src/stdio/vsnprintf.c

   1 #include "stdio_impl.h"
   2 #include <limits.h>
   3 #include <string.h>
   4 #include <errno.h>
   5 #include <stdint.h>
   6
   7 struct cookie {
   8         char *s;
   9         size_t n;
  10 };
  11
  12 #define MIN(a, b) ((a) < (b) ? (a) : (b))
  13
  14 static size_t sn_write(FILE *f, const unsigned char *s, size_t l)
  15 {
  16         struct cookie *c = f->cookie;
  17         size_t k = MIN(c->n, f->wpos - f->wbase);
  18         if (k) {
  19                 memcpy(c->s, f->wbase, k);
  20                 c->s += k;
  21                 c->n -= k;
  22         }
  23         k = MIN(c->n, l);
  24         if (k) {
  25                 memcpy(c->s, s, k);
  26                 c->s += k;
  27                 c->n -= k;
  28         }
  29         *c->s = 0;
  30         f->wpos = f->wbase = f->buf;
  31         /* pretend to succeed, even if we discarded extra data */
  32         return l;
  33 }
  34
  35 int vsnprintf(char *restrict s, size_t n, const char *restrict fmt, va_list ap)
  36 {
  37         unsigned char buf[1];
  38         char dummy[1];
  39         struct cookie c = { .s = n ? s : dummy, .n = n ? n-1 : 0 };
  40         FILE f = {
  41                 .lbf = EOF,
  42                 .write = sn_write,
  43                 .lock = -1,
  44                 .buf = buf,
  45                 .cookie = &c,
  46         };
  47
  48         if (n > INT_MAX) {
  49                 errno = EOVERFLOW;
  50                 return -1;
  51         }
  52
  53         *c.s = 0;
  54         return vfprintf(&f, fmt, ap);
  55 }