[APUE] NO BLOCK IO

a.c

/* [APUE] P356 Polling Example */
/* ./a.out < /var/log/daily.out 2>stderr.out */
/* Environments OSX */
/* gcc -v */
/* Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/usr/include/c++/4.2.1 */
/* Apple LLVM version 8.0.0 (clang-800.0.42.1) */
/* Target: x86_64-apple-darwin15.6.0 */
/* Thread model: posix */
/* InstalledDir: /Library/Developer/CommandLineTools/usr/bin */

#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/ioctl.h>

void
clr_fl(int fd, int flags)
{
    int val;

    if ((val = fcntl(fd, F_GETFL, 0)) == -1)
    {
      //syslog(LOG_ERR, __FILE__, __LINE__,"fcntl() error : %s", strerror(errno));
        exit(1);
    }
    val &= ~flags;

    if (fcntl(fd, F_SETFL, val) == -1)
    {
      //syslog(LOG_ERR, __FILE__, __LINE__,"fcntl() error : %s", strerror(errno));
        exit(1);
    }
    return;
}

void
set_fl(int fd, int flags)
{
    int        val;

    if ( (val = fcntl(fd, F_GETFL, 0)) < 0)
        {
            printf("fcntl F_GETFL error");
            exit(1);
        }

    val |= flags;       

    if (fcntl(fd, F_SETFL, val) < 0)
        {
            printf("fcntl F_SETFL error");
            exit(1);
        }
}

char buf[50000];
int
main(void){
  int ntowrite, nwrite;
  char *ptr;
  ntowrite = read(STDIN_FILENO, buf, sizeof(buf));
  fprintf(stderr, "read %d bytes \n", ntowrite);

  set_fl(STDOUT_FILENO, O_NONBLOCK);

  ptr = buf;
  while (ntowrite > 0){
    errno = 0;
    nwrite = write(STDOUT_FILENO, ptr, ntowrite);
    fprintf(stderr, "nwrite = %d, errno = %d\n", nwrite, errno);

    if (nwrite > 0){
      ptr += nwrite;
      ntowrite -= nwrite;
    }
  }
  clr_fl(STDOUT_FILENO, O_NONBLOCK);
  exit(0);
}