i3-input: Avoid compiler warning (#5480)

Also updates the function to use proper types, const and style.

The warning:

../i3/i3-input/main.c: In function ‘finish_input’:
../i3/i3-input/main.c:187:13: warning: ‘__builtin_strncat’ specified bound depends on the length of the source argument [-Wstringop-overflow=]
  187 |             strncat(dest, command, inputlen);
      |             ^
../i3/i3-input/main.c:175:20: note: length computed here
  175 |     int inputlen = strlen(command);

Which is triggered because gcc thinks it's bad that `input_len` (the
length of the source in the copy) is used instead of a length that is
inside the limits of the allocated size for the destination. However, in
practice, `full_len` is always than `input_len`.

1 files changed, 21 insertions, 18 deletions

diff --git a/i3-input/main.c b/i3-input/main.c
index ef9e0701..afe16ccc 100644
--- a/i3-input/main.c
+++ b/i3-input/main.c
@@ -163,29 +163,32 @@ static void finish_input(void) {
     char *command = (char *)concat_strings(glyphs_utf8, input_position);
 
     /* count the occurrences of %s in the string */
-    int c;
-    int len = strlen(format);
-    int cnt = 0;
-    for (c = 0; c < (len - 1); c++)
-        if (format[c] == '%' && format[c + 1] == 's')
+    const size_t len = strlen(format);
+    size_t cnt = 0;
+    for (size_t c = 0; c < (len - 1); c++) {
+        if (format[c] == '%' && format[c + 1] == 's') {
             cnt++;
-    printf("occurrences = %d\n", cnt);
+        }
+    }
+    printf("occurrences = %ld\n", cnt);
 
     /* allocate space for the output */
-    int inputlen = strlen(command);
-    char *full = scalloc(strlen(format) - (2 * cnt) /* format without all %s */
-                             + (inputlen * cnt)     /* replaced %s */
-                             + 1,                   /* trailing NUL */
-                         1);
+    const size_t input_len = strlen(command);
+    const size_t full_len = MAX(input_len,                 /* avoid compiler warning */
+                                strlen(format) - (2 * cnt) /* format without all %s */
+                                    + (input_len * cnt)    /* replaced %s */
+                                    + 1                    /* trailing NUL */
+    );
+    char *full = scalloc(full_len, 1);
     char *dest = full;
-    for (c = 0; c < len; c++) {
-        /* if this is not % or it is % but without a following 's',
-         * just copy the character */
-        if (format[c] != '%' || (c == (len - 1)) || format[c + 1] != 's')
+    for (size_t c = 0; c < len; c++) {
+        /* if this is not % or it is % but without a following 's', just copy
+         * the character */
+        if (format[c] != '%' || (c == (len - 1)) || format[c + 1] != 's') {
             *(dest++) = format[c];
-        else {
-            strncat(dest, command, inputlen);
-            dest += inputlen;
+        } else {
+            strncat(dest, command, input_len);
+            dest += input_len;
             /* skip the following 's' of '%s' */
             c++;
         }