Avoid warning in wrapper around printf

Question

I have an error reporting functionality in my little C library I'm writing. I want to provide an `errorf` function in addition to the plain `error` function to allow embedding information in error messages easily. ``` /* * Prints a formatted error message. Use it as you would use 'printf'. See the * 'sio_error' function. */ void sio_errorf(const char *format, ...) { // Print the error prefix if (g_input == STDIN) fputs("error: ", stderr); else fprintf(stderr, "%s: ", g_progname); // Pass on the varargs on to 'vfprintf'. va_list arglist; va_start(arglist, format); // This may produce the following warning -- ignore it: // warning: format string is not a string literal vfprintf(stderr, format, arglist); va_end(arglist); fputc('
', stderr); } ``` The problem is, I get this warning (compiling with clang 4.0 with the `-Weverything` switch): warning: format string is not a string literal I understand why doing this would be bad. Is there any way I can get rid of this warning? Can I somehow…

Accepted Answer

If you're using GCC or one of its relatives, try an attribute on the declaration:

```
void sio_errorf(const char *format, ...) __attribute__((format(printf, 1, 2)));
```

To add the attribute to a definition, you can use this:

```
__attribute__((format(printf, 1, 2)))
    static void sio_errorf(const char *format, ...) {
      ....
```

Avoid warning in wrapper around printf

Solution

Problem