CodeDweller/child.cpp

// \file child.cpp
//
// Copyright (C) 2014 MicroNeil Research Corporation.
//
// This program is part of the MicroNeil Research Open Library Project. For
// more information go to http://www.microneil.com/OpenLibrary/index.html
//
// This program is free software; you can redistribute it and/or modify it
// under the terms of the GNU General Public License as published by the
// Free Software Foundation; either version 2 of the License, or (at your
// option) any later version.
//
// This program is distributed in the hope that it will be useful, but WITHOUT
// ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
// FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
// more details.
//
// You should have received a copy of the GNU General Public License along with
// this program; if not, write to the Free Software Foundation, Inc., 59 Temple
// Place, Suite 330, Boston, MA 02111-1307 USA
//==============================================================================

#ifndef _WIN32
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <signal.h>
#include <sys/select.h>

#include <cstring>
#include <cerrno>
#endif

#include <stdexcept>
#include <utility>

#include "CodeDweller/timing.hpp"
#include "CodeDweller/child.hpp"

namespace CodeDweller {

  ChildStream::ChildStream(std::vector<std::string> const &args,
                           size_t bufSize) :
    childStreambuf(bufSize),
    std::iostream(&childStreambuf),
    cmdArgs(args) {

    init();
    run();

  }

  ChildStream::ChildStream(std::string const &childpath, size_t bufSize) :
    childStreambuf(bufSize),
    std::iostream(&childStreambuf) {

    cmdArgs.push_back(childpath);
    init();
    run();
  }

  ChildStream::ChildStream(size_t bufSize) :
    childStreambuf(bufSize),
    std::iostream(&childStreambuf) {

    init();
  }

  ChildStream::~ChildStream() {
    try {
      close();
    } catch (std::exception &e) {
      ;
    }
  }

  void ChildStream::init() {

    childStarted = false;
    childExited = false;
    exitCodeObtainedFlag = false;
    exitCode = 0;
  }

  void ChildStream::open(std::vector<std::string> const &args) {
    cmdArgs = args;
    init();
    run();
  }

  void ChildStream::open(std::string const &childpath) {
    cmdArgs.clear();
    cmdArgs.push_back(childpath);
    init();
    run();
  }

  size_t ChildStream::numBytesAvailable() const {

    return childStreambuf.numBytesAvailable();

  }

  bool ChildStream::isRunning() const {
    return childStarted && !childExited;
  }

  void ChildStream::run() {

    if (childStarted) {
      throw std::logic_error("Child process was active when "
			     "run() was called");
    }

    if (cmdArgs.empty()) {
      throw std::invalid_argument("A child executable must be specified.");
    }

#ifdef _WIN32
    // Set the bInheritHandle flag so pipe handles are inherited. 
    SECURITY_ATTRIBUTES securityAttributes; 

    securityAttributes.nLength = sizeof(SECURITY_ATTRIBUTES); 
    securityAttributes.bInheritHandle = true; 
    securityAttributes.lpSecurityDescriptor = NULL; 

    // Create a pipe for the child process's STDOUT.
    HANDLE childStdOutAtChild;
    HANDLE childStdOutAtParent;
    HANDLE childStdInAtChild;
    HANDLE childStdInAtParent;
    int bufferSize = 0;

    if (!CreatePipe(&childStdOutAtParent,
		    &childStdOutAtChild,
		    &securityAttributes,
		    bufferSize)) {
      throw std::runtime_error("Error from CreatePipe for stdout:  " +
			       getErrorText());
    }

    // Ensure the read handle to the pipe for STDOUT is not inherited.
    int inheritFlag = 0;
    if (!SetHandleInformation(childStdOutAtParent,
			      HANDLE_FLAG_INHERIT,
			      inheritFlag) ) {
      throw std::runtime_error("Error from GetHandleInformation for stdout:  " +
			       getErrorText());
    }

    // Create a pipe for the child process's STDIN. 
    if (! CreatePipe(&childStdInAtChild,
		     &childStdInAtParent,
		     &securityAttributes,
		     bufferSize)) {
      throw std::runtime_error("Error from CreatePipe for stdin:  " +
			       getErrorText());
    }

    // Ensure the write handle to the pipe for STDIN is not inherited. 
    if (!SetHandleInformation(childStdInAtParent,
			      HANDLE_FLAG_INHERIT,
			      inheritFlag)) {
      throw std::runtime_error("Error from GetHandleInformation for stdin:  " +
			       getErrorText());
    }
 
    // Set up members of the PROCESS_INFORMATION structure. 
    PROCESS_INFORMATION processInfo; 

    std::fill((char *) &processInfo,
	      ((char *) &processInfo) + sizeof(PROCESS_INFORMATION),
	      0);
 
    // Set up members of the STARTUPINFO structure.  This structure
    // specifies the STDIN and STDOUT handles for redirection.
    STARTUPINFO startInfo;

    std::fill((char *) &startInfo,
	      ((char *) &startInfo) + sizeof(STARTUPINFO),
	      0);
    startInfo.cb = sizeof(STARTUPINFO); 
    startInfo.hStdError = childStdOutAtChild;
    startInfo.hStdOutput = childStdOutAtChild;
    startInfo.hStdInput = childStdInAtChild;
    startInfo.dwFlags |= STARTF_USESTDHANDLES;
 
