IC221: Systems Programming (SP18)


Home Policy Calendar Units Assignments Resources

Unit 5: Process Management

Table of Contents

1 Unix Processes

A process is an executing instance of a program. We have been discussing processes in one form or another throughout the class, but in this lesson we dive into the lifecycle of a process. How does a process get created? How is it managed? How does it die?

For starters, let's return to the definition of a process: an executing instance of a program. A program is the set of instructions for how a process operates when run, while a process is a current instance of that program as it executes. There can be multiple instances of the same program running; for example, multiple users can be logged into a computer at the same time, each running a shell, which is the same program with multiple executing instances.

A process is also an Operating System abstraction. It's a way to manage varied programs and contain them within individual units. It is via this abstraction that the O.S. can provide isolation, ensuring that one process cannot interfere with another. A process is also the core unit of the O.S. resource of Process Management which the main goal is to determine which process has access to the CPU and at what time.

In this lesson, we are going to trace the life and death of a Unix process, from the first nubile invocation to the final mortal termination. We'll begin at the end, with death, and work backwards towards birth. Throughout this lesson, we'll use the diagram below to explain these concepts. You can find a version of this diagram in APUE on page 201, Section 7.3.

process_lifecycle.png

Figure 1: C Program Life Cycle: Invocation through Termination

2 Process Exiting

Let's start with a fairly simple question: How do we make a program terminate in code? There are illogical ways to do this – dereferencing NULL and forcing a segfault – but the standard way to do this is to allow the main() function to return. What do we do if want our program to logically terminate from another point in the program, somewhere other than main()? We need a separate mechanism to do that, and the solution is an exit call, like we did in bash programming.

2.0.1 exit() and _exit() and _Exit()

There are three ways to forcefully exit a program:

  1. _exit() : System calls that request the O.S. to terminate a process immediately without any additional code execution.
  2. exit() : C Standard Library exit procedure that will cleanly terminate a process by invoking additional code as requested by the user and to manage ongoing I/O.
  3. _Exit() : C Standard Library exit procedure that will immediately terminate a process, essentially a wrapper to _exit()

To better understand these differences we can refer back to the diagram:

process_lifecycle_exit.png

Figure 2: C Program Life Cycle: exit() vs. _exit()

The flow of the green arrows refer to the system call _exit() which leads to direct termination of the process. Once a process is terminated, the kernel is invoked to choose which process should run next, which is why flow points towards the kernel. However, a call to exit() (no underscore), at any point in the program execution, starts a separate set of actions, which include running exit handlers and I/O procedures. This is indicated in red in the diagram, and eventually, once exit procedures are complete, exit() calls _exit() to do the final termination. Not pictured is _Exit(), which has the same action as _exit().

Like many of the O.S. procedures we've been discussing so far in this class, the exit procedure has both a system call version and a library call version. In general, when you program you will likely only use the library function, which is fine, but to be an effective programmer you need to understand the underlying O.S. procedures that enable the library routines.

2.0.2 I/O Buffering and Exit Procedures

One aspect of the standard library exit procedure is to handle I/O buffering that occurs within the C standard library. This depicted in the diagram like so:

process_lifecycle_IO.png

Figure 3: C Program Life Cycle: Standard I/O Cleanup

I/O bufferring in the C standard library is an attempt to limit the number of system calls, i.e., calls to write(), which are costly. A system call requires that the entire process be paused, saved, and swapped out in favor of the OS, which then performs the action, and once complete, the user process must be swapped back in to complete its operation. Just enumerating all the steps of a context switch is exhausting, and it is an expensive operation, one that should be avoided except when absolutely necessary.

Buffering is one way to avoid excessive calls. When you call printf() or another standard I/O library function that is printing to the terminal or a file, the print does not occur right away. Instead, it is buffered, or temporarily stored and waiting to be completed. A print will become unbuffered whenever the buffer is full, e.g., there is enough bytes to write and cannot store any more, or when a line completes, as defined by printing a newline symbol.

So if we were to look at the hello-world program:

int main(){
   printf("Hello World!\n");
}

The printing of "Hello World!" includes a new line symbol, and thus the buffer is flushed and "Hello world!" is printed to the terminal. However, consider this version of the hello-world program:

int main(){
   printf("Hello World!");
}

This time there is no newline, but "Hello World" is still printed to the terminal. How?

When the main() function returns, it actually returns to another function within the C startup routines, which calls exit(). Then exit() will perform a cleanup of standard I/O, flushing all the buffered writes.

However, when you call _exit(), buffers are not cleared. The process will exit immediately. You can see the difference between these two procedures in these two programs:

/*exit_IO_demo.c*/
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>

int main(){

  // I/O buffered since no newline
  printf("Will print");

  exit(0); //C standard exit

}
/*_exit_IO_demo.c*/
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>

int main(){

  // I/O buffered since no newline
  printf("Does not print"); 

  _exit(0); //immediate exit!

}

You do not need to rely on the exit procedures to clear the I/O buffers. Instead, you can directly flush the buffers with fflush() library function. For example:

int main(){

  // I/O buffered since no newline
  printf("Will print once flushed"); 
  fflush(stdout); // flushing stdout's buffer, printing

  _exit(0); //immediate exit!

}

The fflush() function takes a FILE * and will read/write all data that is currently being buffered. There is also an analogous function, fpurge(), which will delete all data in the buffers.

2.0.3 Changing I/O Buffering policy

The buffering policy at the process level varies by input/output mechanisms. For example, let's consider a program that prints to stderr instead of stdout.

