FBB::Process

FBB::Process

libbobcat1-dev_2.00.1-x.tar.gz

2005-2008


FBB::Process(3bobcat)

FBB::Process(3bobcat)

libbobcat1-dev_2.00.1-x.tar.gz Running Child Processes

2005-2008

NAME

FBB::Process - Runs external programs

SYNOPSIS

#include <bobcat/process>
Linking option: -lbobcat

DESCRIPTION

The FBB::Process class offers an elaborate interface to external programs and/or scripts from a C++ program (so-called child-processes). The class offers an easy to use, stream-based interface to the child process's standard input, standard output and standard error streams.

Objects of the FBB::Process class use standard process-executing functions, like sh(1) and members of the execl(1) family to execute the child process, and allow shell-scripts to be executed as well.

The standard input, output and error streams of executed child processes may be accessed via their Process parent objects. Input expected by the child process may be inserted into the Process object, output generated by the child process may be extracted from the Process object. Process objects will not be able to access these streams when child processes themselves redirect their standard streams.

When using (output) redirection with the USE_SHELL path specification (see below for the path and IOMode specifications) the IGNORE_COUT IOMode (and possibly IGNORE_CERR) will normally be specified as well.

The same Process object may be used repeatedly to execute the same or different child processes in sequence. If a previously started child process is still active it will first be terminated. It is also possible (using the stop() member) to end a child process explicitly.

Programs to call as child processes may be specified using Process's constructors. They are not started by the constructor. To start a child process the start() members or the assignment operator may be used.

Some child processes continue until their standard input streams are exhausted. The close() member is provided to close such streams.

The class Process should not be used to construct daemons. To create daemon processes the FBB::Fork class can be used.

Command line arguments passed to child processes may be surrounded by double or single quotes. Arguments surrounded by double quotes will have their double quotes removed, interpreting any escape-sequences that may have been used within. Arguments surrounded by single quotes will have their single quotes removed, accepting their contents unmodified.

Child processes may be allowed a limited amount of time (in seconds) to complete. Alternatively, child processes may have no time limit imposed upon then. A child process will forcefully be terminated when its parent Process object goes out of scope. Since Process inherits from FBB::Fork it may use the latter class's member waitForChild() to prevent premature termination of a child process.

NAMESPACE

FBB
All constructors, members, operators and manipulators, mentioned in this man-page, are defined in the namespace FBB.

INHERITS FROM

FBB::Fork(3bobcat) (private),
FBB:IOStream(3bobcat)

ENUMERATIONS

enum ProcessType:
This enumeration has the following values:

enum IOMode:
The IOMode specification is used to define which of the standard streams used by child processes are accessed through the Process object. Sensible combinations may be formed using the bit_or operator. This enumeration defines the following values:

enum ChildOutput:
The ChildOutput enumeration defines values returned by the available() member (see below) indicating to which standard stream the child process has written information. This enumeration defines the following values:

The latter two values may be combined using the bit_or operator indicating that information on both standard streams is available.

TYPE

PROCESS PARAMETERS

When running a child process three process parameters may be specified: the child streams to access from the Process object (as an IOMode value); the way to locate or start the child program (as a ProcessType value); and the maximum time (in seconds) the child program is allowed to run.
Unless specified otherwise, all the child's standard streams (standard input, output and error) will be accessible from the Parent process; the PATH environment variable will not be used to locate the child program to be executed (often resulting in the requirement to provide an absolute path to the intended program) and the child processes will be allowed unlimited time to run.
Following the construction of a Process object all default parameter values may be modified. Process parameters may be altered for a single process or the general defaults may be modified. The setXXX() members (see below) may be used to change the default process parameters. When process parameters are specified otherwise they will be active for the next process only.

CONSTRUCTORS

The command that may be provided to the following constructors may be the (initial part of the) specification of an external program to run. When the program is eventually started it may start and end with a back-tick (`). The back-ticks will be removed just before the specified program is executed.

A child process is not started automatically following the object construction. A start() member or the assignment operator (see below) can be used to start the specified child process.

The class Process does not offer a copy constructor.

OVERLOADED OPERATORS

  • Process &operator<<(Type value):
    This operator will insert value into the child's standard input stream. I.e., the child process will read value from its standard input. A value of any type that can be inserted into an ostream can be inserted into a Process object. Nothing happens if the member is used when the child process has terminated. The behavior of this operator is undefined unless IOMode CIN was specified.