    // Assemble the command line.
    std::string cmdline;

    if (cmdArgs.size() == 1) {
      cmdline = cmdArgs[0];
    } else {

      // Append all but last command-line arguments.
      for (size_t i = 0; i < cmdArgs.size() - 1; i++) {
	cmdline += cmdArgs[i] + " ";
      }
      cmdline += cmdArgs.back(); // Append last command-line argument.
    }

    // Create the child process. 
    bool status;
 
    status = CreateProcess(NULL, 
			   (char *) cmdline.c_str(),
			   NULL,          // process security attributes 
			   NULL,          // primary thread security attributes 
			   true,          // handles are inherited 
			   0,             // creation flags 
			   NULL,          // use parent's environment 
			   NULL,          // use parent's current directory 
			   &startInfo,    // STARTUPINFO pointer 
			   &processInfo); // receives PROCESS_INFORMATION 

    // If an error occurs, exit the application. 
    if (!status ) {
      throw std::runtime_error("Error from CreateProcess with "
			       "command line \"" + cmdline + "\":  " +
			       getErrorText());
    }

    // Provide the stream buffers with the handles for communicating
    // with the child process.
    childStreambuf.setInputHandle(childStdOutAtParent);
    childStreambuf.setOutputHandle(childStdInAtParent);

    // Save the handles to the child process and its primary thread.
    childProcess = processInfo.hProcess;
    childThread = processInfo.hThread;

    // Close the child's end of the pipes.
    if (!CloseHandle(childStdOutAtChild)) {
      throw std::runtime_error("Error closing the child process "
			       "stdout handle:  " + getErrorText());
    }
     
    if (!CloseHandle(childStdInAtChild)) {
      throw std::runtime_error("Error closing the child process "
			       "stdin handle:  " + getErrorText());
    }

#else

    // Create the pipes for the stdin and stdout.
    int childStdInPipe[2];
    int childStdOutPipe[2];

    if (pipe(childStdInPipe) != 0) {
      throw std::runtime_error("Error creating pipe for stdin:  " +
			       getErrorText());
    }
    if (pipe(childStdOutPipe) != 0) {
      ::close(childStdInPipe[0]);
      ::close(childStdInPipe[1]);
      throw std::runtime_error("Error creating pipe for stdout:  " +
			       getErrorText());
    }

    // Create the child process. 
    childPid = fork();
    if (-1 == childPid) {
      for (int i = 0; i < 2; i++) {
	::close(childStdInPipe[i]);
	::close(childStdOutPipe[i]);
      }
      throw std::runtime_error("Error creating child process:  " +
			       getErrorText());
    }
 
    if (0 == childPid) {

      // The child executes this.  Redirect stdin.
      if (dup2(childStdInPipe[0], STDIN_FILENO) == -1) {
	std::string errMsg;

	// Send message to parent.
	errMsg = "Error redirecting stdin in the child:  " + getErrorText();
	::write(childStdOutPipe[1], errMsg.data(), errMsg.size());
	exit(-1);
      }

      // Redirect stdout.
      if (dup2(childStdOutPipe[1], STDOUT_FILENO) == -1) {
	std::string errMsg;

	// Send message to parent.
	errMsg = "Error redirecting stdout in the child:  " + getErrorText();
	::write(childStdOutPipe[1], errMsg.data(), errMsg.size());
	exit(-1);
      }

      // Close pipes.
      if ( (::close(childStdInPipe[0]) != 0) ||
	   (::close(childStdInPipe[1]) != 0) ||
	   (::close(childStdOutPipe[0]) != 0) ||
	   (::close(childStdOutPipe[1]) != 0) ) {
	std::string errMsg;

	// Send message to parent.
	errMsg = "Error closing the pipes in the child:  " + getErrorText();
	::write(STDOUT_FILENO, errMsg.data(), errMsg.size());
	exit(-1);

      }

      // Prepare the arguments.
      std::vector<const char *> execvArgv;

      for (auto &arg : cmdArgs) {
	execvArgv.push_back(arg.c_str());
      }
      execvArgv.push_back((char *) NULL);

      // Run the child process image.
      (void) execv(execvArgv[0], (char * const *) &(execvArgv[0]));

      // Error from exec.
      std::string errMsg;

      // Send message to parent.
      errMsg = "Error from exec:  " + getErrorText();
      ::write(STDOUT_FILENO, errMsg.data(), errMsg.size());
      exit(-1);

    }

    // Provide the stream buffers with the file descriptors for
    // communicating with the child process.
    childStreambuf.setInputFileDescriptor(childStdOutPipe[0]);
    childStreambuf.setOutputFileDescriptor(childStdInPipe[1]);

    // Close the child's end of the pipes.
    if ( (::close(childStdInPipe[0]) != 0) ||
	 (::close(childStdOutPipe[1]) != 0) ) {
      std::string errMsg;

      throw std::runtime_error("Error closing child's end of pipes in "
			       "the parent:  " + getErrorText());
    }

#endif

    childStarted = true;

  }

  void ChildStream::close() {

    if (!childStarted) {
      throw std::logic_error("Child process was not started "
			     "when close() was called");
    }

    // Terminate the child if it's running.
    if (isRunning()) {

      std::string errorString;
      bool errorOccurred = false;

