source: azure_iot_hub/trunk/musl-1.1.18/src/stdio/vsnprintf.c@ 388

#include "stdio_impl.h"
#include <limits.h>
#include <string.h>
#include <errno.h>
#include <stdint.h>

struct cookie {
        char *s;
        size_t n;
};

#define MIN(a, b) ((a) < (b) ? (a) : (b))

static size_t sn_write(FILE *f, const unsigned char *s, size_t l)
{
        struct cookie *c = f->cookie;
        size_t k = MIN(c->n, f->wpos - f->wbase);
        if (k) {
                memcpy(c->s, f->wbase, k);
                c->s += k;
                c->n -= k;
        }
        k = MIN(c->n, l);
        if (k) {
                memcpy(c->s, s, k);
                c->s += k;
                c->n -= k;
        }
        *c->s = 0;
        f->wpos = f->wbase = f->buf;
        /* pretend to succeed, even if we discarded extra data */
        return l;
}

int vsnprintf(char *restrict s, size_t n, const char *restrict fmt, va_list ap)
{
        unsigned char buf[1];
        char dummy[1];
        struct cookie c = { .s = n ? s : dummy, .n = n ? n-1 : 0 };
        FILE f = {
                .lbf = EOF,
                .write = sn_write,
                .lock = -1,
                .buf = buf,
                .cookie = &c,
        };

        if (n > INT_MAX) {
                errno = EOVERFLOW;
                return -1;
        }

        *c.s = 0;
        return vfprintf(&f, fmt, ap);
}

#ifdef __c2__
int _vsnprintf(char *restrict s, size_t n, const char *restrict fmt, va_list ap)
{
        return vsnprintf(s, n, fmt, ap);
}
#endif