Formating C string with va_list - c

Question

Is there a va_list equivalent of snprintf which takes a va_list of variable arguments? I'm trying to implement two functions:
char * __HYP format_cstring(const char * format, ...);
chat * __HYP format_cstringv(const char * format, var_list args);
But I'm not sure how to apply snprintf to this situation. Something like this (notice the question marks):
char * __HYP format_cstring(const char * format, ...)
{
int size = snprintf(NULL, 0, format, ??);
char * buffer = (char *)malloc(size * sizeof(char));
if (snprintf(buffer, size, format, ??) < 0) {
free(buffer);
return NULL;
}
return buffer;
}
And what about its format_cstringv counterpart?

Answer 1

Here's how I ended up doing it:
// .h
char * sformat(const char * format, ...) __attribute__((format (printf, 1, 2)));
char * vsformat(const char * format, va_list args) __attribute__((format (printf, 1, 0)));
And the implementation:
char * __HYP sformat(const char * format, ...)
{
char * buffer;
va_list args;
va_start(args, format);
buffer = __HYP vsformat(format, args);
va_end(args);
return buffer;
}
char * __HYP vsformat(const char * format, va_list args)
{
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wformat-nonliteral"
int size = vsnprintf(NULL, 0, format, args);
if (size <= 0) {
return NULL;
}
char * buffer = new char[size + 1];
if (buffer == NULL) {
return NULL;
}
if (vsnprintf(buffer, static_cast<size_t>(size), format, args) <= 0) {
free(buffer);
return NULL;
}
#pragma GCC diagnostic pop
return buffer;
}
I've been finding out how forgetful I am about C++, after a few years without touching it.

Answer 2

You are interested in the %r format that exists since ~1980 on UNOS and on DECUS.
See: http://austingroupbugs.net/view.php?id=800
There is a 30+ years old implementation in libschily, see: `http://sourceforge.net/projects/schilytools/files/ and a not-yet published implementation as an enhancement to libc on Solaris.
%r permits any printf() based function to be called by a wrapper function that then can offer the full printf() feature set.
%r takes two arguments:
1) a format string
2) a va_list type parameter
It needs an enhancement to the vararg macros called: va_arg_list() similar to and for the same reason why Sun/Solaris had to introduce va_copy() in the early 1990s in order to implement %n$.
BTW: There have been discussions about %r in the usenet around 1985 but the people that have been discussing the feature at that time believed it was not portable. My code has been tested on all existing CPU types and I am interested to have this added to the ISO C standard.
Here is some example code:
int
error(const char *fmt, ...)
{
va_list args;
int ret;
va_start(args, fmt);
ret = js_fprintf(stderr, "%r", fmt, args);
va_end(args);
return (ret);
}