FBB::Fork(3bobcat)

Template Design Pattern around fork(2)
(libbobcat-dev_6.04.00)

2005-2023

NAME

SYNOPSIS

DESCRIPTION

Terminating child processes send SIGCHLD signals to their parents. The C library offers the following macros to analyze the status values received by the parent process using a wait(2) or waitpid(2) system call:

• int WIFEXITED(int status):
  This macro returns a nonzero value if the child process terminated normally with `exit' or `_exit'.
• int WEXITSTATUS(int status):
  If `WIFEXITED' is true of `status', this macro returns the low-order 8 bits of the exit status value from the child process.
• int WIFSIGNALED(int status):
  This macro returns a nonzero value if the child process terminated because it received a signal that was not handled.
• int WTERMSIG(int status):
  If `WIFSIGNALED' is true of `status', this macro returns the signal number of the signal that terminated the child process.
• int WCOREDUMP(int status):
  This macro returns a nonzero value if the child process terminated and produced a core dump.
• int WIFSTOPPED(int status):
  This macro returns a nonzero value if the child process is stopped.
• int WSTOPSIG(int status):
  If `WIFSTOPPED' is true of `status', this macro returns the signal number of the signal that caused the child process to stop.

NAMESPACE

INHERITS FROM

CONSTRUCTORS

Only the default constructor is available.

DESTRUCTOR

• virtual ~Fork():
  Derived classes may define their own destructor, which is called when the Fork destructor is activated.

MEMBER FUNCTIONS

• void fork():
  Performs the actual forking. It is implemented in such a way that the corresponding parent- and child- processes are activated from virtual members of Fork. If the forking operation fails, an FBB::Exception exception is thrown.

PROTECTED MEMBER FUNCTIONS

• int endChild() const:
  This member may be called by parentProcess to end the child process. To end the child process a SIGTERM is sent to the child process, followed by a SIGKILL (cf. signal(7)). If the child process has already ended then the child process's exit value is returned, otherwise the child process's end is forced (by calling kill) and -2 is returned. This member also calls waitForChild (see below).

• pid_t pid() const:
  Returns the child's process id in the parent's code (i.e., in the parent-members below), and 0 in the child's code (i.e., in the child-members below). The pid member returns -1 when called before the member fork has been called.

• void prepareDaemon() const:
  Prepares for a daemon childprocess. This function may (should) be called from childProcess to ensure that the child process changes its current working directory to the root (/) directory, thus freeing up mount points; that the child process starts a new session/process group to allow the parent (group leader) to kill all its processes without terminating the daemon; and makes sure that the child process closes and reopens the standard streams by associating them with /dev/null to prevent ghost input and output actions from interfering with the daemon's actions. An FBB::Exception is thrown if changing directory to the root directory fails.

• void prepareDaemon(std::string const &out, std::string const &err, mode_t mode = 0600) const:
  Prepares for a daemon childprocess like the previous member function, but allows redirection of the standard output (out) and standard error (err) streams to files. Specify empty strings to redirect these streams to /dev/null. With non-empty strings the specified files are opened in append-mode (and created if not yet existing), by default using mode 0600 (read/write mode for the user only). An FBB::Exception is thrown if changing directory to the root directory or if using the specified s fails.

• int waitForChild() const:
  This member may be called by parentProcess to wait for the completion of the child-process. The return value (exit-code) of the child process is returned as a value between 0 and 255. If the child process terminates before the completion of the parent process, then waitForChild should be called to prevent zombies from occurring. Alternatively, the parent process may terminate (e.g., using exit(2)) while the child process is still alive. This is the normal way to create a daemon process.

PRIVATE (VIRTUAL) MEMBER FUNCTIONS

• virtual void childProcess() = 0:
  This member must be implemented by derived classes. It defines the actions that are performed by the child process, following the fork(2) system call. Just before childProcess is called, childRedirections (see below) has been executed. The childProcess() function should terminate the child process. A good way to do this is to throw an exception which is caught by main()'s function try block. Terminating a process using exit(2) is deprecated in C++.

