Ravensburg, 10-02-21

`spawn` – Call an External Executable

This a sample to call an external application. It is usually done with a combination of fork and exec on *ix systems. Unfortunately, fork is not available in Microsoft's C API.

The glibmm provides the spawn functions which might be a portable alternative. However, there are some issues which have to be clarified before successful usage in professional application development:

What about the standard channels?
What about the environment?
What happens if called executable is not available?
What happend if called executable dies?

Solving these questions is especially interesting in MS Windows.

Source Code

spawn.cc
/** @file sample for call of an external executable * * Last CVS checkin: * $Date: $ * $Author: scheff $ / /***********************************************************************/ // standard C/C++ header: #include <iostream> #include <string> #include <vector> // Gnome header: #include <glibmm/miscutils.h> #include <glibmm/spawn.h> using namespace std; /**********************************************************************/ / provides an output operator for vector<string>. * * @param out the output stream * @param strv the string vector * @return the output stream / ostream& operator << (ostream &out, const vector<string> &strv) { if (!strv.empty()) { vector<string>::const_iterator it; for (it = strv.begin();;) { out << it; ++it; if (it != strv.end()) out << ' '; else break; } } return out; } /** waits for user confirm. * * @param action text with next action / static void pause(const char action) { cerr << "spawn: Press [ENTER] to " << action << ": " << flush; string str; getline(cin, str); } /** provides the main function of application. * * @param argc number of command line arguments * @param argv pointer to array of pointers to strings with command line * arguments * @return 0 ... application exited regularly\n * else ... execution of application aborted / int main(int argc, char argv[]) { cout << "spawn: start" << endl; // build file path #ifdef WIN32 string file; { gchar dir = g_win32_get_package_installation_directory_of_module(0); file = Glib::build_filename(dir, "test-spawn.exe"); g_free(dir); } #else // WIN32 string file = Glib::build_filename(Glib::get_current_dir(), "test-spawn"); #endif // WIN32 // spawn regular { vector<string> args; args.push_back(file); // spawn cout << "spawn: spawn_sync(" << args << "):" << endl; try { Glib::spawn_sync(".", args, Glib::SPAWN_CHILD_INHERITS_STDIN); } catch (const Glib::SpawnError &error) { cerr << "spawn: ERROR: " << error.what() << endl; } } pause("continue"); // spawn with missing app. { vector<string> args; // spawn cout << "spawn: spawn_sync(" << args << "):" << endl; try { Glib::spawn_sync(".", args, Glib::SPAWN_CHILD_INHERITS_STDIN); } catch (const Glib::SpawnError &error) { cerr << "spawn: ERROR: " << error.what() << endl; } } pause("continue"); // spawn with error return { vector<string> args; args.push_back(file); args.push_back("-return"); args.push_back("1"); // spawn cout << "spawn: spawn_sync(" << args << "):" << endl; try { int exitCode; Glib::spawn_sync(".", args, Glib::SPAWN_CHILD_INHERITS_STDIN, sigc::slot<void>(), 0, 0, &exitCode); cout << "spawn: Received exit code: " << exitCode << endl; } catch (const Glib::SpawnError &error) { cerr << "spawn: ERROR: " << error.what() << endl; } } pause("continue"); // spawn with exception { vector<string> args; args.push_back(file); args.push_back("-throw"); // spawn cout << "spawn: spawn_sync(" << args << "):" << endl; try { int exitCode; Glib::spawn_sync(".", args, Glib::SPAWN_CHILD_INHERITS_STDIN, sigc::slot<void>(), 0, 0, &exitCode); cout << "spawn: Received exit code: " << exitCode << endl; } catch (const Glib::SpawnError &error) { cerr << "spawn: ERROR: " << error.what() << endl; } } pause("continue"); // spawn with standard channels { vector<string> args; args.push_back(file); args.push_back("-nopause"); // spawn