Lecture 06: execvp, Pipes, and Interprocess Communication

Principles of Computer Systems

Spring 2019

Stanford University

Computer Science Department

Lecturer: Chris Gregg

  • First example using execvp? An implementation mysystem to emulate the behavior of the libc function called system.
    • Here we present our own implementation of the mysystem function, which executes the supplied command as if we typed it out in the terminal ourselves, ultimately returning once the surrogate command has finished.
    • If the execution of command exits normally (either via an exit system call, or via a normal return statement from main), then our mysystem implementation should return that exact same exit value.
    • If the execution exits abnormally (e.g. it segfaults), then we'll assume it aborted because some signal was ignored, and we'll return that negative of that signal number (e.g. -11 for SIGSEGV).

Lecture 06: execvp, Pipes, and Interprocess Communication

  • Instead of calling a subroutine to perform some task and waiting for it to complete,
    mysystem spawns a child process to perform some task and waits for it to complete.
  • We don't bother checking the return value of execvp, because we know that if it returns at all, it returns a -1. If that happens, we need to handle the error and make sure the child process terminates, via an exposed exit(0) call.
  • Why not call execvp inside parent and forgo the child process altogether? Because
    execvp would consume the calling process, and that's not what we want.
static int mysystem(const char *command) {
  pid_t pid = fork();
  if (pid == 0) {
    char *arguments[] = {"/bin/sh", "-c", (char *) command, NULL};
    execvp(arguments[0], arguments);
    printf("Failed to invoke /bin/sh to execute the supplied command.");
    exit(0);
  }
  int status;
  waitpid(pid, &status, 0);
  return WIFEXITED(status) ? WEXITSTATUS(status) : -WTERMSIG(status);
}
  • Here's the implementation, with minimal error checking (the full version is right here, and a working version is on the next slide):

Lecture 06: execvp, Pipes, and Interprocess Communication

Lecture 06: execvp, Pipes, and Interprocess Communication

  • fgets is a somewhat overflow-safe variant on scanf that knows to read everything up through and including the newline character.
    • The newline character is retained, so we need to chomp that newline off before calling mysystem.
static const size_t kMaxLine = 2048;
int main(int argc, char *argv[]) {
  char command[kMaxLine];
  while (true) {
    printf("> ");
    fgets(command, kMaxLine, stdin);
    if (feof(stdin)) break; 
    command[strlen(command) - 1] = '\0'; // overwrite '\n'
    printf("retcode = %d\n", mysystem(command));
  }
  
  printf("\n");
  return 0;
}
  • Here's a test harness that we can run to confirm our mysystem implementation is working as expected:

Lecture 06: execvp, Pipes, and Interprocess Communication

  • The mysystem function is the first example I've provided where fork, execvp, and waitpid all work together to do something genuinely useful. 
    • The test harness we used to exercise mysystem is operationally a miniature terminal.
    • We need to continue implementing a few additional mini-terminals to fully demonstrate how fork, waitpid, and execvp work in practice.
    • All of this is paying it forward to your fourth assignment, where you'll implement your own shell—we call it stsh for Stanford shell—to imitate the functionality of the shell (csh, bash, zsh, tcsh, etc.) you've been using since you started using Unix.
  • We'll introduce the notion of a pipe, the pipe and dup2 system calls, and how they can be used to introduce communication channels between the different processes.

Lecture 06: execvp, Pipes, and Interprocess Communication

  • Let's work through the implementation of a more sophisticated shell: the simplesh.
    • This is the best example of fork, waitpid, and execvp I can think of: a miniature shell not unlike those you've been using since the day you first logged into a myth machine.
    • simplesh operates as a read-eval-print loop—often called a repl—which itself responds to the many things we type in by forking off child processes.
      • Each child process is initially a deep clone of the simplesh process.
      • Each proceeds to replace its own process image with the new one we specify, e.g. ls, cp, our own CS110 search (which we wrote during our second lecture), or even emacs.
      • As with traditional shells, a trailing ampersand—e.g. as with emacs &—is an instruction to execute the new process in the background without forcing the shell to wait for it to finish.
    • Implementation of simplesh is presented on the next slide. Where helper functions don't rely on CS110 concepts, I omit their implementations.