/*_exit_IO_stderr.c*/
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>

int main(){

  // I/O not buffered for stderr                                                                                                                                             
  fprintf(stderr, "Will print b/c stderr is unbuffered");

  _exit(0); //immediate exit!                                                                                                                                                

}

In some ways, this is good policy. You want errors to be reported immediately, not when it is most convenient for the buffer. There is a side effect, however, writes to stderr are more expensive since it requires an immediate context switch.

We can change the policy for how an input/output stream is buffered. By default, stdout and stdin is line buffered which means that input and output is buffered until a newline is read/written. stderr is unbuffered, as described above. There is also a third choice, fully buffered, which means writes/reads occur once the buffer is full.

The library call to change the buffering policy is setvbuf(), which has the following function declaration:

int setvbuf(FILE *stream, char *buf, int mode, size_t size);

You select which input/output stream is affected, such as stderr or stdout, and you can also provide memory for the buffer with its size. The mode option can have the following choices:

  • _IONBF unbuffered : data is written immediately to the device via the system call write()
  • _IOLBF line buffered : data is written to the device using write() once a newline is found or the buffer is full
  • _IOFBF fully buffered : data is only written to the device using write() once the buffer is full

In general, you do not need to specify a new buffer, instead just want to affect the mode. For example, if we want to set stderr to be line buffered, we can alter the program from above like so, and the result would be that it would no longer print

/*_exit_IO_setvbuf.c*/
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>

int main(){

  //stderr is now Line buffered                                                                                                                                              
  setvbuf(stderr, NULL, _IOLBF, 0);

  // I/O now buffered for stderr                                                                                                                                             
  fprintf(stderr, "Will NOT print b/c stderr is now line buffered");

  _exit(0); //immediate exit!                                                                                                                                                

}

2.0.4 atexit() Exit Handlers

So far, we've seen within the context of I/O buffering, that the exit() procedure will perform actions before executing the final exit, with _exit(). What if you wanted to add an additional exit processing? You do that with exit handlers.

process_lifecycle_exit_handler.png

Figure 4: C Program Life Cycle: exit handlers

Exit handers are functions that are automatically called whenever a program is exit()'ing. You can register, in general, 32 exit handlers using the atexit() library function. The order each exit handler executes is in reverse order of their registration. Consider the below program

/*exit_handler_demo.c*/
#include <stdio.h>
#include <stdlib.h>

void my_exit1(){
  printf("FIRST Exit Handler\n");
}

void my_exit2(){
  printf("SECOND Exit Handler\n");
}

int main(){

  //exit handers execute in reverse order of registration                                                                                                                    
  atexit(my_exit1);
  atexit(my_exit2);
  //my_exit2 will run before my_exit1                                                                                                                                        

  return; //implicitly calls exit()                                                                                                                                         
}

The output of this program is:

#> ./exit_hander_demo
SECOND Exit Handler
FIRST Exit Handler

A couple of other things to note here is that the argument to atexit() is a function. This is the first time we are using a function pointer, or a reference to a function. We will do something similar later in the class when setting up signal handlers, again registering a function to be called when an event occurs.

2.0.5 Exit Statuses

The last bit of the exit() puzzle is exit status. Every program exits with some status. This status can then be checked to learn information about the execution of the process, for example, did it exit successfully or on failure or error?

You can set the exit status of a program via the various exit() calls:

_exit(int status);

exit(int status);

_Exit(int status);

Additionally, the return value of main() implicitly sets the exit status. Convention indicates the following exit status:

  • 0 : success
  • 1 : failure
  • 2 : error

We've seen this before when programming with bash scripting. Recall that an if statement in bash executes a program:

if cmd  #<--- executes command and checks exit status
then

else

fi

When cmd succeeds, i.e., returns with exit status 0, then the then block is executed. If the cmd fails or there was an error, the else block is executed. The special variable, $?, also stores the exit status of the last executed program.

An exit status actually is more than just the argument to _exit(). The kernel also prepares a termination status for a process, one of those parts is the exit status. The termination status also contains information about how the program terminated, and we will concern ourselves more with termination status when we discuss process management.

3 Process Creation and Management

3.1 The Birth of a Process

Now that we have good understanding about the death of a process, let's look at the birth of a process.

process_lifecycle_exit_exec.png

Figure 5: C Program Life Cycle: exec and startups

Forever, you have always learned that a program begins in main(), but that's not exactly true. The part of the program you wrote starts in main(), but, in fact, when you compile a program, there is a set of additional functions that will execute first to set up the environment. Once those complete, main() is called, and when main() returns, it returns to those startup routines, which eventually calls exit() … and we already know how that story ends.

There is a more fundamental questions that we are not addressing: How does a program actually load and execute as a process? This is accomplished with the exec family of system calls, in particular execv() and

3.2 Exec System Calls

The exec family of system calls simply loads a program from memory and executes it, replacing the current program of the process. Once the loading of instructions is complete, exec will start execution through the startup procedures. We'll just discuss the execv() system call, which has the following function definition. Read the manual page to find the other forms of exec, including execve() and execvp().

int execv(const char *path, char *const argv[]);

execv() takes a path to a program, as it lives within the file system, as well as the arguments to that program. It's easier to understand by looking at an example. The program below, will execute ls.

/* exec_ls.c*/
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>

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

  //arguments for ls
  char * ls_args[2] = { "/bin/ls", NULL} ;

  //execute ls
  execv( ls_args[0], ls_args);

  //only get here if exec failed                                                                                                                                             
  perror("execve failed");

  return 2; //return error status                                                                                                                                            
}

The execv() system call takes a path to the program, in this case that is "/bin/ls", as well as an arg array. For this exec, the only argument is the name of the program, ls_args[0]. You might notice that the arguments to execv() match the arguments to main() with respect to the argv array, and that's intentional. In some ways, you can think of an exec calling the main() function of the program directly with those arguments. You have to NULL the last argument so that exec can count the total arguments, to set argc.

