a/tests/osmux/osmux_test.c b/tests/osmux/osmux_test.c
index bf6174b..09b7a9a 100644
--- a/tests/osmux/osmux_test.c
+++ b/tests/osmux/osmux_test.c
@@ -65,8 +65,8 @@ static struct timeval last;
 
 static void tx_cb(struct msgb *msg, void *data)
 {
-	char buf[4096];
 	struct rtp_hdr *rtph = (struct rtp_hdr *)msg->data;
+	char buf[4096];
 #if OSMUX_TEST_USE_TIMING
 	struct timeval now, diff;
 
@@ -102,9 +102,9 @@ static struct osmux_out_handle h_output;
 
 static void osmux_deliver(struct msgb *batch_msg, void *data)
 {
-	char buf[2048];
 	struct osmux_hdr *osmuxh;
 	LLIST_HEAD(list);
+	char buf[2048];
 
 	osmux_snprintf(buf, sizeof(buf), batch_msg);
 	fprintf(stderr, "OSMUX message (len=%d) %s\n", batch_msg->len, buf);
@@ -182,11 +182,11 @@ static void osmux_test_marker(int ccid) {
 
 static void osmux_test_loop(int ccid)
 {
+	struct rtp_hdr *rtph = (struct rtp_hdr *)rtp_pkt;
 	struct msgb *msg;
+	int i, j, k = 0;
 	char buf[1024];
-	struct rtp_hdr *rtph = (struct rtp_hdr *)rtp_pkt;
 	uint16_t seq;
-	int i, j, k = 0;
 
 	for (i = 1; i < 65; i++) {
 		msg = msgb_alloc(1500, "test");