-Werror=format: how can the compiler know
c, c++, compiler-construction, libc
Solution
As far as I can see, there's no way the compiler can tell that the code is wrong, because the format string isn't parsed until runtime.
As long as the format string is a string literal, it can be parsed at compile time. If it isn't (which is usually a bad idea anyway), then you can get a warning about that from `-Wformat-security`.
does the compiler have a special feature that kicks in for printf and similar libc functions?
Yes.
or is this a feature I could use for my own functions?
Yes, as long as you're using the same style of format string as `printf` (or various other standard functions like `scanf` or `strftime`).
void my_printf(Something, char const * format, SomethingElse, ...)
__attribute__ ((format (printf,2,4)));
to indicate that the second argument is a `printf`-style format string, and the values to format begin with the fourth. See http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html.
Problem
I wrote this intentionally wrong code ``` printf("%d %d", 1); ``` compiling with `g++` and `-Werror=format`. The compiler gives this very impressive warning: ``` error: format '%d' expects a matching 'int' argument [-Werror=format] ``` As far as I can see, there's no way the compiler can tell that the code is wrong, because the format string isn't parsed until runtime. My question: does the compiler have a special feature that kicks in for printf and similar libc functions, or is this a feature I could use for my own functions? String literals?