cout << "spawn: spawn_sync(" << args << "):" << endl; try { string output, error; int exitCode; Glib::spawn_sync(".", args, Glib::SpawnFlags(0), sigc::slot<void>(), &output, &error, &exitCode); cout << "spawn: Received on standard output:" << endl << output << endl; cout << "spawn: Received on standard error:" << endl << error << endl; cout << "spawn: Received exit code: " << exitCode << endl; } catch (const Glib::SpawnError &error) { cerr << "spawn: ERROR: " << error.what() << endl; } } #if 0 // This problem isn't solved, satisfying! pause("continue"); // spawn with merged standard output and error { vector<string> args; args.push_back(file); try { string output; int exitCode = 0; / This doesn't work: Glib::spawn_sync(".", args, Glib::SpawnFlags(0), sigc::slot<void>(), &output, &output, &exitCode); * Var. output receives only error channel! * Output channel gets lost. / / This doesn't work, also: Glib::spawn_sync(".", args, Glib::SpawnFlags(0), sigc::slot<void>(), &output, 0, &exitCode); * Var. output receives only output channel! * Error channel goes to console. */ cout << "spawn: Received on standard output:" << endl << output << endl; cout << "spawn: Received exit code: " << exitCode << endl; } catch (const Glib::SpawnError &error) { cerr << "spawn: ERROR: " << error.what() << endl; } } #endif // 0 // done pause("finish"); return 0; }
test-spawn.cc
/** @file sample which is called by spwan * * Last CVS checkin: * $Date: $ * $Author: scheff $ / /***********************************************************************/ // standard C/C++ header: #include <exception> #include <iostream> #include <string> // Gnome header: #include <glib/gtimer.h> #include <glibmm/timer.h> using namespace std; /***********************************************************************/ /// provides an exception for abort. class Abort: public exception { /// returns text with exception description. public: virtual const char what(void) const; }; /*************************************************************************/ const char Abort::what(void) const { return "Intentional abort thrown."; } /************************************************************************/ /// @name Settings & Options //@{ static double tWait = 3.0; ///< sleep time static int ret = 0; ///< main function return value static bool forceAbort = false; ///< flag: true ... throw Abort() static bool noPause = false; ///< flag: true ... skip user confirm //@} / waits for user confirm. * * @param action text with next action / static void pause(const char action) { if (!noPause) { cout << "test-spawn: Press [ENTER] to " << action << ": " << flush; string str; getline(cin, str); } } /** provides the main function of application. * * @param argc number of command line arguments * @param argv pointer to array of pointers to strings with command line * arguments * @return 0 ... application exited regularly\n * else ... execution of application aborted / int main(int argc, char argv[]) { cout << "test-spawn: start" << endl; // process options for (int i = 1; i < argc; ++i) { string arg = argv[i]; /* unused: if (arg == "-wait") { ++i; tWait = strtod(argv[i], 0); cout << "test-spawn: wait = " << tWait << endl; } else/ if (arg == "-return") { ++i; ret = atoi(argv[i]); cout << "test-spawn: return = " << ret << endl; } else if (arg == "-throw") { forceAbort = true; cout << "test-spawn: will abort" << endl; } else if (arg == "-nopause") noPause = true; } // write to console pause("continue"); cout << "test-spawn: Output to cout" << endl; cerr << "test-spawn: Output to cerr" << endl; cout << "test-spawn: Another output to cout" << endl; cerr << "test-spawn: Another output to cerr" << endl; // consume some time cout << "test-spawn: Waiting for " << tWait << " s..." << endl; Glib::usleep((gulong)(G_USEC_PER_SEC tWait)); // abort? if (forceAbort) throw Abort(); // done pause("finish"); return ret; }