For example, we can extend this program to take an argument for ls where it will long list the contents of the directory /bin. If we were to call ls from the shell to perform this task, we would do so like this:

ls -l /bin

We can translate that into an argv array with the following values:

//arguments for ls, will run: ls  -l /bin                                                                                                                                  
  char * ls_args[4] = { "/bin/ls", "-l", "/bin", NULL} ;
  //             ^                   ^      ^                                                                                                                                
  //             '                   |      |                                                                                                                                
  //     Now with an argument to ls -'------'

In this case, ls_argv has 3 fields set, not including NULL. We can now pass this through to exec:

/*exec_ls_arg.c*/ 
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>

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

  //arguments for ls, will run: ls  -l /bin                                                                                                                                  
  char * ls_args[4] = { "/bin/ls", "-l", "/bin", NULL} ;
  //             ^                   ^      ^                                                                                                                                
  //             '                   |      |                                                                                                                                
  //     Now with an argument to ls -'------'                                                                                                                                

  //execute ls                                                                                                                                                               
  execv( ls_args[0], ls_args);

  //only get here if exec failed                                                                                                                                             
  perror("execve failed");

  return 2; //return error status                                                                                                                                            
}

Another thing to note is that upon success, an exec does not return. Instead, the whole program is replaced with the exec'ed program, when the exec'ed program returns, that's the final return. To check if an exec fails, you don't need an if statement.

3.3 Forking Processes

So far, we've only loaded programs and executed them as an already running process. This is not creating a new process, and for that we need a new system call. The fork() system call will duplicate the calling process and create a new process with a new process identifier.

3.3.1 fork()

With the exception of two O.S. processes, the kernel and init process, all process are spawned from another process. The procedure of creating a new process is called forking: An exact copy of the process, memory values and open resources, is produced. The original process that forked, is called the parent, while the newly created, duplicate process is called the child. Let's look at an example of a process forking using the fork() system call:

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

int main(){

  pid_t c_pid;


  c_pid = fork(); //duplicate                                                                                                                                                
  if( c_pid == 0 ){
    //child: The return of fork() is zero                                                                                                                                    
    printf("Child: I'm the child: %d\n", c_pid);

  }else if (c_pid > 0){
    //parent: The return of fork() is the process of id of the child                                                                                                         

    printf("Parent: I'm the parent: %d\n", c_pid);

  }else{
    //error: The return of fork() is negative                                                                                                                                

    perror("fork failed");
    _exit(2); //exit failure, hard                                                                                                                                           

  }

  return 0; //success                                                                                                                                                        
}

The fork() system call is unlike any other function call you've seen so far. It returns twice, once in the parent and once in child, and it returns different values in the parent and the child.

To follow the logic, you first need to realize that once fork() is called, the Operating System is creating a whole new process which is an exact copy of the original process. At this point, fork() still hasen't returned because the O.S. is context switched in, and now it must return from fork() twice, once in the child process and once in the parent, where execution in both process can continue.

3.3.2 Process identifiers or pid

Every process has a unique identifier, the process identifier or pid. This value is assigned by the operating system when the process is created and is a 2-byte number (or a short). There is a special typedef for the process identifier, pid_t, which we will use.

In the above sample code, after the call to fork(), the parent's return value from fork() is the process id of the newly created child process. The child, however, has a return value of 0. On error, fork(), returns -1. Then you should bail with _exit() because something terrible happened.

One nice way to see a visual of the parent process relationship is using the bash command pstree:

#> pstree -ah
init
  ├─NetworkManager
  │   ├─dhclient -d -4 -sf /usr/lib/NetworkManager/nm-dhcp-client.action -pf /var/run/sendsigs.omit.d/network-manager.dhclient-eth0.pid -lf...
  │   └─2*[{NetworkManager}]
  ├─accounts-daemon
  │   └─{accounts-daemon}
(...)

At the top is the init process, which is the parent of all proces. Somewhere down tree is my login shell

(...)
├─sshd -D
  │   └─sshd   
  │       └─sshd    
  │           └─bash
  │               ├─emacs get_exitstatus.c
  │               ├─emacs foursons.c
  │               ├─emacs Makefile
  │               ├─emacs get_exitstatus.c
  │               ├─emacs fork_exec_wait.c
  │               ├─emacs mail_reports.py
  │               └─pstree -ah
(...)

And you can see that the process of getting to a bash shell via ssh requires a number of forks and child process.

3.3.3 Retrieving Process Identifiers: getpid() and getppid()

With fork(), the parent can learn the process id of the child, but the child doesn't know its own process id (or pid) after the fork nor does it know its parents process id. For that matter, the parent doesn't know its own process id either. There are two system calls to retrieve this information:

//retrieve the current process id
pid_t getpid(void);

//retrieve the parent's process id
pid_t getppid(void);

There is no way for a process to directly retrieve its child pid because any process may have multiple children. Instead, a process must maintain that information directly through the values returned from a fork(). Here is a sample program that prints the current process id and the parent's process id.

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

int main(){
  pid_t pid, ppid;

  //get the process'es pid
  pid = getpid();

  //get the parrent of this process'es pid
  ppid = getppid();


  printf("My pid is: %d\n",pid);
  printf("My parent's pid is %d\n", ppid);

  return 0;
}

If we run this program a bunch of times, we will see output like this:

#> ./get_pid_ppid 
My pid is: 14307
My parent's pid is 13790

#> ./get_pid_ppid 
My pid is: 14308
My parent's pid is 13790

#> ./get_pid_ppid 
My pid is: 14309
My parent's pid is 13790

Every time the program runs, it has a different process id (or pid). Every process must have a unique pid, and the O.S. applies a policy for reusing process id's as processes terminate. But, the parent's pid is the same. If you think for a second, this makes sense: What's the parent of the program? The shell! We can see this by echo'ing $$, which is special bash variable that stores the pid of the shell:

#> echo $$
13790

Whenever you execute a program on the shell, what's really going on is the shell is forking, and the new child is exec'ing the new program. One thing to consider, though, is that when a process forks, the parent and the child continue executing in parallel: Why doesn't the shell come back immediately and ask the user to enter a new command? The shell instead waits for the child to finish process before prompting again, and there is a system call called wait() to just do that.

