fake_outputs: Use %n format specifier instead of sprintf

fake_outputs_init used a sprintf invocation with a throw-away buffer
to estimate how many characters the sscanf invocation consumed. This
was unnecessary, and also potentially incorrect, as differences
between the read and formatted strings (such as leading zeros) could
lead to fake_outputs_init to lose its track.

Instead, use the %n format specifier which allows saving the number of
characters consumed by sscanf so far. %n is part of C99.

diff --git a/src/fake_outputs.c b/src/fake_outputs.c
index 6639b3611005b3a393a1c493558724ff0122490c..447409d2a024503f6f83cd6d494070349b66d88a 100644 (file)
--- a/src/fake_outputs.c
+++ b/src/fake_outputs.c
@@ -33,10 +33,10 @@ static Output *get_screen_at(unsigned int x, unsigned int y) {
  *
  */
 void fake_outputs_init(const char *output_spec) {
-    char useless_buffer[1024];
     const char *walk = output_spec;
     unsigned int x, y, width, height;
-    while (sscanf(walk, "%ux%u+%u+%u", &width, &height, &x, &y) == 4) {
+    int chars_consumed;
+    while (sscanf(walk, "%ux%u+%u+%u%n", &width, &height, &x, &y, &chars_consumed) == 4) {
         DLOG("Parsed output as width = %u, height = %u at (%u, %u)\n",
              width, height, x, y);
         Output *new_output = get_screen_at(x, y);
@@ -68,8 +68,7 @@ void fake_outputs_init(const char *output_spec) {
             num_screens++;
         }
 
-        /* Figure out how long the input was to skip it */
-        walk += sprintf(useless_buffer, "%ux%u+%u+%u", width, height, x, y) + 1;
+        walk += chars_consumed + 1;
     }
 
     if (num_screens == 0) {