spawn.cc

/** @file sample for call of an external executable
 *
 * Last CVS checkin:
 * $Date: $
 * $Author: scheff $
 */

/**************************************************************************/

// standard C/C++ header:
#include <iostream>
#include <string>
#include <vector>

// Gnome header:
#include <glibmm/miscutils.h>
#include <glibmm/spawn.h>

using namespace std;

/**************************************************************************/

/** provides an output operator for vector<string>.
 *
 * @param out the output stream
 * @param strv the string vector
 * @return the output stream
 */
ostream& operator << (ostream &out, const vector<string> &strv)
{
  if (!strv.empty()) {
    vector<string>::const_iterator it;
    for (it = strv.begin();;) {
      out << *it; ++it; if (it != strv.end()) out << ' '; else break;
    }
  }
  return out;
}

/** waits for user confirm.
 *
 * @param action text with next action
 */
static void pause(const char *action)
{
  cerr << "spawn: Press [ENTER] to " << action << ": " << flush;
  string str; getline(cin, str);
}

/** provides the main function of application.
 *
 * @param argc number of command line arguments
 * @param argv pointer to array of pointers to strings with command line
 *        arguments
 * @return 0 ... application exited regularly\n
 *         else ... execution of application aborted
 */
int main(int argc, char *argv[])
{
  cout << "spawn: start" << endl;
  // build file path
#ifdef WIN32
  string file;
  { gchar *dir = g_win32_get_package_installation_directory_of_module(0);
    file = Glib::build_filename(dir, "test-spawn.exe");
    g_free(dir);
  }
#else // WIN32
  string file
    = Glib::build_filename(Glib::get_current_dir(), "test-spawn");
#endif // WIN32
  // spawn regular
  { vector<string> args;
    args.push_back(file);
    // spawn
    cout << "spawn: spawn_sync(" << args << "):" << endl;
    try {
      Glib::spawn_sync(".", args,
        Glib::SPAWN_CHILD_INHERITS_STDIN);
    } catch (const Glib::SpawnError &error) {
      cerr << "spawn: ERROR: " << error.what() << endl;
    }
  }
  pause("continue");
  // spawn with missing app.
  { vector<string> args;
    // spawn
    cout << "spawn: spawn_sync(" << args << "):" << endl;
    try {
      Glib::spawn_sync(".", args,
        Glib::SPAWN_CHILD_INHERITS_STDIN);
    } catch (const Glib::SpawnError &error) {
      cerr << "spawn: ERROR: " << error.what() << endl;
    }
  }
  pause("continue");
  // spawn with error return
  { vector<string> args;
    args.push_back(file); args.push_back("-return"); args.push_back("1");
    // spawn
    cout << "spawn: spawn_sync(" << args << "):" << endl;
    try {
      int exitCode;
      Glib::spawn_sync(".", args,
        Glib::SPAWN_CHILD_INHERITS_STDIN,
        sigc::slot<void>(), 0, 0, &exitCode);
      cout << "spawn: Received exit code: " << exitCode << endl;
    } catch (const Glib::SpawnError &error) {
      cerr << "spawn: ERROR: " << error.what() << endl;
    }
  }
  pause("continue");
  // spawn with exception
  { vector<string> args;
    args.push_back(file); args.push_back("-throw");
    // spawn
    cout << "spawn: spawn_sync(" << args << "):" << endl;
    try {
      int exitCode;
      Glib::spawn_sync(".", args,
        Glib::SPAWN_CHILD_INHERITS_STDIN,
        sigc::slot<void>(), 0, 0, &exitCode);
      cout << "spawn: Received exit code: " << exitCode << endl;
    } catch (const Glib::SpawnError &error) {
      cerr << "spawn: ERROR: " << error.what() << endl;
    }
  }
  pause("continue");
  // spawn with standard channels
  { vector<string> args;
    args.push_back(file); args.push_back("-nopause");
    // spawn
    cout << "spawn: spawn_sync(" << args << "):" << endl;
    try {
      string output, error; int exitCode;
      Glib::spawn_sync(".", args,
        Glib::SpawnFlags(0),
        sigc::slot<void>(), &output, &error, &exitCode);
      cout << "spawn: Received on standard output:" << endl
	<< output << endl;
      cout << "spawn: Received on standard error:" << endl
	<< error << endl;
      cout << "spawn: Received exit code: " << exitCode << endl;
    } catch (const Glib::SpawnError &error) {
      cerr << "spawn: ERROR: " << error.what() << endl;
    }
  }
#if 0 // This problem isn't solved, satisfying!
  pause("continue");
  // spawn with merged standard output and error
  { vector<string> args;
    args.push_back(file);
    try {
      string output; int exitCode = 0;
      /* This doesn't work:
      Glib::spawn_sync(".", args, Glib::SpawnFlags(0),
        sigc::slot<void>(), &output, &output, &exitCode);
       * Var. output receives only error channel!
       * Output channel gets lost.
       */
      /* This doesn't work, also:
      Glib::spawn_sync(".", args, Glib::SpawnFlags(0),
        sigc::slot<void>(), &output, 0, &exitCode);
       * Var. output receives only output channel!
       * Error channel goes to console.
       */
      cout << "spawn: Received on standard output:" << endl
        << output << endl;
      cout << "spawn: Received exit code: " << exitCode << endl;
    } catch (const Glib::SpawnError &error) {
      cerr << "spawn: ERROR: " << error.what() << endl;
    }
  }
#endif // 0
  // done
  pause("finish");
  return 0;
}

test-spawn.cc

/** @file sample which is called by spwan
 *
 * Last CVS checkin:
 * $Date: $
 * $Author: scheff $
 */

/**************************************************************************/

// standard C/C++ header:
#include <exception>
#include <iostream>
#include <string>

// Gnome header:
#include <glib/gtimer.h>
#include <glibmm/timer.h>

using namespace std;

/**************************************************************************/

/// provides an exception for abort.
class Abort: public exception {
  /// returns text with exception description.
  public: virtual const char* what(void) const;
};

/**************************************************************************/

const char* Abort::what(void) const
{
  return "Intentional abort thrown.";
}

/**************************************************************************/

/// @name Settings & Options
//@{
static double tWait = 3.0; ///< sleep time
static int ret = 0; ///< main function return value
static bool forceAbort = false; ///< flag: true ... throw Abort()
static bool noPause = false; ///< flag: true ... skip user confirm
//@}