#ifdef _WIN32
      if (!TerminateProcess(childProcess, terminateExitCode)) {
        errorString += (errorOccurred ? "\n" : "");
        errorString += "Error terminating the child process:  " +
          getErrorText();
        errorOccurred = true;
      }
#else
      if (kill(childPid, SIGKILL) != 0) {
        errorString += (errorOccurred ? "\n" : "");
        errorString += "Error terminating the child process:  " +
          getErrorText();
        errorOccurred = true;
      } else if (waitpid(childPid, NULL, 0) != childPid) {
        errorString += (errorOccurred ? "\n" : "");
        errorString += "Error waiting for the child process:  " +
          getErrorText();
        errorOccurred = true;
      }
#endif

      if (errorOccurred) {
        throw std::runtime_error(errorString);
      }

    }

    // Reset.
    init();

  }

  bool ChildStream::isDone() {

    if (childExited) {

      return true;

    }

    if (!childStarted) {
      throw std::logic_error("Child process was not started "
			     "when isDone() was called");
    }

    int result;

#ifdef _WIN32
    if (!GetExitCodeProcess(childProcess, (LPDWORD) &result)) {
      throw std::runtime_error("Error checking status of child process:  " +
			       getErrorText());
    }

    if (STILL_ACTIVE == result) {

      return false;

    }

    // Child process has exited.  Save the exit code.
    exitCode = result;
    exitCodeObtainedFlag = true;

#else
    int status = 0;

    result = waitpid(childPid, &status, WNOHANG);
    if (-1 == result) {
      throw std::runtime_error("Error checking status of child process:  " +
			       getErrorText());
    } else if (0 == result) {

      // Child is still running.
      return false;

    }

    if (WIFEXITED(status)) {

      // Child exited normally.
      exitCode = WEXITSTATUS(status);
      exitCodeObtainedFlag = true;

    }

#endif

    childExited = true;

    return true;

  }

  int32_t ChildStream::result() {

    if (exitCodeObtainedFlag) {

      return exitCode;

    }

    // Check whether the process is running, and get the exit code.
    if (!isDone()) {
      throw std::logic_error("Child process was still running "
			     "when result() was called");
    }

    // Child process has exited.
    if (!exitCodeObtainedFlag) {

      // Exit code is not available.
      throw std::runtime_error("Child process has exited but the exit "
			       "code is not available");

    }

    return exitCode;

  }

  std::string ChildStream::getErrorText() {

#ifdef _WIN32
    LPVOID winMsgBuf;
    DWORD lastError = GetLastError(); 

    FormatMessage(
		  FORMAT_MESSAGE_ALLOCATE_BUFFER | 
		  FORMAT_MESSAGE_FROM_SYSTEM |
		  FORMAT_MESSAGE_IGNORE_INSERTS,
		  NULL,
		  lastError,
		  MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
		  (char *) &winMsgBuf,
		  0, NULL );

    std::string errMsg((char *) winMsgBuf);

    LocalFree(winMsgBuf);
    return errMsg;
#else
    return strerror(errno);
#endif
  }

  ChildStream::ChildStreambuf::ChildStreambuf(std::size_t bufSize) :
#ifdef _WIN32
    inputHandle(0),
    outputHandle(0),
#else
    inputFileDescriptor(-1),
    outputFileDescriptor(-1),
#endif
    bufferSize(bufSize),
    readBuffer(bufferSize + 1),
    writeBuffer(bufferSize + 1) {

    // Read buffer initialization.
    char *end = &(readBuffer.front()) + readBuffer.size();

    // Indicate to underflow that underflow has not been called.
    setg(end, end, end);

    // Write buffer initialization.
    char *base = &(writeBuffer.front());

    // Indicate to overflow that overflow has not been called.
    setp(base, base + writeBuffer.size() - 1);

  }

#ifdef _WIN32
  void ChildStream::ChildStreambuf::setInputHandle(HANDLE inHandle) {

    inputHandle = inHandle;

  }
#else
  void ChildStream::ChildStreambuf::setInputFileDescriptor(int inFd) {

    inputFileDescriptor = inFd;

  }
#endif

  size_t ChildStream::ChildStreambuf::numBytesAvailable() const {

    size_t nBytesAvailable =  egptr() - gptr();

#ifdef _WIN32
    DWORD lpTotalBytesAvail;

    if (!PeekNamedPipe(inputHandle,
		       NULL,
		       0,
		       NULL,
		       &lpTotalBytesAvail,
		       NULL)) {
      throw std::runtime_error("Error from PeekNamedPipe:  " +
			       getErrorText());
    }

    if (lpTotalBytesAvail > 0) {
      nBytesAvailable++;
    }
#else
    fd_set readFd;
    int retVal;
    struct timeval timeout = {0, 0};

    FD_ZERO(&readFd);
    FD_SET(inputFileDescriptor, &readFd);

    // Check if input is available.
    retVal = select(inputFileDescriptor + 1, &readFd, NULL, NULL, &timeout);

    if (-1 == retVal) {
      throw std::runtime_error("Error from select(): " + getErrorText());
    } else if (retVal > 0) {
	nBytesAvailable++;
    }
#endif

    return nBytesAvailable;