  • Process &operator>>(Type value):
    This operator will extract value from the child's standard output stream and optionally (if IOMode MERGE_COUT_CERR was specified) from the child's error stream. I.e., value may be extracted from Process objects. A value of any type that can be extracted from an istream can be extracted from a Process object. Nothing happens if the member is used when the child process has terminated. The behavior of this operator is undefined unless IOMode COUT or MERGE_COUT_CERR was specified.

  • Process &operator+=(std::string const &):
    This operator adds the provided std::string object to the command specification currenly stored in a Process object. The currently stored command specification may be redefined using the member setCommand() (see below). The member operator+=() does not add a separating blank space between the currently stored command specification and the text to append. It merely adds its right-hand side string to the command stored so far. It does not affect a currently running child process.

  • int operator=(std::string const &cmd):
    The operator=() member defines cmd as the stored command in a Process object. If the command starts and ends with a back-tick (`) then the back-ticks are removed, and the resulting contents are interpreted as a command to execute.
    Next it will call stop() (see below) to end any ongoing process followed by calling start() to execute the newly defined command using the current default process parameters. It returns stop()'s return value.

  • Process &operator()(iomode mode):
    This operator changes the IOMode of the next child process. It returns the Process object allowing constructions like
    
        process(COUT) = "/bin/cat"; 
            
    
    to start a new child process with the specified IOMode.

  • Process &operator()(iomode mode, ProcessType type):
    This operator changes the IOMode and ProcessType process parameters of the next child process.

  • Process &operator()(iomode mode, ProcessType type, size_t timeLimit):
    This operator changes all three process parameters of the next child process. Time limit 0 prevents a time limit from being imposed upon the next child process.

    )

    The default overloaded assignment operator is not available.

    MEMBERS

    EXAMPLE

    #include "../process"
    #include <string>
    #include <iostream>
    
    using namespace std;
    using namespace FBB;
    
    void prompt(char const *task)
    {
        cout << "Press Enter to start " << task << endl;
        cin.ignore(INT_MAX, '\n');
    }
    
    int main(int argc, char **argv)
    try
    {
        string line;
        Process process(Process::CIN | Process::COUT,
                        "/usr/bin/sha1sum");
    
        prompt("sha1sum");
        process.start();
        process << "Hello world\n";         // input to sha1sum
        process.close();
        process >> line;                    // retrieve the value
        cout << line << endl;
        process.stop();
    
        if (argc > 1)                       // sending an e-mail
        {
            cout << "Sending mail to " << argv[1] << endl;
            prompt("/usr/bin/mail");
            process.setCommand("/usr/bin/mail -s 'from Process' ");
            process += argv[1];
            process.start(Process::CIN);
            process << "This mail was sent by the process drive\n";
            process << "It consists of multiple lines of text\n";
            process.close();
            process.waitForChild();
        }
    
        prompt("5 seconds IO to /bin/cat");
        process.setTimeLimit(5);            // change time limit
        process = "/bin/cat";
        while (process.active())
        {
            cout << "? ";
            getline(cin, line);
            process << line << endl;        // to /bin/cat
            line.clear();
            if (!getline(process, line))    // from /bin/cat
                break;
            cout << "Received: " << line << endl;
        }
        cout << "/bin/cat forcefully terminated\n";
    
        process.setTimeLimit(0);
        for (size_t trial = 0; trial < 5; ++trial)
        {
            prompt("ls");
    
            process(Process::COUT) = "/bin/ls";
    
            cerr << process.str() << endl;
            size_t count = 0;
            while (getline(process, line))
                cout << ++count << ": " << line << endl;
        }
    }
    catch (Errno const &err)
    {
        cerr << "EXCEPTION CAUGHT: " << err.what() << endl;
        return 1;
    }
    catch (...)
    {
        cerr << "Unrecognized exception in main()\n";
        return 0;
    }
    

    FILES

    bobcat/process - defines the class interface

    SEE ALSO

    bobcat(7), execle(3), fork(3bobcat), iostream(3fork), sh(1)

    BUGS

    With the release of Bobcat 1.21.1 the class Process was completely rewritten. The new implementation, however, should not affect existing programs other than that Process will no longer impose a limited time-to-live upon child processes. The interface was enlarged, but this should not affect existing programs. The internal organization of the Process class has changed though, requiring recompilation of sources defining Process class type objects and linking dynamically to the Bobcat library.

    DISTRIBUTION FILES

    BOBCAT

    Bobcat is an acronym of `Brokken's Own Base Classes And Templates'.

    COPYRIGHT

    This is free software, distributed under the terms of the GNU General Public License (GPL).

    AUTHOR

    Frank B. Brokken (f.b.brokken@rug.nl).