3.4 Waiting on a child with wait()

The wait() system call is used by a parent process to wait for the status of the child to change. A status change can occur for a number of reasons, the program stopped or continued, but we'll only concern ourselves with the most common status change: the program terminated or exited. (We will discuss stopped and continued in later lessons.)

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

pid_t wait(int *status);

Once the parent calls wait(), it will block until a child changes state. In essence, it is waiting on its children to terminate. This is described as a blocking function because it blocks and does not continue until an event is complete.

Once it returns, wait() will returns the pid of the child process that terminated (or -1 if the process has no children), and wait() takes an integer pointer as an argument. At that memory address, it will set the termination status of the child process. As mentioned in the previous lesson, part of the termination status is the exit status, but it also contains other information for how a program terminated, like if it had a SEGFAULT.

3.4.1 Checking the Status of children

To learn about the exit status of a program we can use the macros from sys/wait.h which check the termination status and return the exit status. From the main page:

WIFEXITED(status)
       returns true if the child terminated normally, that is, 
       by calling exit(3) or _exit(2), or by returning from main().

WEXITSTATUS(status)
       returns  the  exit  status of the child.  This consists of the least significant 
       8 bits of the status argument that the child specified in a call to exit(3) or _exit(2) or as the
       argument for a return statement in main().  
       This macro should only be employed if WIFEXITED returned true.

There are other checks of the termination status, and refer to the manual page for more detail. Below is some example code for checking the exit status of forked child. You can see that the child delays its exit by 2 seconds with a call to sleep.

/*get_exitstatus.c*/
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

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

int main(){

  pid_t c_pid, pid;
  int status;

  c_pid = fork(); //duplicate

  if( c_pid == 0 ){
    //child
    pid = getpid();

    printf("Child: %d: I'm the child\n", pid, c_pid);
    printf("Child: sleeping for 2-seconds, then exiting with status 12\n");

    //sleep for 2 seconds
    sleep(2);

    //exit with statys 12
    exit(12);

  }else if (c_pid > 0){
    //parent

    //waiting for child to terminate
    pid = wait(&status);

    if ( WIFEXITED(status) ){
      printf("Parent: Child exited with status: %d\n", WEXITSTATUS(status));
    }

  }else{
    //error: The return of fork() is negative
    perror("fork failed");
    _exit(2); //exit failure, hard
  }

  return 0; //success                                                                                                                                                        
}

3.5 Fork/Exec/Wait Cycle

We now have all the parts to write a program that will execute another program and wait for that program to finish. This reminds me of another program we've already used in this class… the shell, but you'll get to that later in the lab.

For now, consider the example code below which executes ls on the /bin directory:

/*fork_exec_wait.c*/
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>

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

int main(int argc, char * argv[]){
  //arguments for ls, will run: ls  -l /bin                                                                                                                                  
  char * ls_args[3] = { "ls", "-l", NULL} ;
  pid_t c_pid, pid;
  int status;

  c_pid = fork();

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

    printf("Child: executing ls\n");

    //execute ls                                                                                                                                                               
    execvp( ls_args[0], ls_args);
    //only get here if exec failed                                                                                                                                             
    perror("execve failed");
  }else if (c_pid > 0){
    /* PARENT */

    if( (pid = wait(&status)) < 0){
      perror("wait");
      _exit(1);
    }

    printf("Parent: finished\n");

  }else{
    perror("fork failed");
    _exit(1);
  }

  return 0; //return success
}

And the execution:

aviv@saddleback: demo $ ./fork_exec_wait
Child: executing ls
total 5120
-rwxr-xr-x  2 root  wheel    18480 Sep  9 18:44 [
-r-xr-xr-x  1 root  wheel   628736 Sep 26 22:03 bash
-rwxr-xr-x  1 root  wheel    19552 Sep  9 18:57 cat
-rwxr-xr-x  1 root  wheel    30112 Sep  9 18:50 chmod
-rwxr-xr-x  1 root  wheel    24768 Sep  9 18:49 cp
-rwxr-xr-x  2 root  wheel   370096 Sep  9 18:40 csh
-rwxr-xr-x  1 root  wheel    24400 Sep  9 18:44 date
-rwxr-xr-x  1 root  wheel    27888 Sep  9 18:50 dd
-rwxr-xr-x  1 root  wheel    23472 Sep  9 18:49 df
-r-xr-xr-x  1 root  wheel    14176 Sep  9 19:27 domainname
-rwxr-xr-x  1 root  wheel    14048 Sep  9 18:44 echo
-rwxr-xr-x  1 root  wheel    49904 Sep  9 18:57 ed
-rwxr-xr-x  1 root  wheel    19008 Sep  9 18:44 expr
-rwxr-xr-x  1 root  wheel    14208 Sep  9 18:44 hostname
-rwxr-xr-x  1 root  wheel    14560 Sep  9 18:44 kill
-r-xr-xr-x  1 root  wheel  1394560 Sep  9 19:59 ksh
-rwxr-xr-x  1 root  wheel    77728 Sep  9 19:32 launchctl
-rwxr-xr-x  2 root  wheel    14944 Sep  9 18:49 link
-rwxr-xr-x  2 root  wheel    14944 Sep  9 18:49 ln
-rwxr-xr-x  1 root  wheel    34640 Sep  9 18:49 ls
-rwxr-xr-x  1 root  wheel    14512 Sep  9 18:50 mkdir
-rwxr-xr-x  1 root  wheel    20160 Sep  9 18:49 mv
-rwxr-xr-x  1 root  wheel   106816 Sep  9 18:49 pax
-rwsr-xr-x  1 root  wheel    46688 Sep  9 18:59 ps
-rwxr-xr-x  1 root  wheel    14208 Sep  9 18:44 pwd
-r-sr-xr-x  1 root  wheel    25216 Sep  9 19:27 rcp
-rwxr-xr-x  2 root  wheel    19760 Sep  9 18:49 rm
-rwxr-xr-x  1 root  wheel    14080 Sep  9 18:49 rmdir
-r-xr-xr-x  1 root  wheel   628800 Sep 26 22:03 sh
-rwxr-xr-x  1 root  wheel    14016 Sep  9 18:44 sleep
-rwxr-xr-x  1 root  wheel    28064 Sep  9 18:59 stty
-rwxr-xr-x  1 root  wheel    34224 Sep  9 21:59 sync
-rwxr-xr-x  2 root  wheel   370096 Sep  9 18:40 tcsh
-rwxr-xr-x  2 root  wheel    18480 Sep  9 18:44 test
-rwxr-xr-x  2 root  wheel    19760 Sep  9 18:49 unlink
-rwxr-xr-x  1 root  wheel    14112 Sep  9 19:32 wait4path
-rwxr-xr-x  1 root  wheel   551232 Sep  9 19:19 zsh
Parent: finished

The parent first forks a child process. In the child process, the execution is replaced by ls which prints the output. Meanwhile, the parent wait's for the execution to complete before continuing.

Imagine now this process occurring in a loop, and instead of running ls, the user provides the program that should run. That's a shell, and that's what you will be doing in the next lab.

4 Pipelines and Process Groups

In the last lesson and lab, we've been discussing job control and the mechanisms that enable it. Generally, job control is a feature of the shell and supported by the terminal device driver. The shell manages which jobs are stopped or running and notifies the terminal driver which job is currently in the foreground. The terminal device driver listens for special keys, like Ctrl-c or Ctrl-z, and delivers the appropriate signal to the foreground process, like terminate or stop.

That narrative is fairly straightforward, as long as there is only one process running within a job, but a job may contain more than one process, which could complicate the actions of the terminal device driver. Additionally, jobs can be further grouped together into sessions, and the mechanisms that enable all this interaction requires further discussion. In this lesson, we will explore process grouping and how this operating system services support job control and shell features we've grown to rely on (and love?).

4.1 Pipeline of processes

Consider the following pipeline:

sleep 10 | sleep 20 | sleep 30 | sleep 50 &

Here we have four different sleep commands running in a pipeline. The sleep command doesn't read or write to the terminal; it just sleeps for that many seconds and then exits. None of the sleep commands are blocking or waiting on input from another sleep command, so they can all run independently. We just happend to put them in a pipeline, but what is the impact of that? How long will this job take to complete?

One possibility is that each sleep command will run in sequence. First sleep 10 runs, then sleep 20, then sleep 30 runs, and finally sleep 50 runs, and thus it would take 10+20+30+50 = 110 seconds for the pipeline to finish. Another possibility is that they run all at the same time, or concurrently or in parallel, in which case the job would complete when the loggest sleep finishes, 50 seconds.

These two possibilities, in sequence and in parallel, also describe two possibilities for how a pipeline is executed. In sequence would imply that the shell forks the first item in the pipeline, lets that run, then the second item in the pipeline, lets that run, and so on. Or, in parallel: the shell forks all the items in the pipeline at once and lets the run concurrently. The major difference between these two choices is that a pipeline executing in sequence would have a single process running at a time for each total job while executing in parallel, however, would have multiple currently running processes per job.

By now, hopefully, you've already plugged that pipeline into the shell and found out that, yes, the pipeline executes in parallel, not in sequence. We can see this as well using the ps command.

sleep 10 | sleep 20 | sleep 30 | sleep 50 &
[1] 4128
aviv@saddleback: ~ $ ps -o pid,args
  PID COMMAND
 3981 -bash
 4125 sleep 10
 4126 sleep 20
 4127 sleep 30
 4128 sleep 50
 4129 ps -o pid,args

4.2 Process Grouping for Jobs

The implication of this discovery, that all process in the pipeline run concurrently, is that the shell must use a procedure for forking each of the process individually. But, then, how are these process linked? They are suppose to be a single job after all, and we also know that the terminal device driver is responsible for delivering signals to the foreground job. There must be some underlying procedure and process to enable this behavior, and, of course, there is.

The operating system provides a number of ways to group processes together. Process can be grouped into both process groups and sessions. A process group is a way to group processes into distinct jobs that are linked, and a session is way to link process groups under a single interruptive unit, like the terminal.

The key to understanding how the pipeline functions is that all of these process are places in the same process group, and we can see that by running the pipeline again. This time, however, we can also request that ps outputs the parent pid (ppid) and the process group (pgid) in addition to the process id (pid) and the command arguments (args).