  }

  std::streambuf::int_type ChildStream::ChildStreambuf::underflow() {

    // Check for empty buffer.
    if (gptr() < egptr()) {

      // Not empty.
      return traits_type::to_int_type(*gptr());

    }

    // Need to fill the buffer.
    char *base = &(readBuffer.front());
    char *start = base;

    // Check whether this is the first fill.
    if (eback() == base) {

      // Not the first fill.  Copy one putback character.
      *(eback()) = *(egptr() - 1);
      start++;
    }

    // start points to the start of the buffer.  Fill buffer.
#ifdef _WIN32
    DWORD nBytesRead;

    if (!ReadFile(inputHandle,
		  start,
		  readBuffer.size() - (start - base),
		  &nBytesRead,
		  NULL)) {
      return traits_type::eof();
    }
#else
    ssize_t nBytesRead;

    nBytesRead = ::read(inputFileDescriptor,
                        start,
                        readBuffer.size() - (start - base));
    if (-1 == nBytesRead) {
      return traits_type::eof();
    }
#endif

    // Check for EOF.
    if (0 == nBytesRead) {
      return traits_type::eof();
    }

    // Update buffer pointers.
    setg(base, start, start + nBytesRead);

    return traits_type::to_int_type(*gptr());

  }

#ifdef _WIN32
  void ChildStream::ChildStreambuf::setOutputHandle(HANDLE outHandle) {

    outputHandle = outHandle;

  }
#else
  void ChildStream::ChildStreambuf::setOutputFileDescriptor(int outFd) {

    outputFileDescriptor = outFd;

  }
#endif

  void ChildStream::ChildStreambuf::flushBuffer() {

    // Write.
    std::ptrdiff_t nBytes = pptr() - pbase();

#ifdef _WIN32
    DWORD nBytesWritten;

    if (!WriteFile(outputHandle,
		   pbase(),
		   nBytes,
		   &nBytesWritten,
		   NULL)) {
      // Clear the output buffer.
      pbump(-nBytes);
      throw std::runtime_error("Error writing to child process:  " +
			       getErrorText());
    }
#else
    ssize_t nBytesWritten;

    nBytesWritten = ::write(outputFileDescriptor, pbase(), nBytes);

#endif

    // Clear the output buffer.
    pbump(-nBytes);

    if (nBytes != nBytesWritten) {
      throw std::runtime_error("Not all data was written to to child "
			       "process:  " + getErrorText());
    }

    return;

  }

  std::streambuf::int_type ChildStream::ChildStreambuf::overflow(int_type ch) {

    // Check whether we're writing EOF.
    if (traits_type::eof() != ch) {

      // Not writing EOF.
      *(pptr()) = ch;
      pbump(1);

      // Write.
      flushBuffer();

      // Success.
      return ch;

    }

    return traits_type::eof();

  }

  int ChildStream::ChildStreambuf::sync() {

    flushBuffer(); // Throws exception on failure.

    // Success.
    return 1;

  }

  Child::CircularBuffer::CircularBuffer(size_t maxSize) :
    buffer(maxSize + 1),
    capacity(maxSize) {

      iBegin = 0;
      iEnd = 0;
  }

  bool Child::CircularBuffer::empty() const {
    return (iBegin == iEnd);
  }

  size_t Child::CircularBuffer::nUsed() const {
    return capacity - nFree();
  }

  void Child::CircularBuffer::clear() {
    iBegin = 0;
    iEnd = 0;
  }

  size_t Child::CircularBuffer::nFree() const {

    size_t iLocalBegin = iBegin;

    // Correct an invalid value.
    if (iLocalBegin >= capacity + 1) {
      iLocalBegin = 0;
    }

    if (iLocalBegin <= iEnd) {
      return capacity - (iEnd - iLocalBegin);
    }
    return iLocalBegin - iEnd - 1;

  }

  void Child::CircularBuffer::put(char const *ptr, size_t nBytes) {

    for (size_t i = 0; i < nBytes; i++) {
      buffer[iEnd] = *ptr;
      ptr++;
      nextIndex(iEnd);
    }

  }

  void Child::CircularBuffer::getAndErase(std::string &buf, size_t nBytes) {

    if ( (0 == nBytes) || (nBytes > nUsed()) ) {
      nBytes = nUsed();
    }

    buf.clear();
    if (buf.capacity() < nBytes) {
      buf.reserve(nBytes);
    }

    for (size_t i = 0; i < nBytes; i++) {
      buf.push_back(buffer[iBegin]);
      nextIndex(iBegin);
    }

  }

  Child::Child(std::vector<std::string> const &args,
               size_t bufSize,
               std::uint16_t nominalAboveMinPollTime_ms,
               std::uint16_t deltaPollTime_ms) :
    bufferCapacity(bufSize),
    readBuffer(bufferCapacity),
    writeBuffer(bufferCapacity),
    nominalPollTime_ms(nominalAboveMinPollTime_ms +
                       CodeDweller::MinimumSleeperTime),
    maximumPollTime_ms(nominalPollTime_ms + deltaPollTime_ms) {

    init();
    open(args);
  }

  Child::Child(std::string const &childpath,
               size_t bufSize,
               std::uint16_t nominalAboveMinPollTime_ms,
               std::uint16_t deltaPollTime_ms) :
    bufferCapacity(bufSize),
    readBuffer(bufferCapacity),
    writeBuffer(bufferCapacity),
    nominalPollTime_ms(nominalAboveMinPollTime_ms +
                       CodeDweller::MinimumSleeperTime),
    maximumPollTime_ms(nominalPollTime_ms + deltaPollTime_ms) {

