linux_programming/linux_programming_interface/solutions/chapter_5/exercise_5_6.c

/*
After each of the calls to write() in the following code, explain what the content of
the output file would be, and why:
fd1 = open(file, O_RDWR | O_CREAT | O_TRUNC, S_IRUSR | S_IWUSR);
fd2 = dup(fd1);
fd3 = open(file, O_RDWR);
write(fd1, "Hello,", 6);
write(fd2, "world", 6);
lseek(fd2, 0, SEEK_SET);
write(fd1, "HELLO,", 6);
write(fd3, "Gidday", 6);
*/

#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <assert.h>
#include <stdio.h>

int
main (int argc, char *argv[])
{
  if (argc != 2) return EXIT_FAILURE;
  int fd1, fd2, fd3;
  fd1 = open (argv[1], O_RDWR | O_CREAT | O_TRUNC, S_IRUSR | S_IWUSR);
  fd2 = dup (fd1);
  fd3 = open (argv[1], O_RDWR);
  write (fd1, "Hello,", 6);
  // Hello,
  write (fd2, "world", 6);
  // Hello,world?
  printf ("lseek %lld\n", lseek (fd2, 0, SEEK_CUR));
  lseek (fd2, 0, SEEK_SET);
  write (fd1, "HELLO,", 6);
  // HELLO,world?
  assert (lseek (fd1, 0, SEEK_CUR) != lseek (fd3, 0, SEEK_CUR));
  // lseek for dup and opened fd1 == fd2, where fd3 is still 0
  write (fd3, "Gidday", 6);
  // Giddayworld?

  return EXIT_SUCCESS;
}