#> sleep 10 | sleep 20 | sleep 30 | sleep 50 &
[1] 4134
#> ps -o pid,pgid,ppid,args
  PID  PGID  PPID COMMAND
 3981  3981  3980 -bash
 4131  4131  3981 sleep 10
 4132  4131  3981 sleep 20
 4133  4131  3981 sleep 30
 4134  4131  3981 sleep 50
 4135  4135  3981 ps -o pid,pgid,ppid,args

Notice first that the shell, bash, has a pid of 3981 and process group id (pgid) that is the same. The shell is in it's own process group. Similarly, the ps command itself also has a pid that is the same as its process group. However, the sleep commands, are in the process group id of 4131, which also is the pid of the first process in the pipeline. We can visualize this relationship like so:

process_grouping.png

Figure 6: Processes grouping in pipelines

As you can see, the rule of thumb for process grouping is that process executing as the same job, e.g., a single input to the shell as a pipeline, are placed in the same group. Also, the choice of process group id is the pid of the process.

5 Programming with Process Groups

Below, we will look at how we program with process groups using system calls, and we will investigate this from the perspective of the programmer as well as how the shell automatically groups process. We will use series of fairly straight forward system calls, and to bootstrap that discussion, we outline them below with brief descriptions.

Retrieving pid's or pgid's:

  • pid_t getpid() : get the process id for the calling process
  • pid_t getppid() : get the process id of the parent of the calling proces
  • pid_t getpgrp() : get the prcesso group id of the calling process
  • pid_t getpgid(pid_t pid) : get the process group id for the proces identified by pid

Setting pgid's:

  • pid_t setpgrp() : set the process group of the calling process to iteself, i.e. after a call to setpgr(), the following condition holds getpid() == getpgrp().
  • pid_t setpgid(pid_t pid, pid_t pgid) : set the process group id of the process identified by pid to the pgid, if pid is 0, then set the process group id of the calling process, and if pgid is 0, then the pid of the process identified by pid and is made the same as its process group, i.e., setpgid(0,0) is equivalent to calling setpgrp().

5.1 Retrieving the Process Group

Each process group has a unique process group identifier, or pgid, which are typically a pid of a process that is a member of the group. Upon a fork(), the child process inherits the parent's process group. We can see how this works with a small program that forks a child and prints the proces group identifies of both parent and child.

int main(int argc, char * argv[]){
/*inherit_pgid.c*/

  pid_t c_pid,pgid,pid;

  c_pid = fork();

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

    pgid = getpgrp();
    pid = getpid();

    printf("Child:  pid: %d pgid: *%d*\n", pid, pgid);

  }else if (c_pid > 0){
    /* PARRENT */

    pgid = getpgrp();
    pid = getpid();

    printf("Parent: pid: %d pgid: *%d*\n", pid, pgid);

  }else{
    /* ERROR */
    perror(argv[0]);
    _exit(1);
  }

  return 0;
}

Here is the output of running this program.

#> ./inherit_pgid
Parent: pid: 3630 pgid: *3630*
Child:  pid: 3631 pgid: *3630*

Notice that the process groups are the same, and that's because a child inherits the process group of its parent. Now let's look at a similar program that doesn't fork, and instead just prints the process group identifier of itself and its parent, which is the shell.

/*getpgrp.c*/
int main(int argc, char * argv[]){

  pid_t pid, pgid; //process id and process group for this program                                                                         
  pid_t ppid, ppgid; //process id and proces group for the _parent_                                                                        

  //current 
  pid = getpid();
  pgid = getpgrp();

  //parent
  ppid = getppid();
  ppgid = getpgid(ppid);

  //print this parent's process pid and pgid                                                                                               
  printf("%s: (current) pid:%d pgid:%d\n", argv[0], pid, pgid);
  printf("%s: (parrent) ppid:%d pgid:%d\n", argv[0], ppid, ppgid);

  return 0;
}

If we were to run this program in the shell, you might expect that both the child and the parent would print the same process group. Of course, why shouldn't this be the case? The program is a result of a fork from the shell, and thus the parent is the shell and the child is the program, and that's what just happened before, the parent and child had the same process group. But, looking at the output, that is not what occurs here.

#> ./getpgrp
./getpgrp: (current) pid:3760 pgid:3760
./getpgrp: (parrent) ppid:369 pgid:369

Instead, we find that the parent, which is the shell, is not in the same process group as the child, the getpgrp program. Why is that? This is because the new process is also a job in the shell and each job needs to run in its own process group for the purpose of terminal signaling. What we can now recognize from these examples, starting with the pipeline of sleep commands, is that a shell will fork each process separately in a job and assign the process group id based on the first child forked, as is clear upon further inspection of the output of these two examples:

#> sleep 10 | sleep 20 | sleep 30 | sleep 50 &
[1] 4134
#> ps -o pid,pgid,ppid,args
  PID  PGID  PPID COMMAND
 3981  3981  3980 -bash
 4131  4131  3981 sleep 10
 4132  4131  3981 sleep 20
 4133  4131  3981 sleep 30
 4134  4131  3981 sleep 50
 4135  4135  3981 ps -o pid,pgid,ppid,args
#> ./inherit_pgid
Parent: pid: 3630 pgid: *3630*
Child:  pid: 3631 pgid: *3630*

5.2 Setting the Process Group

Finally, now that we have learned to identify the process group, the next thing to do is to assign new process groups. There are two functions that do this: setpgrp() and setpgid().

  • setpgrp() : sets the process group of the calling process to itself. That is the calling process joins a process group of one, containing itself, where its pid is the as its pgid.
  • setpgid(pid_t pid, pid_t pgid) : set the process group of the process identified by pid to pgid. If pid is 0, then sets the process group of the calling process to pgid. If pgid is 0, then sets the process group of the process identified by pid to pid. Thus, setgpid(0,0) is the same as setpgid().

Let's consider a small program that sets the process group of the child after a fork using setpgrp() call from the child. The program below will print the process id's and process groups from the child's and parent's perspective.

