So I'm trying to override the fscanf function in c, but I only want different behavior to occur if certain conditions are met; if those conditions are not met, I want to just call the original fscanf. I know that you can use dlsym to use the original version of a function while interpositioning it, but the problem with fscanf is that it's variadic. I can get all of the parameters passed into my function using va_list and va_arg, but how am I supposed to call the original fscanf on these parameters when I don't actually know how many parameters there are?
You can't call the original fscanf from the interposed fscanf. You just have to call vfscanf(). Your interposed function would look like:
int fscanf(FILE *stream, const char *format, ...)
{
....
....
va_list ap;
va_start(ap, format);
int rc = vfscanf(stream, format, ap);
va_end(ap);
return rc;
}