photon/libc/stdio/vsnprintf.c

#include <stdio.h>
#include "__printf.h"

struct __vsnprintf_arg {
    char *buf;
    size_t idx;
    size_t max;
};

static void _out_buf(struct __printf_backend *bk, char ch)
{
    struct __vsnprintf_arg *arg = bk->arg;
    if (arg->idx < arg->max) {
        arg->buf[arg->idx] = ch;
    }

    arg->idx++;
}

int vsnprintf(char *buf, size_t sz, const char *restrict format, va_list arg)
{
    struct __vsnprintf_arg bk_arg = {};
    bk_arg.buf = buf;
    bk_arg.max = sz;

    struct __printf_backend bk = {};
    bk.out = _out_buf;
    bk.arg = &bk_arg;

    __xprintf(&bk, format, arg);
    if (!bk_arg.idx) {
        return 0;
    }

    return bk_arg.idx - 1;
}