All values duplicate in a process are duplicated across a fork, which brings up some interesting situations, like what happens when the process has open file descriptors. For example:

int fd = open( .... );
cpid = fork();
if( cpid == 0){
/*CHILD*/

//reading from same file as parent?
read(fd, ....)

}else if (cpid > 0){
/*PARENT*/

//reading from same file as child?
read(fd, ....)

}

First, consider that the reference to an open file, a file descriptor, is just an integer number (fd above) that is used by the operating system to look up the open file in the file descriptor table. The entry in the table contains a number of information, including what point in the file is currently being referenced. For example, if you read 10 bytes from a file descriptor, and then some point in later in the program read 10 bytes again, you do not reread the same 10 bytes, instead you read the next 10 bytes. This is accomplished via the data stored in the file descriptor table which tracks the current place in the file.

Returning to the example above, the value of fd should be duplicated from parent to child. The question is, how does this affect the data stored in the file descriptor table. Technically, the value of fd, e.g., a number like 3, is the same for parent and child, and should then reference the same entry in the file descriptor table, and it does. Interestingly, it not only references the same entry in the table, but that there is nothing wrong with two differences processes reading from the same file, which will stay in sync.

You can see this in the program below where a parent and child process alternate between reading 1 byte at a time from a file.

int main(int argc, char * argv[]){

  int fd, status;
  pid_t cpid;
  char c;

  if ( argc < 2){
    fprintf(stderr, "ERROR: Require path\n");
    return 1;
  }

  //shared between all children and parent
  if( (fd = open(argv[1], O_RDONLY)) < 0){
    perror("open");
    return 1;
  }

  while (1){

    cpid = fork();

    if( cpid == 0 ){
      /* CHILD */

      //try and read 1 byte from file
      if( read(fd, &c, 1) > 0){
        printf("c: %c\n", c); // print the char

        _exit(0); //exit with status 0 on sucess read
      }else{
        //no more to read
        _exit(1); //exit with status 1 on failed read
      }

    }else if ( cpid > 0){
      /* PARENT */

      //wait for child to read first
      wait(&status);

      //if exit status 1, break the loop, no more to read
      if( WEXITSTATUS(status) ){
        break; 
      }

      //now parent reads a byte
      if( read(fd, &c, 1) > 0){
        printf("p: %c\n", c); // print the char
      }

    }else{
      /* ERROR */
      perror("fork");
      return 1;
    }
  }

  //done reading the file
  close(fd);

  return 0;
}

Prior to entering the loop, a file is open, and after each fork, the file descriptor is duplicated to the child, which tries to read a byte and print it, returning either success or failure if there is no more bytes to read. Meanwhile, the parent is waiting for the child terminate, checks the status, and if there is more of the file to read, the parent then reads and prints a byte. The result is that the program alternates between reading 1 byte at a time from a file between parent and a sequence of children. Here's the output of running the program, "c:" is a print from a child and "p:" is a print from the parent.

#> cat helloworld.txt 
Hello World!
#> ./shared_files helloworld.txt 
c: H
p: e
c: l
p: l
c: o
p:  
c: W
p: o
c: r
p: l
c: d
p: !
c:

The pipe() system call is used to create a set of connected file descriptors, one for reading and one for writing. Whatever data is written to the write end of the pipe can be read from the read end of the pipe. Here's the generally setup:

int pfd[2]; //pfd[0] reading end of pipe
            //pfd[1] writing end of pipe


//open the pipe
if( pipe(pfd) < 0){
  perror("pipe");
  return 1;
}

Now pfd and array of two integers for file descriptors is set such that pfd[0] is the reading end (like 0 for stdin) and pfd[1] is the writing end (like 1 for stdout). We can now use the two file descriptors to transfer data. For example, here is the hello-world program for pipes:

int main(int argc, char * argv[]){

  //print hello world through a pipe!
  char hello[] = "Hello World!\n";
  char c;

  int pfd[2]; //pfd[0] reading end of pipe
              //pfd[1] writing end of pipe


  //open the pipe
  if( pipe(pfd) < 0){
    perror("pipe");
    return 1;
  }

  //write hello world to pipe
  write(pfd[1], hello, strlen(hello));

  //close write end of pipe
  close(pfd[1]);

  //read hello world from pipe, write to stdout
  while( read(pfd[0], &c, 1)){
    write(1, &c, 1);
  }

  //close the read end of the pipe
  close(pfd[0]);

  return 0;
}

The program writes "Hello World!\n" to the write end of the pipe, and then reads it back from read end of the pipe, writing the result to stdout.

