ref: c2a5d7e3f28e86aeef1e4e37e2b88e46af90b62c
dir: /sys/src/libc/fmt/vsnprint.c/
#include <u.h>
#include <libc.h>
int
vsnprint(char *buf, int len, char *fmt, va_list args)
{
Fmt f;
if(len <= 0)
return -1;
f.runes = 0;
f.start = buf;
f.to = buf;
f.stop = buf + len - 1;
f.flush = nil;
f.farg = nil;
f.nfmt = 0;
f.args = args;
dofmt(&f, fmt);
*(char*)f.to = '\0';
return (char*)f.to - buf;
}