Lecture 06: execvp, Pipes, and Interprocess Communication

  • Here's the core implementation of simplesh (full implementation is right here, and you can run the code on the following slide):

Lecture 06: execvp, Pipes, and Interprocess Communication

int main(int argc, char *argv[]) {
  while (true) {
    char command[kMaxCommandLength + 1];
    readCommand(command, kMaxCommandLength);
    char *arguments[kMaxArgumentCount + 1];
    int count = parseCommandLine(command, arguments, kMaxArgumentCount);
    if (count == 0) continue;
    if (strcmp(arguments[0], "quit") ==) break; // hardcoded builtin to exit shell
    bool isbg = strcmp(arguments[count - 1], "&") == 0;
    if (isbg) arguments[--count] = NULL; // overwrite "&"
    pid_t pid = fork();
    if (pid == 0) execvp(arguments[0], arguments);
    if (isbg) { // background process, don't wait for child to finish
      printf("%d %s\n", pid, command);
    } else {    // otherwise block until child process is complete
      waitpid(pid, NULL, 0);
    }
  }
  printf("\n");
  return 0;
}

Lecture 06: execvp, Pipes, and Interprocess Communication

  • Introducing the pipe system call.
    • The pipe system call takes an uninitialized array of two integers—let’s call it fds—and populates it with two file descriptors such that everything written to fds[1]can be read from fds[0].
    • Here's the prototype:

       
    • pipe is particularly useful for allowing parent processes to communicate with spawned child processes.
      • That's because the file descriptor table of the parent is cloned, and that clone is installed in the child.
      • That means the open file table entries references by the parent's pipe endpoints are also referenced by the child's copies of them.

Lecture 06: execvp, Pipes, and Interprocess Communication

int pipe(int fds[]);
  • How does pipe work?
    • To illustrate how pipe works and how arbitrary data can be passed over from one process to a second, let's consider the following program (which you can find here, or run on the next slide):

Lecture 06: execvp, Pipes, and Interprocess Communication

int main(int argc, char *argv[]) {
  int fds[2];
  pipe(fds);
  pid_t pid = fork();
  if (pid == 0) {
    close(fds[1]);
    char buffer[6];
    read(fds[0], buffer, sizeof(buffer));
    printf("Read from pipe bridging processes: %s.\n", buffer);
    close(fds[0]);
    return 0;
  }
  close(fds[0]);
  write(fds[1], "hello", 6);
  waitpid(pid, NULL, 0);
  close(fds[1]);
  return 0;
}

Lecture 06: execvp, Pipes, and Interprocess Communication

  • How do pipe and fork work together in this example?
    • The base address of a small integer array called fds is shared with the call to pipe
    • pipe allocates two descriptors, setting the first to draw from a resource and the second to publish to that same resource. 
    • pipe then plants copies of those two descriptors into indices 0 and 1 of the supplied array before it returns.
    • The fork call creates a child process, which itself inherits a shallow copy of the parent's fds array. 
      • The reference counts in each of the two open file entries is promoted from 1 to 2 to reflect the fact that two descriptors—one in the parent, and a second in the child—reference each of them.
      • Immediately after the fork call, anything printed to fds[1]is readable from the parent's fds[0]and the child's fds[0].
      • Similarly, both the parent and child are capable of publishing text to the same       resource via their copies of fds[1].

Lecture 06: execvp, Pipes, and Interprocess Communication

  • How do pipe and fork work together in this example?
    • The parent closes fds[0]before it writes to anything to fds[1]to emphasize the fact that the parent has no interest in reading anything from the pipe.
    • Similarly, the child closes fds[1] before it reads from fds[0] to emphasize the fact that the it has zero interest in publishing anything to the pipe. It's imperative all write endpoints of the pipe be closed if not being used, else the read end will never know if more text is to come or not.
    • For simplicity, I assume the one call to write in the parent presses all six bytes of "hello"('\0'included) in a single call. Similarly, I assume the one call to read pulls in those same six bytes into its local buffer with just the one call.
    • As is the case with all programs, I make the concerted effort to donate all resources back to the system before I exit. That's why I include as many close calls as I do in both the child and the parent before allowing them to exit.

