cprint.c

#include <stdint.h>
#include <unistd.h>
#include <stdio.h>
#include <errno.h>
#include <stdarg.h>
#include <stdlib.h>

int cprint(intptr_t first, ...) {
  va_list args, args2;
  va_start(args, first);

  size_t bufsiz, wrotelen, wholelen;
  int fd, strresp, ret, malloc_buf_p;
  char *buf, *fmt, a_buf[BUFSIZ];

  buf = a_buf;
  bufsiz = BUFSIZ;
  strresp = 0;
  malloc_buf_p = 0;

  if (first == 0) {
    buf = va_arg(args, char *);
    bufsiz = va_arg(args, size_t);
    fmt = va_arg(args, char *);
    strresp = 1;
  }
  else if (first < FD_SETSIZE) {
    fd = first;
    fmt = va_arg(args, char *);
  }
  else {
    fd = 1;
    fmt = (char *)first;
  }

retry:
  va_copy(args2, args);
  ret = vsnprintf(buf, bufsiz, fmt, args2);
  va_end(args2);
  if (ret <= 0 || strresp) {
    goto final;
  }
  else if (ret >= bufsiz) {
    buf = malloc(ret + 1);
    if (buf == NULL)
      return -errno;
    bufsiz = ret + 1;
    malloc_buf_p = 1;
    goto retry;
  }

  wholelen = ret;
  wrotelen = 0;
  while (wrotelen < wholelen) {
    ret = write(fd, buf + wrotelen, wholelen - wrotelen);
    if (ret <= 0) return -errno;
    wrotelen += ret;
  }
  ret = wholelen;

final:
  if (malloc_buf_p)
    free(buf);
  va_end(args);
  return ret;
}

cprint.h

#include <stdint.h>

int cprint(intptr_t first, ...);
/*
cprint(const char *fmt, ...); (1)
cprint(0, char *buf, size_t bufsiz, const char *fmt, ...); (2)
cprint(intptr_t fd, const char *fmt, ...) (3)

(1) same as printf
(2) except the first argument must be 0, same as snprintf
(3) The first argument must be a writable fd below FD_SETSIZE, like fprintf, but write to the fd
*/