/*setpgrp.c*/
int main(int argc, char * argv[]){

  pid_t cpid, pid, pgid, cpgid; //process id's and process groups

  cpid = fork();

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

    //set process group to itself
    setpgrp();

    //print the pid, and pgid of child from child
    pid = getpid();
    pgid = getpgrp();
    printf("Child:          pid:%d pgid:*%d*\n", pid, pgid);

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

    //print the pid, and pgid of parent
    pid = getpid();
    pgid = getpgrp();
    printf("Parent:         pid:%d pgid: %d \n", pid, pgid);    

    //print the pid, and pgid of child from parent
    cpgid = getpgid(cpid);
    printf("Parent: Child's pid:%d pgid:*%d*\n", cpid, cpgid);

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

  return 0;
}

And, here's the output:

#> ./setpgrp
Parent:         pid:20178 pgid: 20178 
Parent: Child's pid:20179 pgid:*20178*
Child:          pid:20179 pgid:*20179*

Clearly, something is not right. The child sees a different pgid is different than the parent. What we have here is a race condition, which is when you have two processes running in parallel, you don't know which is going to finish the race first.

Consider that there are two possibility for how the above program will execute following the fork. In one possibility, after the fork, the child runs before the parent and the process group is set properly, and in the other scenario, the parent runs first reads the process group before the child gets a chance to set it. It is the later that we see above, the parent running before the child, thus the wrong pgid.

To avoid these issues, when setting the process group of a child, you should call setpgid()=/=setpgrp() in both the parent and the child before anything depends on those values. In this way, you can disambiguate the runtime process, it will not matter which runs first, the parent or the child, the result is always the same, the child is placed in the appropriate process group. Below is an example of that and the output.

/*setpgid.c*/
int main(int argc, char * argv[]){

  pid_t cpid, pid, pgid, cpgid; //process id's and process groups

  cpid = fork();

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

    //set process group to itself
    setpgrp(); //<---------------------------!

    //print the pid, and pgid of child from child
    pid = getpid();
    pgid = getpgrp();
    printf("Child:          pid:%d pgid:*%d*\n", pid, pgid);

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

    //set the proccess group of child 
    setpgid(cpid, cpid); //<------------------!

    //print the pid, and pgid of parent
    pid = getpid();
    pgid = getpgrp();
    printf("Parent:         pid:%d pgid: %d \n", pid, pgid);    

    //print the pid, and pgid of child from parent
    cpgid = getpgid(cpid);
    printf("Parent: Child's pid:%d pgid:*%d*\n", cpid, cpgid);


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


  return 0;
}
#> ./setpgid
Parent:         pid:20335 pgid: 20335 
Parent: Child's pid:20336 pgid:*20336*
Child:          pid:20336 pgid:*20336*

6 Process Groups and Terminal Signaling

Where process groups fit into the ecosystem of process settings is within the terminal settings. Let's return the terminal control function, tcsetpgrp(). Before, we discussed this function as setting the foreground processes, but just from its name tcsetpgrp(), it actually sets the foreground process group.

6.1 Foreground Process Group

This distinction is important because of terminal signaling. We know now that when we execute a pipeline, the shell will fork all the process in the job and place them in the same process group. We also know that when we use special control keys, like Ctrl-c or Ctrl-z that the terminal will deliver special signals to the foreground job, such as indicating to terminate or stop. For example, this sequence of shell interaction makes sense:

#> sleep 10 | sleep 20 | sleep 30 | sleep 50 &
[1] 24253
#> ps
  PID TTY          TIME CMD
 4038 pts/3    00:00:00 bash
24250 pts/3    00:00:00 sleep
24251 pts/3    00:00:00 sleep
24252 pts/3    00:00:00 sleep
24253 pts/3    00:00:00 sleep
24254 pts/3    00:00:00 ps
#> fg
sleep 10 | sleep 20 | sleep 30 | sleep 50
^C
#> ps
  PID TTY          TIME CMD
 4038 pts/3    00:00:00 bash
24255 pts/3    00:00:00 ps

We started the sleep commands in the background, we see that there are 4 instances of sleep running, and we can move them from the background to the foreground, were they are signaled with Ctrl-c to terminate via the terminal. All good, right? There is something missing: Given that there are multiple processes running in the foreground, how does the terminal know which of those to signal to stop or terminate signal? How does it differentiate which processes are in the foreground?

The answer is, the terminal does not identify foreground process individually. Instead, it identifies a foreground process group. All processes associated with the foreground job are in the foreground process group, and instead of signalling processes individually both shell and the terminal think of execution in terms of process groups.

6.2 Orphaned Stopped Process Groups

Process group interaction has other side effects when you consider programs that fork children. For example, consider the program (orphan) below which simply forks a child, and then both child a parent loop forever:

   /*orphan.c*/
   int main(int argc, char * argv[]){

  pid_t cpid;

  cpid = fork();

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

    //child loops forever!                                                                                                                 
    while(1);

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

    //Parrent loops forever                                                                                                                
    while(1);

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

  return 0;
}

If we were to run this program, we can see that, yes, indeed, it forks and now we have two versions of orphan running in the same process group.

#> ./orphan &
[1] 24468
#> ps -o pid,pgid,ppid,comm
  PID  PGID  PPID COMMAND
 4038  4038  4037 bash
24468 24468  4038 orphan
24469 24468 24468 orphan
24470 24470  4038 ps

Moving the orphan program to the foreground, it can then be terminated by the terminal using Ctrl-c.