Lecture 06: execvp, Pipes, and Interprocess Communication

  • Here's a more sophisticated example:
    • Using pipe, fork, dup2, execvp, close, and waitpid, we can implement the subprocess function, which relies on the following record definition and is implemented to the following prototype (full implementation of everything is right here):

 

 

  • The child process created by subprocess executes the provided command (assumed to be a '\0'-terminated C string) by calling "/bin/sh -c <command>"as we did in our mysystem implementation. 
    • Rather than waiting for command to finish, subprocess returns a subprocess_t with the command process’s pid and a single descriptor called supplyfd.
    • By design, arbitrary text can be published to the return value’s supplyfd field with          the understanding that that same data can be ingested verbatim by the child's                stdin.

Lecture 06: execvp, Pipes, and Interprocess Communication

typedef struct {
  pid_t pid;
  int supplyfd;
} subprocess_t;
subprocess_t subprocess(const char *command);
  • Let's first implement a test harness to illustrate how subprocess should work.
    • By understanding how subprocess works for us, we'll have an easier time understanding the details of its implementation.
    • Here's the program, which spawns a child process that reads from stdin and publishes everything it reads to its stdout in sorted order:

Lecture 06: execvp, Pipes, and Interprocess Communication

int main(int argc, char *argv[]) {
  subprocess_t sp = subprocess("/usr/bin/sort");
  const char *words[] = {
    "felicity", "umbrage", "susurration", "halcyon", 
    "pulchritude", "ablution", "somnolent", "indefatigable"
  };
  for (size_t i = 0; i < sizeof(words)/sizeof(words[0]); i++) {
    dprintf(sp.supplyfd, "%s\n", words[i]);
  }
  close(sp.supplyfd);
  int status;
  pid_t pid = waitpid(sp.pid, &status, 0);
  return pid == sp.pid && WIFEXITED(status) ? WEXITSTATUS(status) : -127;
}
  • Key features of the test harness:
    • The program creates a subprocess_t running sort and publishes eight fancy SAT words to supplyfd, knowing those words flow through the pipe to the child's stdin
    • The parent shuts the supplyfd down by passing it to close. The reference count of the open file entry referenced by supplyfd is demoted from 1 to 0 with that close call. That sends an EOF to the process that tries in ingest data from the other end of the pipe.
    • The parent then blocks within a waitpid call until the child exits. When the child exits, the parent assumes all of the words have been printed in sorted order to stdout.

Lecture 06: execvp, Pipes, and Interprocess Communication

cgregg@myth60$ ./subprocess 
ablution
felicity
halcyon
indefatigable
pulchritude
somnolent
susurration
umbrage
cgregg@myth60$
  • Implementation of subprocess (error checking intentionally omitted for brevity):








     
    • The write end of the pipe is embedded into the subprocess_t. That way, the parent knows where to publish text so it flows to the read end of the pipe, across the parent process/child process boundary. This is bonafide interprocess communication.
    • The child process uses dup2 to bind the read end of the pipe to its own standard input. Once the reassociation is complete, fds[0] can be closed.

Lecture 06: execvp, Pipes, and Interprocess Communication

subprocess_t subprocess(const char *command) {
  int fds[2];
  pipe(fds);
  subprocess_t process = { fork(), fds[1] };
  if (process.pid == 0) {
    close(fds[1]);
    dup2(fds[0], STDIN_FILENO);
    close(fds[0]);
    char *argv[] = {"/bin/sh", "-c", (char *) command, NULL};
    execvp(argv[0], argv);
  }
  close(fds[0]);
  return process;
}
  • Introduction to Signals
    • A signal is a small message that notifies a process that an event of some type occurred. Signals are often sent by the kernel, but they can be sent from other processes as well.
    • A signal handler is a function that executes in response to the arrival and consumption of a signal.
    • You're already familiar with some types of signals, even if you've not referred to them by that name before.
      • You haven't truly programmed in C before unless you've unintentionally dereferenced a NULL pointer.
      • When that happens, the kernel delivers a signal of type SIGSEGV, informally known as a segmentation fault (or a SEGmentation Violation, or SIGSEGV, for short).
      • Unless you install a custom signal handler to manage the signal differently, a SIGSEGV terminates the program and generates a core dump.
    • Each signal category (e.g. SIGSEGV) is represented internally by some number (e.g. 11). In fact, C #defines SIGSEGV to be the number 11.