    init();
    open(childpath);
  }

  Child::Child(size_t bufSize,
               std::uint16_t nominalAboveMinPollTime_ms,
               std::uint16_t deltaPollTime_ms) :
    bufferCapacity(bufSize),
    readBuffer(bufferCapacity),
    writeBuffer(bufferCapacity),
    nominalPollTime_ms(nominalAboveMinPollTime_ms +
                       CodeDweller::MinimumSleeperTime),
    maximumPollTime_ms(nominalPollTime_ms + deltaPollTime_ms) {

      init();
  }

  Child::~Child() {
    try {
      close();
    } catch (std::exception &e) {
      ;
    }
  }

  void Child::open(std::vector<std::string> const &args) {
    cmdArgs = args;

    if (isRunning()) {
      throw std::logic_error("The child process was already active.");
    }

    init();
    run();
  }

  void Child::open(std::string const &childpath) {

    if (isRunning()) {
      throw std::logic_error("The child process was already active.");
    }

    cmdArgs.clear();
    cmdArgs.push_back(childpath);
    init();
    run();
  }

  void Child::init() {

    childStarted = false;
    childExited = false;
    exitCodeObtainedFlag = false;
    exitCode = 0;

    readBuffer.clear();
    nWriteBytes = 0;
    nTransmitBytes = 0;

    threadsAreRunning = false;
    stopFlag = true;
    errorText.clear();

  }

  bool Child::write(std::string const &data) {

    if (!isRunning()) {
      throw std::logic_error("No child process is running.");
    }

    // The free space in the write buffer can be checked without
    // locking the mutex because:
    //
    //    1) bufferCapacity is unchanging, and
    //
    //    2) nWriteBytes is only decreased by the writer thread.
    //
    // This means that the calculated free space can only increase by
    // the action of the writer thread; the calculated free space is a
    // minimum value.  In the worst case, this method would return
    // false unnecessarily.
    if (data.size() > bufferCapacity - nWriteBytes) {
      return false;
    }

    std::lock_guard<std::mutex> lock(writeBufferMutex);

    std::copy(data.data(),
              data.data() + data.size(),
              &(writeBuffer[nWriteBytes]));

    nWriteBytes += data.size();

    return true;

  }

  size_t Child::writeAndShrink(std::string &data) {

    if (!isRunning()) {
      throw std::logic_error("No child process is running.");
    }

    // See the comment above regarding checking the free space in the
    // write buffer without locking the mutex.
    size_t nFree = bufferCapacity - nWriteBytes;

    if (0 == nFree) {
      return 0;
    }

    std::lock_guard<std::mutex> lock(writeBufferMutex);

    size_t nBytesToCopy = data.size();

    if (nBytesToCopy > nFree) {
      nBytesToCopy = nFree;
    }

    std::copy(data.data(),
              data.data() + nBytesToCopy,
              &(writeBuffer[nWriteBytes]));
    nWriteBytes += nBytesToCopy;

    data.erase(0, nBytesToCopy);

    return nBytesToCopy;

  }

  bool Child::isFinishedWriting() const {
    return ( (0 == nWriteBytes) &&
             (0 == nTransmitBytes) );
  }

  size_t Child::read(std::string &data, size_t nBytes) {

    if (!isRunning()) {
      throw std::logic_error("No child process is running.");
    }

    data.clear();

    // Whether the read circular buffer is empty can be checked
    // without locking the mutex because:
    //
    if (readBuffer.empty()) {
      return 0;
    }

    std::lock_guard<std::mutex> lock(readBufferMutex);

    size_t nBytesToRead = nBytes;

    if (nBytesToRead > readBuffer.nUsed()) {
      nBytesToRead = readBuffer.nUsed();
    }
    
    readBuffer.getAndErase(data, nBytesToRead);

    return data.size();

  }

  void Child::close() {

    if (!childStarted) {
      throw std::logic_error("Child process was not started "
			     "when close() was called");
    }

    // Stop the reader and writer threads.  Note: None of the error
    // conditions that cause an exception to be thrown by join()
    // can ever occur.
    stopFlag = true;

    // Terminate the child if it's running.
    if (isRunning()) {

      std::string errorString;
      bool errorOccurred = false;

#ifdef _WIN32
      if (!CloseHandle(inputHandle)) {
        errorString = "Error closing input from the child:  " + getErrorText();
        errorOccurred = true;
      }
      if (!CloseHandle(outputHandle)) {
        errorString += (errorOccurred ? "\n" : "");
        errorString += "Error closing output to the child:  " + getErrorText();
        errorOccurred = true;
      }

      if (!TerminateProcess(childProcess, terminateExitCode)) {
        errorString += (errorOccurred ? "\n" : "");
        errorString += "Error terminating the child process:  " +
          getErrorText();
        errorOccurred = true;
      }
#else
      if (0 != ::close(inputFileDescriptor)) {
        errorString = "Error closing input from the child:  " + getErrorText();
        errorOccurred = true;
      }
      if (0 != ::close(outputFileDescriptor)) {
        errorString += (errorOccurred ? "\n" : "");
        errorString += "Error closing output to the child:  " + getErrorText();
        errorOccurred = true;
      }