There are a number of very reasonable questions you should be asking at this point:

• Where does the data go that's written to the pipe? It must be stored somewhere because we read it back later.
• How much data can we write to the pipe before we have to read it? Computers are finite machines, at some point, the pipe must burst!

To answer the first question, where does the data go, we can refer back to our discussions of I/O buffering. We know that the O.S. and the C standard library provides some amount of buffering on reads and writes. From the perspective of the program, it sees a pipe file descriptor like any other file descriptor, but instead of being hooked into a file in the file system, it actually points to a buffer, a storage space, in the kernel. Reading and writing from the pipe is just a matter of adding data to the buffer and remove data from the buffer.

To answer the second question, how much data, we can write a program and find out. Below, here is a program that opens a pipe, and writes 'A' to the pipe in an loop, which will break when the write fails. It also maintains a count of how many times 'A' was written.

int main(int argc, char * argv[]){

  char c = 'A';
  int i;
  int pfd[2]; //pfd[0] reading end of pipe
              //pdf[1] writing end of pipe

  //open the pipe
  if( pipe(pfd) < 0){
    perror("pipe");
    return 1;
  }

  //write A's to pipe until it's full
  i = 0;
  while( write(pfd[1], &c, 1) > 0){
    printf("%d\n",i);
    i++;
  }
  perror("write");

  //close write end of pipe
  close(pfd[1]);

  //read from pipe?!?
  while( read(pfd[0], &c, 1)){
    write(1, &c, 1);
  }

  close(pfd[0]);

  return 0;
}

Before we give the output, let's consider some possibilities. Two come to mind.

1. First, we'll write as many 'A's as possible counting and printing all along, and then the write() will fail, -1 is returned. The result is the count and that many A's. Essentially, once the pipe is full, a write fails.
2. Another possibilities is that once the pipe is full, and you try and do that last write, it doesn't fail, it just makes the program wait until the pipe is no longer full, that is, another program has read from it. Essentially, once the pipe is full, the write blocks.

Two possibilities, what happens? We don't need to squint, we can run the program, and here's the output.

#>./pipe_block
1
2
(...)
65534
65535
^C

It's number 2: when the pipe is full, the write will block. We can see this because the program printed all the numbers and the it reached a max, hung, and was terminated with Ctrl-c. The number 65535 is also meaningful, it is 2¹⁶ -1, which is as good a choice for the pipe size.

However, it could have been number 1, a failed write. The Operating System allows you to change the way read/write call function depending on the file descriptor. The way to do this is using the fcntl() or file descriptor control. This system call allows you to set a number of options (or flags) about the interaction with a file descriptor. One such flag is an option to set the file descriptor to be non-blocking:

//set pipe to not block
fcntl(pfd[1], F_SETFL, O_NONBLOCK);

You can read more about other flags in the man pages man 2 fcntl, but the above call just sets the write end of the pipe to be non-blocking. If we rerun the program with that setting:

#>./pipe_nonblock
1
2
(...)
65534
65535
write: Resource temporarily unavailable
AAAAAAAAAAAAAAAAAAAAAAA (...)

The write() will fail with a resource unavailable error, the pipe is full, and then the rest of the program can proceed, printing an absurd number of 'A's.

We now have a pretty good understanding of a pipe, so let's see how to use them to do inter-process communication. Like before, we are going to leverage the fact that after a fork, the process is fully duplicated, which would include the pipe. With the pipe file descriptors in the parent and child, it becomes fairly intuitive to read and write data between them. Here's a really simple program that will send data from the parent to the child across a pipe, and then the child will write what it read to stdout.

int main(int argc, char * argv[]){


  //print hello world through a pipe! To child!
  char hello[] = "Hello World!\n";
  char c;

  int pfd[2]; //pfd[0] reading end of pipe
              //pfd[1] writing end of pipe

  pid_t cpid;
  int status;

  //open a pipe, pfd[0] for reading, pfd[1] for writing
  if ( pipe(pfd) < 0){
    perror("pipe");
    return 1;
  }

  cpid = fork();

  if( cpid == 0 ){
    /* CHILD */

    //close the writing end in child
    close(pfd[1]);

    //try and read 1 byte from pipe, write byte stdout
    while( read(pfd[0], &c, 1) > 0){
      write(1, &c,1); 
    }

    //close pipe
    close(pfd[0]);

    _exit(0);    
  }else if ( cpid > 0){
    /* PARENT */

    //close reading end in parent
    close(pfd[0]);

    //write hello world to pipe
    write(pfd[1], hello, strlen(hello));

    //close the pipe
    close(pfd[1]);

    //wait for child
    wait(&status);

  }else{
    /* ERROR */
    perror("fork");
    return 1;
  }

  return 0;
}

The program is just like the hello_pipe program, but this time, the parent writes "Hello World\n" to the pipe, but it's the child that reads it out and writes it to standard output. While this is a silly example, it is easy to start to see how pipe's can be sued to communicate across process.

