vsprintf and vsnprintf [-Wformat-nonliteral] warning on Clang 5.0

Question

i have this chunk of code ``` static void err_doit(int errnoflag, int level, const char *fmt, va_list ap) { int errno_save; unsigned long n; char buf[MAXLINE]; errno_save = errno; #ifdef HAVE_VSNPRINTF vsnprintf(buf, sizeof(buf), fmt, ap); /* this is safe */ #else vsprintf(buf ,fmt, ap); /* this is not safe */ #endif n = strlen(buf); if (errnoflag) snprintf(buf + n, sizeof(buf) - n, ": %s", strerror(errno_save)); strcat(buf, "
"); if (daemon_proc) { syslog(level,"%s", buf); } else { fflush(stdout); fputs(buf, stderr); fflush(stderr); } return; } ``` when i compile it (Clang 5.0.0 with -Weverything) i obtain those warnings: ``` Building C object lib/netutils/CMakeFiles/netutils.dir/error.c.o /Users/User/Desktop/project.cmake/lib/netutils/error.c:98:16: warning: format string is not a string literal [-Wformat-nonliteral] vsprintf(buf ,fmt, ap); /* this is not safe */ ^~~ /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk/usr/include/secur…

Accepted Answer

Apparently the solution is to tell Clang that your `vsnprintf` is called within a function that implements the behaviour of the `printf` family of functions by, well, calling `vsnprintf`. You do this with an attribute, as described here:

```
__attribute__((__format__ (__printf__, 3, 0)))
static void err_doit(int errnoflag, int level, const char *fmt, va_list ap)
{
    ...
}
```

This checks whether the format string is a literal on the calling function. As `err_doit` takes already a `va_list`, you should specify the format on the functions that call it, too. The second number, which is 0 here, should then be the argument index of the variadic argument, `...`. See also this discussion.

Function attributes are a non-standard extension of gcc, which is also implemented for Clang. If you want to keep the code compiler independent, you should wrap the attributes in a macro that hides it for compilers that don't know attributes.

(Disclaimer: I couldn't check this with Clang, but it works for gcc. Edit: fixed wrong argument index in example)

vsprintf and vsnprintf [-Wformat-nonliteral] warning on Clang 5.0

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