      if (kill(childPid, SIGKILL) != 0) {
        errorString += (errorOccurred ? "\n" : "");
        errorString += "Error terminating the child process:  " +
          getErrorText();
        errorOccurred = true;
      } else if (waitpid(childPid, NULL, 0) != childPid) {
        errorString += (errorOccurred ? "\n" : "");
        errorString += "Error waiting for the child process:  " +
          getErrorText();
        errorOccurred = true;
      }
#endif

      if (errorOccurred) {
        throw std::runtime_error(errorString);
      }

    }

    if (threadsAreRunning) {

      readerThread.join();
      writerThread.join();
      threadsAreRunning = false;

    }

    // Reset.
    init();

  }

  bool Child::isRunning() const {
    return childStarted && !childExited;
  }

  bool Child::errorOccurred(std::string &errorDescription) const {
    errorDescription = errorText;
    return !errorDescription.empty();
  }

  void Child::run() {

    if (childStarted) {
      throw std::logic_error("Child process was active when "
			     "run() was called");
    }

    if (cmdArgs.empty()) {
      throw std::invalid_argument("A child executable must be specified.");
    }

#ifdef _WIN32
    // Set the bInheritHandle flag so pipe handles are inherited. 
    SECURITY_ATTRIBUTES securityAttributes; 

    securityAttributes.nLength = sizeof(SECURITY_ATTRIBUTES); 
    securityAttributes.bInheritHandle = true; 
    securityAttributes.lpSecurityDescriptor = NULL; 

    // Create a pipe for the child process's STDOUT.
    HANDLE childStdOutAtChild;
    HANDLE childStdOutAtParent;
    HANDLE childStdInAtChild;
    HANDLE childStdInAtParent;
    int bufferSize = 0;

    if (!CreatePipe(&childStdOutAtParent,
		    &childStdOutAtChild,
		    &securityAttributes,
		    bufferSize)) {
      throw std::runtime_error("Error from CreatePipe for stdout:  " +
			       getErrorText());
    }

    // Ensure the read handle to the pipe for STDOUT is not inherited.
    int inheritFlag = 0;
    if (!SetHandleInformation(childStdOutAtParent,
			      HANDLE_FLAG_INHERIT,
			      inheritFlag) ) {
      throw std::runtime_error("Error from GetHandleInformation for stdout:  " +
			       getErrorText());
    }

    // Create a pipe for the child process's STDIN. 
    if (! CreatePipe(&childStdInAtChild,
		     &childStdInAtParent,
		     &securityAttributes,
		     bufferSize)) {
      throw std::runtime_error("Error from CreatePipe for stdin:  " +
			       getErrorText());
    }

    // Ensure the write handle to the pipe for STDIN is not inherited. 
    if (!SetHandleInformation(childStdInAtParent,
			      HANDLE_FLAG_INHERIT,
			      inheritFlag)) {
      throw std::runtime_error("Error from GetHandleInformation for stdin:  " +
			       getErrorText());
    }
 
    // Set up members of the PROCESS_INFORMATION structure. 
    PROCESS_INFORMATION processInfo; 

    std::fill((char *) &processInfo,
	      ((char *) &processInfo) + sizeof(PROCESS_INFORMATION),
	      0);
 
    // Set up members of the STARTUPINFO structure.  This structure
    // specifies the STDIN and STDOUT handles for redirection.
    STARTUPINFO startInfo;

    std::fill((char *) &startInfo,
	      ((char *) &startInfo) + sizeof(STARTUPINFO),
	      0);
    startInfo.cb = sizeof(STARTUPINFO); 
    startInfo.hStdError = childStdOutAtChild;
    startInfo.hStdOutput = childStdOutAtChild;
    startInfo.hStdInput = childStdInAtChild;
    startInfo.dwFlags |= STARTF_USESTDHANDLES;
 
    // Assemble the command line.
    std::string cmdline;

    if (cmdArgs.size() == 1) {
      cmdline = cmdArgs[0];
    } else {

      // Append all but last command-line arguments.
      for (size_t i = 0; i < cmdArgs.size() - 1; i++) {
	cmdline += cmdArgs[i] + " ";
      }
      cmdline += cmdArgs.back(); // Append last command-line argument.
    }

    // Create the child process. 
    bool status;
 
    status = CreateProcess(NULL, 
			   (char *) cmdline.c_str(),
			   NULL,          // process security attributes 
			   NULL,          // primary thread security attributes 
			   true,          // handles are inherited 
			   0,             // creation flags 
			   NULL,          // use parent's environment 
			   NULL,          // use parent's current directory 
			   &startInfo,    // STARTUPINFO pointer 
			   &processInfo); // receives PROCESS_INFORMATION 

    // If an error occurs, exit the application. 
    if (!status ) {
      throw std::runtime_error("Error from CreateProcess with "
			       "command line \"" + cmdline + "\":  " +
			       getErrorText());
    }

    // Provide the stream buffers with the handles for communicating
    // with the child process.
    inputHandle = childStdOutAtParent;
    outputHandle = childStdInAtParent;