• virtual void childRedirections():
  This function may be redefined in derived classes to set up the redirections that are necessary to communicate with the parent process. See also the classes redirector(3bobcat) and pipe(3bobcat). By default, childRedirections does nothing.

• virtual void parentProcess() = 0:
  This member must be implemented by derived classes. It defines the actions that are performed by the parent process, following the fork(2) system call. Just before parentProcess is called, parentRedirections (see below) has been executed.

  When deriving classes from Fork their parentProcess implementation preferably handles all actions to perform by the parent process. If the child process remains active when the parent process decides that the program has performed its duties (e.g., the child process is replaced by a program started by an exec.. function, continuously producing output, interpreted by the parent process) then the parent process can call endChild to end the child process before ending the parentProcess function.

• virtual void parentRedirections():
  This function may be redefined in derived classes to set up the redirections that are necessary to communicate with, e.g., the parent. See, e.g., the classes redirector(3bobcat) and pipe(3bobcat). By default, parentRedirections does nothing.

EXAMPLES

#include <iostream>
#include <unistd.h>

#include <bobcat/fork>

using namespace std;
using namespace FBB;

class Background: public Fork
{
    public:
        void childProcess()     override;
        void parentProcess()    override;
};

void Background::childProcess()
{
    for (int idx = 0; idx < 3; ++idx)
    {
        cout << "Hello world # " << idx << endl;
        sleep(1);
    }
    throw 0;    // caught in main()
}

void Background::parentProcess()
{
    cout << "Waiting for the child process to end...\n"
            "The child returns value " << waitForChild() << endl;
}

int main()
try
{
    Background bg;

    bg.fork();
    cout << "This is from the parent\n";
}
catch(int x)
{
    cout << "The child terminates with: " << x << endl;
    return x;
}

Here's a more extensive example:

#include <iostream>
#include <string>

#include <signal.h>
#include <unistd.h>

#include <sys/types.h>

#include <bobcat/pipe>
#include <bobcat/ofdstream>
#include <bobcat/ifdstream>
#include <bobcat/process>
#include <bobcat/fork>

class ChildIO: public FBB::Fork
{
    FBB::Pipe childInput;   // child reads this
    FBB::Pipe childOutput;   // child writes this

    public:
        void childRedirections()    override;
        void childProcess()         override;
        void parentProcess()        override;
};

using namespace std;
using namespace FBB;


void ChildIO::childRedirections()
{
    childInput.readFrom(Redirector::STDIN);
    childOutput.writtenBy(Redirector::STDOUT);
}

void ChildIO::childProcess()
{
        // The /bin/cat program replaces the
        // child process started by Fork::fork()
    Process process(Process::DIRECT, "/bin/cat");
    process.start();

    // this point is never reached
}

void ChildIO::parentProcess()
{
        // Set up the parent's sides of the pipes
    IFdStream fromChild(childOutput.readOnly());
    OFdStream toChild(childInput.writeOnly());

        // write lines to the child, read its output
    string line;
    while (true)
    {
        cout << "? ";
        line.clear();
        getline(cin, line);

        if (line.empty())
        {
            kill(pid(), SIGTERM);
            break;
        }

        toChild << line << endl;

        getline(fromChild, line);
        cout << "Got: " << line << endl;
    }
    cout << "The child returns value " << waitForChild() << endl;
}

int main()
try
{
    ChildIO io;

    io.fork();

    return 0;
}
catch(exception const &exc)
{
    cerr << "Exception: " << exc.what() << endl;
}
catch(int x)
{
    cout << "The child terminates with: " << x << endl;
    return x;
}

FILES

SEE ALSO

BUGS

BOBCAT PROJECT FILES

• https://fbb-git.gitlab.io/bobcat/: gitlab project page;
• bobcat_6.04.00-x.dsc: detached signature;
• bobcat_6.04.00-x.tar.gz: source archive;
• bobcat_6.04.00-x_i386.changes: change log;
• libbobcat1_6.04.00-x_*.deb: debian package containing the libraries;
• libbobcat1-dev_6.04.00-x_*.deb: debian package containing the libraries, headers and manual pages;

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