Lecture 06: Signals

  • Other signal types:
    • Whenever a process commits an integer-divide-by-zero (and, in some cases, a floating-point divide by zero on older architectures), the kernel hollers and issues a SIGFPE signal to the offending process. By default, the program handles the SIGFPE by printing an error message announcing the zero denominator and generating a core dump.
    • When you type ctrl-c, the kernel sends a SIGINT to the foreground process (and by default, that foreground is terminated).
    • When you type ctrl-z, the kernel issues a SIGTSTP to the foreground process (and by default, the foreground process is halted until a subsequent SIGCONT signal instructs it to continue).
    • When a process attempts to publish data to the write end of a pipe after the read end has been closed, the kernel delivers a SIGPIPE to the offending process. The default SIGPIPE handler prints a message identifying the pipe error and terminates the program.

Lecture 06: Signals

  • One signal type most important to multiprocessing:
    • Whenever a child process changes state—that is, it exits, crashes, stops, or resumes from a stopped state, the kernel sends a SIGCHLD signal to the process's parent.
      • By default, the signal is ignored. In fact, we've ignored it until right now and gotten away with it.
      • This particular signal type is instrumental to allowing forked child processes to run in the background while the parent process moves on to do its own work without blocking on a waitpid call.
      • The parent process, however, is still required to reap child processes, so the parent will typically register a custom SIGCHLD handler to be asynchronously invoked whenever a child process changes state.
      • These custom SIGCHLD handlers almost always include calls to waitpid, which can be used to surface the pids of child processes that've changed state. If the child process of interest actually terminated, either normally or abnormally, the waitpid also culls the zombie the relevant child process has become.

Lecture 06: Signals

  • Our first signal handler example: Disneyland
    • Here's a carefully coded example that illustrates how to implement and install a SIGCHLD handler.
    • The premise? Dad takes his five kids out to play. Each of the five children plays for a different length of time. When all five kids are done playing, the six of them all go home.
    • The parent process is modeling dad, and the five child processes are modeling his children. (Full program, with error checking, is right here.)

Lecture 06: Signals

static const size_t kNumChildren = 5;
static size_t numDone = 0;

int main(int argc, char *argv[]) {
  printf("Let my five children play while I take a nap.\n");
  signal(SIGCHLD, reapChild);
  for (size_t kid = 1; kid <= 5; kid++) {
    if (fork() == 0) {
      sleep(3 * kid); // sleep emulates "play" time
      printf("Child #%zu tired... returns to dad.\n", kid);
      return 0;
    }
  }
  • Our first signal handler example: Disneyland
    • The program is crafted so each child process exits at three-second intervals. reapChild, of course, handles each of the SIGCHLD signals delivered as each child process exits.
    • The signal prototype doesn't allow for state to be shared via parameters, so we have no choice but to use global variables.

Lecture 06: Signals

  // code below is a continuation of that presented on the previous slide
  while (numDone < kNumChildren) {
    printf("At least one child still playing, so dad nods off.\n");
    sleep(5);
    printf("Dad wakes up! ");
  }
  printf("All children accounted for.  Good job, dad!\n");
  return 0;
}

static void reapChild(int unused) {
  waitpid(-1, NULL, 0);
  numDone++;
}
  • Here's the output of the above program.
    • Dad's wakeup times (at t = 5 sec, t = 10 sec, etc.) interleave the various finish times (3 sec, 6 sec, etc.) of the children, and the output published below reflects that.
    • Understand that the SIGCHLD handler is invoked 5 times, each in response to some child process finishing up.

