[libgpiod][PATCH] gpiomon: add option to set line buffered output

[Martin Hundebøll <martin@xxxxxxxxxx>]

Some applications call gpiomon in a sub process, in which case glibc
defaults to block buffered output on stdout. This makes the output
arrive to the calling process only when the (4kB) buffer is filled (or
when gpiomon exists), making the information obsolete and pretty much
useless.

Support such scenarios by adding a switch to configure line buffered
output on stdout. Similar switches are available in other applications
(e.g. `rsync`s --output argument).

Signed-off-by: Martin Hundebøll <martin@xxxxxxxxxx>
---
 src/tools/gpiomon.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/src/tools/gpiomon.c b/src/tools/gpiomon.c
index c13ab38..e79b458 100644
--- a/src/tools/gpiomon.c
+++ b/src/tools/gpiomon.c
@@ -26,11 +26,12 @@ static const struct option longopts[] = {
 	{ "silent",		no_argument,		NULL,	's' },
 	{ "rising-edge",	no_argument,		NULL,	'r' },
 	{ "falling-edge",	no_argument,		NULL,	'f' },
+	{ "linebuffered",	no_argument,		NULL,	'b' },
 	{ "format",		required_argument,	NULL,	'F' },
 	{ GETOPT_NULL_LONGOPT },
 };
 
-static const char *const shortopts = "+hvln:srfF:";
+static const char *const shortopts = "+hvln:srfbF:";
 
 static void print_help(void)
 {
@@ -46,6 +47,7 @@ static void print_help(void)
 	printf("  -s, --silent:\t\tdon't print event info\n");
 	printf("  -r, --rising-edge:\tonly process rising edge events\n");
 	printf("  -f, --falling-edge:\tonly process falling edge events\n");
+	printf("  -l, --linebuffered:\tset line buffered output\n");
 	printf("  -F, --format=FMT\tspecify custom output format\n");
 	printf("\n");
 	printf("Format specifiers:\n");
@@ -242,6 +244,7 @@ int main(int argc, char **argv)
 {
 	unsigned int offsets[GPIOD_LINE_BULK_MAX_LINES], num_lines = 0, offset;
 	bool active_low = false, watch_rising = false, watch_falling = false;
+	bool linebuffered = false;
 	struct timespec timeout = { 10, 0 };
 	int optc, opti, ret, i, event_type;
 	struct mon_ctx ctx;
@@ -278,6 +281,9 @@ int main(int argc, char **argv)
 		case 'f':
 			watch_falling = true;
 			break;
+		case 'b':
+			linebuffered = true;
+			break;
 		case 'F':
 			ctx.fmt = optarg;
 			break;
@@ -298,6 +304,9 @@ int main(int argc, char **argv)
 	else
 		event_type = GPIOD_CTXLESS_EVENT_BOTH_EDGES;
 
+	if (linebuffered && setvbuf(stdout, NULL, _IOLBF, 0) != 0)
+		die("failed to set line buffering");
+
 	if (argc < 1)
 		die("gpiochip must be specified");
 
-- 
2.20.1