/** waits for user confirm.
 *
 * @param action text with next action
 */
static void pause(const char *action)
{
  if (!noPause) {
    cout << "test-spawn: Press [ENTER] to " << action << ": " << flush;
    string str; getline(cin, str);
  }
}

/** provides the main function of application.
 *
 * @param argc number of command line arguments
 * @param argv pointer to array of pointers to strings with command line
 *        arguments
 * @return 0 ... application exited regularly\n
 *         else ... execution of application aborted
 */
int main(int argc, char *argv[])
{
  cout << "test-spawn: start" << endl;
  // process options
  for (int i = 1; i < argc; ++i) {
    string arg = argv[i];
    /* unused:
    if (arg == "-wait") {
      ++i; tWait = strtod(argv[i], 0);
      cout << "test-spawn: wait = " << tWait << endl;
    } else*/ if (arg == "-return") {
      ++i; ret = atoi(argv[i]);
      cout << "test-spawn: return = " << ret << endl;
    } else if (arg == "-throw") {
      forceAbort = true;
      cout << "test-spawn: will abort" << endl;
    } else if (arg == "-nopause") noPause = true;
  }
  // write to console
  pause("continue");
  cout << "test-spawn: Output to cout" << endl;
  cerr << "test-spawn: Output to cerr" << endl;
  cout << "test-spawn: Another output to cout" << endl;
  cerr << "test-spawn: Another output to cerr" << endl;
  // consume some time
  cout << "test-spawn: Waiting for " << tWait << " s..." << endl;
  Glib::usleep((gulong)(G_USEC_PER_SEC * tWait));
  // abort?
  if (forceAbort) throw Abort();
  // done
  pause("finish");
  return ret;
}

Output

Console:
`spawn: start spawn: spawn_sync(c:\...\glibmm\Debug\test-spawn.exe): test-spawn: start test-spawn: Press [ENTER] to continue: ↵ test-spawn: Output to cout test-spawn: Output to cerr test-spawn: Another output to cout test-spawn: Another output to cerr test-spawn: Waiting for 3 s... test-spawn: Press [ENTER] to finish: ↵ spawn: Press [ENTER] to continue: ↵ spawn: spawn_sync():`
Snapshot

Console/Interaction:
click on the buttonspawn: ERROR: Lesen aus Weiterleitung zum Kind (EOF) fehlgeschlagen spawn: Press [ENTER] to continue: ↵ spawn: spawn_sync(c:\Dokumente und Einstellungen\scheff\Eigene Dateien\tests\gli bmm\Debug\test-spawn.exe -return 1): test-spawn: start test-spawn: return = 1 test-spawn: Press [ENTER] to continue: ↵ test-spawn: Output to cout test-spawn: Output to cerr test-spawn: Another output to cout test-spawn: Another output to cerr test-spawn: Waiting for 3 s... test-spawn: Press [ENTER] to finish: ↵ spawn: Received exit code: 1 spawn: Press [ENTER] to continue: ↵ spawn: spawn_sync(c:\Dokumente und Einstellungen\scheff\Eigene Dateien\tests\gli bmm\Debug\test-spawn.exe -throw): test-spawn: start test-spawn: will abort test-spawn: Press [ENTER] to continue: ↵ test-spawn: Output to cout test-spawn: Output to cerr test-spawn: Another output to cout test-spawn: Another output to cerr test-spawn: Waiting for 3 s...
Snapshot

Console/Interaction:
click on the button`spawn: Press [ENTER] to continue: ↵ spawn: spawn_sync(c:\Dokumente und Einstellungen\scheff\Eigene Dateien\tests\gli bmm\Debug\test-spawn.exe -nopause): spawn: Received on standard output: test-spawn: start test-spawn: Output to cout test-spawn: Another output to cout test-spawn: Waiting for 3 s... spawn: Received on standard error: test-spawn: Output to cerr test-spawn: Another output to cerr spawn: Received exit code: 0 spawn: Press [ENTER] to finish: ↵`

Conclusion

This time, I focused on usage of Glib::spawn_sync. Control of external application worked fine even in error situations. I'm not quite satisfied with the handling of standard output/error channnel. I was not able to (chronological) merge both channels in one string. Thus, further investigation is necessary. I'm afraid I have to use Gtk::spawn_async_with_pipes reading the channels "by myself".

Files

spawn.cc	C++ source code
spawn.vcproj	MS Visual C++ 2008 project file
test-spawn.cc	C++ source code
test-spawn.vcproj	MS Visual C++ 2008 project file

dirk/C++/glibmm/spawn

Last modified: Wed Feb 24 20:57:00 2010

spawn – Call an External Executable

Source Code

Output

Conclusion

Files

`spawn` – Call an External Executable