    // Save the handles to the child process and its primary thread.
    childProcess = processInfo.hProcess;
    childThread = processInfo.hThread;

    // Close the child's end of the pipes.
    if (!CloseHandle(childStdOutAtChild)) {
      throw std::runtime_error("Error closing the child process "
			       "stdout handle:  " + getErrorText());
    }
     
    if (!CloseHandle(childStdInAtChild)) {
      throw std::runtime_error("Error closing the child process "
			       "stdin handle:  " + getErrorText());
    }

#else

    // Create the pipes for the stdin and stdout.
    int childStdInPipe[2];
    int childStdOutPipe[2];

    if (pipe(childStdInPipe) != 0) {
      throw std::runtime_error("Error creating pipe for stdin:  " +
			       getErrorText());
    }
    if (pipe(childStdOutPipe) != 0) {
      ::close(childStdInPipe[0]);
      ::close(childStdInPipe[1]);
      throw std::runtime_error("Error creating pipe for stdout:  " +
			       getErrorText());
    }

    // Create the child process. 
    childPid = fork();
    if (-1 == childPid) {
      for (int i = 0; i < 2; i++) {
	::close(childStdInPipe[i]);
	::close(childStdOutPipe[i]);
      }
      throw std::runtime_error("Error creating child process:  " +
			       getErrorText());
    }
 
    if (0 == childPid) {

      // The child executes this.  Redirect stdin.
      if (dup2(childStdInPipe[0], STDIN_FILENO) == -1) {
	std::string errMsg;

	// Send message to parent.
	errMsg = "Error redirecting stdin in the child:  " + getErrorText();
	::write(childStdOutPipe[1], errMsg.data(), errMsg.size());
	exit(-1);
      }

      // Redirect stdout.
      if (dup2(childStdOutPipe[1], STDOUT_FILENO) == -1) {
	std::string errMsg;

	// Send message to parent.
	errMsg = "Error redirecting stdout in the child:  " + getErrorText();
	::write(childStdOutPipe[1], errMsg.data(), errMsg.size());
	exit(-1);
      }

      // Close pipes.
      if ( (::close(childStdInPipe[0]) != 0) ||
	   (::close(childStdInPipe[1]) != 0) ||
	   (::close(childStdOutPipe[0]) != 0) ||
	   (::close(childStdOutPipe[1]) != 0) ) {
	std::string errMsg;

	// Send message to parent.
	errMsg = "Error closing the pipes in the child:  " + getErrorText();
	::write(STDOUT_FILENO, errMsg.data(), errMsg.size());
	exit(-1);

      }

      // Prepare the arguments.
      std::vector<const char *> execvArgv;

      for (auto &arg : cmdArgs) {
	execvArgv.push_back(arg.c_str());
      }
      execvArgv.push_back((char *) NULL);

      // Run the child process image.
      (void) execv(execvArgv[0], (char * const *) &(execvArgv[0]));

      // Error from exec.
      std::string errMsg;

      // Send message to parent.
      errMsg = "Error from exec:  " + getErrorText();
      ::write(STDOUT_FILENO, errMsg.data(), errMsg.size());
      exit(-1);

    }

    // Provide the stream buffers with the file descriptors for
    // communicating with the child process.
    inputFileDescriptor = childStdOutPipe[0];
    outputFileDescriptor = childStdInPipe[1];

    // Close the child's end of the pipes.
    if ( (::close(childStdInPipe[0]) != 0) ||
	 (::close(childStdOutPipe[1]) != 0) ) {
      std::string errMsg;

      throw std::runtime_error("Error closing child's end of pipes in "
			       "the parent:  " + getErrorText());
    }

#endif

    childStarted = true;

    // Start the reader and writer threads.
    stopFlag = false;

    try {

      std::thread readerTemp(&Child::readFromChild, this);
      readerThread = std::move(readerTemp);

    } catch (std::exception &e) {

      throw std::runtime_error("Error starting reader thread:  " +
                               getErrorText());

    }

    try {

      std::thread writerTemp(&Child::writeToChild, this);
      writerThread = std::move(writerTemp);

    } catch (std::exception &e) {

      stopFlag = true;
      readerThread.join();

      throw std::runtime_error("Error starting writer thread:  " +
                               getErrorText());

    }

    threadsAreRunning = true;

  }

  bool Child::isDone() {

    if (childExited) {

      return true;

    }

    if (!childStarted) {
      throw std::logic_error("Child process was not started "
			     "when isDone() was called");
    }

    int result;

#ifdef _WIN32
    if (!GetExitCodeProcess(childProcess, (LPDWORD) &result)) {
      throw std::runtime_error("Error checking status of child process:  " +
			       getErrorText());
    }

    if (STILL_ACTIVE == result) {

      return false;

    }

    // Child process has exited.  Save the exit code.
    exitCode = result;
    exitCodeObtainedFlag = true;

#else
    int status = 0;

    result = waitpid(childPid, &status, WNOHANG);
    if (-1 == result) {
      throw std::runtime_error("Error checking status of child process:  " +
			       getErrorText());
    } else if (0 == result) {

      // Child is still running.
      return false;

    }

    if (WIFEXITED(status)) {

      // Child exited normally.
      exitCode = WEXITSTATUS(status);
      exitCodeObtainedFlag = true;

    }

#endif

    childExited = true;

