Question

I’d like to write a wrapper function for the mvwprint/mvwchgat ncurses functions which prints the message in the specified window and then changes its attributes.

However, mvwchgat needs to know how many characters it should change – and I have no idea how to tell mvwchgat how long the formatted string is, since a strlen() on, for instance, "abc%d" obviously returns 5, because strlen doesn’t know what %d stands for …

Answer 1

In C99 or C11, you can use a line like this:

length = snprintf(NULL, 0, format_string, args);

From the manual of snprintf (emphasis mine):

The functions snprintf() and vsnprintf() do not write more than size bytes (including the terminating null byte (‘\0’)). If the output was truncated due to this limit then the return value is the number of characters (excluding the terminating null byte) which would have been written to the final string if enough space had been available. Thus, a return value of size or more means that the output was truncated.

Since we are giving snprintf 0 as the size, then the output is always truncated and the output of snprintf would be the number of characters that would have been written, which is basically the length of the string.

In C89, you don’t have snprintf. A workaround is to create a temporary file, or if you are in *nix open /dev/null and write something like this:

FILE *throw_away = fopen("/dev/null", "w"); /* On windows should be "NUL" but I haven't tested */
if (throw_away)
{
    fprintf(throw_away, "<format goes here>%n", <args go here>, &length);
    fclose(throw_away);
} /* else, try opening a temporary file */