Lecture 06: Signals

cgregg@myth60$ ./five-children 
Let my five children play while I take a nap.
At least one child still playing, so dad nods off.
Child #1 tired... returns to dad.
Dad wakes up! At least one child still playing, so dad nods off.
Child #2 tired... returns to dad.
Child #3 tired... returns to dad.
Dad wakes up! At least one child still playing, so dad nods off.
Child #4 tired... returns to dad.
Child #5 tired... returns to dad.
Dad wakes up! All children accounted for.  Good job, dad!
cgregg@myth60$
  • Advancing our understanding of signal delivery and handling.
    • Now consider the scenario where the five kids are the same age and run about Disneyland for the same amount of time. Restated, sleep(3 * kid) is now sleep(3) so all five children flashmob dad when they're all done.
    • The output presented below makes it clear dad never detects all five kids are present and accounted for, and the program runs forever because dad keeps going back to sleep.

Lecture 06: Signals

cgregg*@myth60$ ./broken-pentuplets 
Let my five children play while I take a nap.
At least one child still playing, so dad nods off.
Kid #1 done playing... runs back to dad.
Kid #2 done playing... runs back to dad.
Kid #3 done playing... runs back to dad.
Kid #4 done playing... runs back to dad.
Kid #5 done playing... runs back to dad.
Dad wakes up! At least one child still playing, so dad nods off.
Dad wakes up! At least one child still playing, so dad nods off.
Dad wakes up! At least one child still playing, so dad nods off.
Dad wakes up! At least one child still playing, so dad nods off.
^C # I needed to hit ctrl-c to kill the program that loops forever!
cgregg@myth60$
  • Advancing our understanding of signal delivery and handling.
    • The five children all return to dad at the same time, but dad can't tell.
    • Why? Because if multiple signals come in at the same time, the signal handler is only run once.
      • If three SIGCHLD signals are delivered while dad is off the processor, the operating system only records the fact that at one or more SIGCHLDs came in.
      • When the parent is forced to execute its SIGCHLD handler, it must do so on behalf of the one or more signals that may have been deliveredsince the last time it was on the processor.
    • That means our SIGCHLD handler needs to call waitpid in a loop, as with:

Lecture 06: Signals

static void reapChild(int unused) {
  while (true) {
    pid_t pid = waitpid(-1, NULL, 0);
    if (pid < 0) break;
    numDone++;
  }
}
  • Advancing our understanding of signal delivery and handling.
    • The improved reapChild implementation seemingly fixes the pentuplets program, but it changes the behavior of the first five-children program.
      • When the first child in the original program has exited, the other children are still out playing.
      • The SIGCHLD handler will call waitpid once, and it will return the pid of the first child.
      • The SIGCHLD handler will then loop around and call waitpid a second time.
      • This second call will block until the second child exits three seconds later, preventing dad from returning to his nap.
    • We need to instruct waitpid to only reap children that have exited but to return without blocking, even if there are more children still running. We use WNOHANG for this, as with:

Lecture 06: Signals

static void reapChild(int unused) {
  while (true) {
    pid_t pid = waitpid(-1, NULL, WNOHANG);
    if (pid <= 0) break; // note the < is now a <=
    numDone++;
  }
}
  • All SIGCHLD handlers generally have this while loop structure.
    • Note we changed the if (pid < 0)test to if (pid <= 0).
    • A return value of -1 typically means that there are no child processes left.
    • A return value of 0—that's a new possible return value for us—means there are other child processes, and we would have normally waited for them to exit, but we’re returning instead because of the WNOHANG being passed in as the third argument.
  • The third argument supplied to waitpid can include several flags bitwise-or'ed together.
    • WUNTRACED informs waitpid to block until some child process has either ended or been stopped.
    • WCONTINUED informs waitpid to block until some child process has either ended or resumed from a stopped state.
    • WUNTRACED | WCONTINUED | WNOHANG asks that waitpid return information about a child process that has changed state (i.e. exited, crashed, stopped, or continued) but to do so without blocking.

Lecture 06: Signals