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I'm working on a sample program to learn how pipes and forking works. In my very basic implementation, in my child process, i closed 0 and duplicated the read end of the pipe so that file descriptor 0 is now the read end of my pipe.

From my parent process, I write out a string, and in my child process, I read the string using cin as cin essentially is my read end of the pipe and what I observe is the complete string does not print out and I can't seem to understand why!

#include <iostream>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

#define TEST_STRING "Hello, Pipe!"
int main(int argc, char const *argv[]) {
  int fd[2];
  pipe(fd);

  pid_t pid;
  if ((pid = fork()) == 0) {
    //Child
    close(0);
    close(fd[1]);
    int myCin = dup(fd[0]);
    char buf[sizeof(TEST_STRING)];

    // int x;
    // std::cin >> x;
    // std::cout << x << std::endl;
    // read(myCin, buf, sizeof(TEST_STRING));
    std::cin >> buf;

    std::cout << buf << std::endl;

  }
  else {
    //parent
    write(fd[1], TEST_STRING, sizeof(TEST_STRING));
    close(fd[1]);
    waitpid(pid, NULL, 0);
  }
  return 0;
}

Here's my strace as well:

clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7fd895adaa10) = 1904
strace: Process 1904 attached
[pid  1903] write(4, "Hello, Pipe!\0", 13) = 13
[pid  1903] close(4)                    = 0
[pid  1903] wait4(1904,  <unfinished ...>
[pid  1904] close(0)                    = 0
[pid  1904] close(4)                    = 0
[pid  1904] dup(3)                      = 0
[pid  1904] fstat(0, {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0
[pid  1904] read(0, "Hello, Pipe!\0", 4096) = 13
[pid  1904] fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 2), ...}) = 0
[pid  1904] write(1, "Hello,\n", 7)     = 7
[pid  1904] read(0, "", 4096)           = 0
[pid  1904] exit_group(0)               = ?
[pid  1904] +++ exited with 0 +++
Mohit Athwani
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    using std::cin.getline(buf, sizeof(buf)); instead of std::cin >> buf; can get you the desired result. – lordofire Feb 18 '17 at 06:17

1 Answers1

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When you read from cin that way it will discard leading whitespace and then stop at the next whitespace character. So that's why it only returned what it did. Try std:getline.

You shouldn't count on dup() choosing FD 0 for you. Use dup2() so you can specify what descriptor to use.

I also doubt changing the FD from under cin is safe. You could get buffered data from before the FD was duped.

TrentP
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