    // Stop threads.
    stopFlag = true;
    readerThread.join();
    writerThread.join();
    threadsAreRunning = false;

    return true;

  }

  int32_t Child::result() {

    if (exitCodeObtainedFlag) {

      return exitCode;

    }

    // Check whether the process is running, and get the exit code.
    if (!isDone()) {
      throw std::logic_error("Child process was still running "
			     "when result() was called");
    }

    // Child process has exited.
    if (!exitCodeObtainedFlag) {

      // Exit code is not available.
      throw std::runtime_error("Child process has exited but the exit "
			       "code is not available");

    }

    return exitCode;

  }

  std::string Child::getErrorText() {

#ifdef _WIN32
    LPVOID winMsgBuf;
    DWORD lastError = GetLastError(); 

    FormatMessage(
		  FORMAT_MESSAGE_ALLOCATE_BUFFER | 
		  FORMAT_MESSAGE_FROM_SYSTEM |
		  FORMAT_MESSAGE_IGNORE_INSERTS,
		  NULL,
		  lastError,
		  MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
		  (char *) &winMsgBuf,
		  0, NULL );

    std::string errMsg((char *) winMsgBuf);

    LocalFree(winMsgBuf);
    return errMsg;
#else
    return strerror(errno);
#endif
  }

  void Child::readFromChild() {

    std::vector<char> rxBuf(bufferCapacity);

    CodeDweller::PollTimer pollTimer(nominalPollTime_ms, maximumPollTime_ms);

    auto sleepTime = std::chrono::milliseconds(maximumPollTime_ms);

    while (!stopFlag) {

      char *bufferPtr;

      bufferPtr = &(rxBuf[0]);

      // Blocking read from the child.
#ifdef _WIN32
      DWORD nBytesRead;

      if (!ReadFile(inputHandle,
                    bufferPtr,
                    bufferCapacity,
                    &nBytesRead,
                    NULL)) {

        if (stopFlag) {
          break;
        }

        errorText = "Error reading from the child process:  ";
        errorText += getErrorText();
        break;

      }
#else
      ssize_t nBytesRead;

      nBytesRead = ::read(inputFileDescriptor,
                          bufferPtr,
                          bufferCapacity);
      if (-1 == nBytesRead) {

        if (stopFlag) {
          break;
        }

        errorText = "Error reading from the child process:  ";
        errorText += getErrorText();
        break;

      } else if (0 == nBytesRead) {

        // EOF; child exited.
        break;

      }
#endif

      // Copy to the shared read buffer.
      while ((nBytesRead > 0) && !stopFlag) {

        int nBytesToPut, nBytesFree;

        nBytesToPut = nBytesRead;

        // Can be called in the reader thread without locking the
        // mutex.
        nBytesFree = readBuffer.nFree();

        if (nBytesToPut > nBytesFree) {
          nBytesToPut = nBytesFree;
        }

        if (nBytesToPut > 0) {
          std::lock_guard<std::mutex> lock(readBufferMutex);

          readBuffer.put(bufferPtr, nBytesToPut);
          bufferPtr += nBytesToPut;
          nBytesRead -= nBytesToPut;

          pollTimer.reset();

        } else {
          pollTimer.pause();
        }

      }

    }

  }

  void Child::writeToChild() {

    std::vector<char> localWriteBuffer(bufferCapacity);
    size_t nLocalWriteBytes;

    CodeDweller::PollTimer pollTimer(nominalPollTime_ms, maximumPollTime_ms);

    auto sleepTime = std::chrono::milliseconds(maximumPollTime_ms);

    while (!stopFlag) {

      char *bufferPtr;

      // Poll for data in the shared write buffer.
      while ((0 == nWriteBytes) && !stopFlag) {
        pollTimer.pause();
      }

      if (stopFlag) {
        goto exit;
      }

      // Copy from the shared write buffer.
      {
        std::lock_guard<std::mutex> lock(writeBufferMutex);

        localWriteBuffer.swap(writeBuffer);
        nLocalWriteBytes = nWriteBytes;
        nWriteBytes = 0;
      }

      if (stopFlag) {
        goto exit;
      }

      pollTimer.reset();

      // Blocking write to the child.
      bufferPtr = &(localWriteBuffer[0]);

      while (nLocalWriteBytes > 0) {

#ifdef _WIN32
        DWORD nBytesWritten;

        if (!WriteFile(outputHandle,
                       bufferPtr,
                       nLocalWriteBytes,
                       &nBytesWritten,
                       NULL)) {

          if (stopFlag) {
            goto exit;
          }

          errorText = "Error writing to the child process:  ";
          errorText += getErrorText();
          goto exit;

        }

        if (stopFlag) {
          goto exit;
        }
#else
        ssize_t nBytesWritten;

        nBytesWritten = ::write(outputFileDescriptor,
                                bufferPtr,
                                nLocalWriteBytes);

        if (stopFlag) {
          goto exit;
        }

        if (-1 == nBytesWritten) {

          if (ENOSPC != errno) {

            // Some error other than no space.
            errorText = "Error writing to the child process:  ";
            errorText += getErrorText();
            goto exit;

          }

        }
#endif

        nLocalWriteBytes -= nBytesWritten;
        bufferPtr += nBytesWritten;

      }

    }

  exit: return;

  }

}