#> fg
./orphan
^C
#> ps -o pid,pgid,ppid,comm
  PID  PGID  PPID COMMAND
 4038  4038  4037 bash
24471 24471  4038 ps

The resulting termination is for both parent and child, which is as expected since they are both in the foreground process group. While we might expect an orphan to be created, this does not occur. However, let's consider the same program, but this time, the child is placed in a different process group as the parent:

/*orphan_group.c*/
int main(int argc, char * argv[]){

  pid_t cpid;

  cpid = fork();

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

    //set process group to itself
    setpgrp();

    //child loops forever!
    while(1);

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

    //set the proccess group of child 
    setpgid(cpid, cpid); 

    //Parrent loops forever
    while(1);

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


  return 0;
}

Let's do the same experiment as before:

#> ./orphan_group &
[1] 24487
#> ps -o pid,pgid,ppid,comm
  PID  PGID  PPID COMMAND
 4038  4038  4037 bash
24487 24487  4038 orphan_group
24488 24488 24487 orphan_group
24489 24489  4038 ps
#> fg
./orphan_group
^C
#> ps -o pid,pgid,ppid,comm
  PID  PGID  PPID COMMAND
 4038  4038  4037 bash
24488 24488     1 orphan_group
24490 24490  4038 ps

This time, yes, we see that we have created an orphan process. This is clear from the PPID field which indicates that the parent of the orphan_group program is init, which inherits all orphaned processes. This happens because the terminal signal Ctrl-c is delivered to the foreground process group only, but the child is not in that group. The child is in its own process group and never recieves the signal, and, thus, never terminates. It just continues on its merry way never realizing that it just lost its parent. In this examples lies the danger of using process groups; it's very easy to create a bunch of orphans that will just cary on if not killed. To rid yourself of them, you must explictely kill them with a call like killall

#> killall orphan_group
#> ps -o pid,pgid,ppid,comm
  PID  PGID  PPID COMMAND
 4038  4038  4037 bash
24494 24494  4038 ps

And good riddance …

7 Resource Duplication Across Forks

Recall, from our discussion of fork() and process duplication, the entire process is duplicated, not just the code, but also all the state of the process. This includes the entire state of memory, such as the values of variables. For example, consider the program below that will fork 5 children tracked with a variable i, which is printed from the child, then manipulated in the child, and printed again in the parent.

  /*shared_variable.c*/
  int main(int argc, char * argv[]){

  int status;
  pid_t cpid, pid;

  int i=0;

  while(1){ //loop and fork children

    cpid = fork();

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

      pid = getpid();

      printf("Child: %d: i:%d\n", pid, i);

      //set i in child to something differnt
      i *= 3;
      printf("Child: %d: i:%d\n", pid, i);

      _exit(0); //NO FORK BOMB!!!
    }else if ( cpid > 0){
      /* PARENT */

      //wait for child
      wait(&status);

      //print i after waiting
      printf("Parent: i:%d\n", i);

      i++;
      if (i > 5){       
        break; //break loop after 5 iterations
      }

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

    //pretty print
    printf("--------------------\n");
  }

  return 0;
}

And we can see the output of this program

>./shared_variables
Child: 3833: i:0
Child: 3833: i:0
Parent: i:0
--------------------
Child: 3834: i:1
Child: 3834: i:3
Parent: i:1
--------------------
Child: 3835: i:2
Child: 3835: i:6
Parent: i:2
--------------------
Child: 3836: i:3
Child: 3836: i:9
Parent: i:3
--------------------
Child: 3837: i:4
Child: 3837: i:12
Parent: i:4
--------------------
Child: 3838: i:5
Child: 3838: i:15
Parent: i:5

Looking through the output, we can see each of the 5 children identified by their process id, and we can track the state of the variable i. It is initialized in the parent prior to the fork, and that value is duplicated to the child, as indicated by the first child print. The child then multiplies i by 3, and prints the value again, which is indicated by the second child print. Meanwhile, the parent is waiting for the child to terminate with a call to wait(), and then prints its own view of the variable i, which is unchanged.

The program demonstrates how duplication occurs across a fork. The current state of the parent is duplicated to the child, but additional edits by the child are on its own version of the memory not the parents, which also has its own version of the memory.

7.1 File Descriptor's across Forks

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.

/*shared_file.c*/
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:

8 Inter-Process Communication and Pipes

Where duplication of file descriptors becomes interesting is when you consider the possibility for inter-process communication. So far, we've seen very limited inter-process communication through the setting of exit status or termination conditions; a parent can check the status of a terminating child and perform some action based on that. But, how can a child communicate to a parent? Or, how can we communicate more than just a short number?

We know that process can communicate a large amount of information over a text stream with a pipe on the command line, and, moreover, this is vital part of the Unix design philosophy. What we are going to look at now is how we can create pipes to perform inter process communicating, leveraging the duplication of file descriptors across forks.

8.1 Hello pipe()

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.

8.2 Pipes Bursting! and Blocking!

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.

img5.gif

Figure 7: In and Out of a pipe communicate through the kernel (source tldp.org)

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.

/*pipe_block.c*/
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.

8.3 Inter Process Pipes

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.

/*pipe_hello_to_child.c*/
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.

9 Duplicating File Descriptor and Pipelines

The other place we've seen pipes is in the context of bash command lines.

cat sample_db.csv | cut -d "," -f 5 | sort | uniq | wc -l

As we know, he pipe symbol says, connect the standard output of one program with the standard input of another. From the context of the pipe() system call, we can begin to imagine how that might be done, but there's a piece missing. With the pipe() system call, we get the input and output file descriptors which are used for inter process communication, but for the commands above, they read and write from stdout and stdin directly. How do we make them use the pipe instead? This is accomplished by file descriptor duplication, a process that overwrites a file descriptor with another, and this can be done with the standard file descriptors.

9.1 Duplicating a File Descriptor

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:

/*hello_dup.c*/
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.

9.2 Setting up a pipeline

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:

   /*pipe_dup.c*/
   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.