third_party/ulib/musl/src/stdio/vsnprintf.c

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

static size_t sn_write(FILE* f, const unsigned char* s, size_t l) {
    size_t k = f->wend - f->wpos;
    if (k > l)
        k = l;
    memcpy(f->wpos, s, k);
    f->wpos += k;
    /* pretend to succeed, but discard extra data */
    return l;
}

int vsnprintf(char* restrict s, size_t n, const char* restrict fmt, va_list ap) {
    int r;
    char b;
    FILE f = {.lbf = EOF, .write = sn_write, .lock = -1};

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

    /* Ensure pointers don't wrap if "infinite" n is passed in */
    if (n > SIZE_MAX - (uintptr_t)s - 1)
        n = SIZE_MAX - (uintptr_t)s - 1;
    f.buf_size = n;
    f.buf = f.wpos = (void*)s;
    f.wbase = f.wend = (void*)(s + n);
    r = vfprintf(&f, fmt, ap);

    /* Null-terminate, overwriting last char if dest buffer is full */
    if (n)
        f.wpos[-(f.wpos == f.wend)] = 0;
    return r;
}