The system call to duplicate file descriptors is dup(), but we will use the slightly easier to manage dup2() version of the system call. Here is the prototype for =dup2():

int dup2(int filedes, int filedes2);

It will duplicate the file descriptor fildes onto the file descriptor filedes2; essentially, if you were to read and write from fildes2 now, it would be the same as reading and writing from filedes. The two file descriptors do not have the same value, one might be 4 and other 7, but their entries in the file descriptor table has been duplicated.

To see this in action, let's look at a hello world program of file duplication:

int main(int argc, char * argv[]){
  //print hello world to a file with dup
  int fd;

  //check args
  if ( argc < 2){
    fprintf(stderr, "ERROR: Require destination path\n");
    return 1;
  }

  //open destination file
  if( (fd = open(argv[1], O_WRONLY | O_TRUNC | O_CREAT , 0644)) < 0){
    perror("open");
    return 1;
  }

  //close standard out
  close(1);

  //duplicate fd to stdout
  dup2(fd, 1);

  //print to stdout, which is now duplicated to fd
  printf("Hello World!\n");

  return 0;
}

In this program, we open a new file at the file descriptor fd and we wish to make that file descriptor the same as standard out. The first step is to close standard out, and the duplicate the new fd to standard out. Now, whenever we print to standard out it will actually print to the file descriptor. Which is exactly what happens when we run the program:

#> ./hello_dup hello
#> cat hello
Hello World!

You might notice, that this is the same as standard output redirection in the shell using the > symbol, and, in fact, this is how the shell implements that redirection.

Finally, we have all the information we need to set up a basic pipeline between two process that uses the stdin/stdout mechanisms. The procedure is straightforward, we need a pipe and the read and write ends are duplicated to standard output and input of the processes that are communicating.

Let's consider doing this for the simple program which will set up a pipeline like so: parent | child. Here is such a program:

   int main(int argc, char * argv[]){

  int status;
  int pfd[2];
  pid_t cpid;
  char c;

  //open a pipe, pfd[0] for reading, pfd[1] for writing
  if ( pipe(pfd) < 0){
    perror("pipe");
    return 1;
  }

  //Setup a pipe between child 1 and child 2, like:
  // parent | child

  cpid = fork();

  if( cpid == 0 ){
    /* CHILD 1*/

    //close stdin
    close(0);

    //duplicate reading end to stdin
    dup2(pfd[0], 0);

    //close the writing end
    close(pfd[1]);

    //try and read 1 byte from stding and write stdout
    while( read(0, &c, 1) > 0){ //stdin now pipe!
      write(1, &c,1); //this is still stdout
    }

    exit(0);
  } else if ( cpid > 0){
    /* PARENT */

    //close stdout
    close(1);

    //duplicate writing end to stdout
    dup2(pfd[1], 1);

    //close reading end 
    close(pfd[0]);


    //read and read 1 byte from stdin, write byte to pipe
    while( read(0,&c,1) > 0){
      write(1, &c, 1);
    }

    //close the pipe and stdout
    close(pfd[1]);
    close(1);

    //wait for child
    wait(&status);

  }else{
    /* ERROR */
    perror("fork");
    return 1;
  }

  return 0;
}

First, a pipe is created before the fork. In the parent, standard out and the read end of the pipe are closed, and then the write end of the pipe is duplicated onto standard out. Whenever the parent writes to standard out, it is actually writing to the pipe. In the child, it similarly closes the write end of the pipe and standard in, and then duplicates the read end of the pipe to standard input. Now, whenver the child reads from standard input it is actually reading from the pipe, which the parent will write to.

It's a pipeline! Not a very useful pipeline. The parent reads its data from standard input, sends it to the child through the pipe, which writes the data to standard output. It's a glorified cat program that requires two process, but a pipeline none the less. We can see that it is doing that by running it in the command line:

#> ./pipe_dup
hello
hello
^Z
[1]+  Stopped                 ./pipe_dup
#> ps -o pid,pgid,comm
  PID  PGID COMM
  418   418 bash
 4772  4772 emacs
 5188  5188 ./pipe_dup
 5189  5188 ./pipe_dup

The "hello" typed into the terminal was read by the parent, piped to the child, and then printed back to the terminal. If we stop the program and run ps, we can see that there are two processes running. It works! Now, the next step is implementing more complicated pipelines, which is a task for another lesson.