.TH "FBB::Fork" "3bobcat" "2005\-2024" "libbobcat\-dev_6\&.06\&.02" "Template Design Pattern around fork(2)"

.PP 
.SH "NAME"
FBB::Fork \- Implements \fBfork\fP(2) using the Template Design Pattern
.PP 
.SH "SYNOPSIS"
\fB#include <bobcat/fork>\fP
.br 
Linking option: \fI\-lbobcat\fP
.PP 
.SH "DESCRIPTION"
\fBFBB::Fork\fP objects may be used to implement the \fBfork\fP(2) call as part
of the Template Algorithm Design Pattern\&. The class was designed as a virtual
base class for classes implementing the essential parts of the forking
process\&. The class is a virtual base class\&. Derived classes \fImust\fP implement
the members \fIchildProcess\fP and \fIparentProcess\fP as part of the
`Template Method Design Pattern\(cq\& (see Gamma \fIet al\&.\fP, 1995)\&.
.PP 
Terminating child processes send \fISIGCHLD\fP signals to their parents\&. The
\fBC\fP library offers the following \fImacros\fP to analyze the \fIstatus\fP values
received by the parent process using a \fBwait\fP(2) or \fBwaitpid\fP(2) system
call:
.IP o 
\fBint WIFEXITED(int status)\fP:
.br 
This macro returns a nonzero value if the child process terminated
normally with `exit\(cq\& or `_exit\(cq\&\&.
.IP o 
\fBint WEXITSTATUS(int status)\fP:
.br 
If `WIFEXITED\(cq\& is true of `\fIstatus\fP\(cq\&, this macro returns the
low\-order 8 bits of the exit status value from the child process\&.
.IP o 
\fBint WIFSIGNALED(int status)\fP:
.br 
This macro returns a nonzero value if the child process terminated
because it received a signal that was not handled\&.
.IP o 
\fBint WTERMSIG(int status)\fP:
.br 
If `WIFSIGNALED\(cq\& is true of `\fIstatus\fP\(cq\&, this macro returns the
signal number of the signal that terminated the child process\&.
.IP o 
\fBint WCOREDUMP(int status)\fP:
.br 
This macro returns a nonzero value if the child process terminated
and produced a core dump\&.
.IP o 
\fBint WIFSTOPPED(int status)\fP:
.br 
This macro returns a nonzero value if the child process is stopped\&.
.IP o 
\fBint WSTOPSIG(int status)\fP:
.br 
If `WIFSTOPPED\(cq\& is true of `\fIstatus\fP\(cq\&, this macro returns the signal
number of the signal that caused the child process to stop\&.

.PP 
.SH "NAMESPACE"
\fBFBB\fP
.br 
All constructors, members, operators and manipulators, mentioned in this
man\-page, are defined in the namespace \fBFBB\fP\&.
.PP 
.SH "INHERITS FROM"
\-
.PP 
.SH "CONSTRUCTORS"

.PP 
Only the default constructor is available\&.
.PP 
.SH "DESTRUCTOR"
.IP o 
\fBvirtual ~Fork()\fP:
.br 
Derived classes may define their own destructor, which is called when
the \fIFork\fP destructor is activated\&.

.PP 
.SH "MEMBER FUNCTIONS"
.IP o 
\fBvoid fork()\fP:
.br 
Performs the actual forking\&. It is implemented in such a way that the
corresponding parent\- and child\- processes are activated from virtual members
of \fBFork\fP\&. If the forking operation fails, an \fIFBB::Exception\fP exception
is thrown\&.

.PP 
.SH "PROTECTED MEMBER FUNCTIONS"
.IP o 
\fBint endChild() const\fP:
.br 
This member may be called by \fIparentProcess\fP to end the child
process\&. To end the child process a \fISIGTERM\fP is sent to the child process,
followed by a \fISIGKILL\fP (cf\&. \fBsignal\fP(7))\&. If the child process has
already ended then the child process\(cq\&s exit value is returned, otherwise the
child process\(cq\&s end is forced (by calling \fIkill\fP) and \-2 is returned\&. This
member also calls \fIwaitForChild\fP (see below)\&.
.IP 
.IP o 
\fBpid_t pid() const\fP:
.br 
Returns the child\(cq\&s process id in the parent\(cq\&s code (i\&.e\&., in the
\fBparent\-\fPmembers below), and 0 in the child\(cq\&s code (i\&.e\&., in the
\fBchild\-\fPmembers below)\&. The \fIpid\fP member returns \-1 when called \fIbefore\fP
the member \fIfork\fP has been called\&.
.IP 
.IP o 
\fBvoid prepareDaemon() const\fP:
.br 
Prepares for a daemon childprocess\&. This function may (should) be
called from \fIchildProcess\fP 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 \fI/dev/null\fP to prevent ghost input and
output actions from interfering with the daemon\(cq\&s actions\&. An
\fIFBB::Exception\fP is thrown if changing directory to the root directory
fails\&.
.IP 
.IP o 
\fBvoid prepareDaemon(std::string const &out, std::string const &err,
mode_t mode = 0600) const\fP:
.br 
Prepares for a daemon childprocess like the previous member function,
but allows redirection of the standard output (\fIout\fP) and standard error
(\fIerr\fP) streams to files\&. Specify empty strings to redirect these streams to
\fI/dev/null\fP\&. 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 \fIFBB::Exception\fP is thrown if
changing directory to the root directory or if using the specified \fBs\fP
fails\&.
.IP 
.IP o 
\fBint waitForChild() const\fP:
.br 
This member may be called by \fIparentProcess\fP 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
\fIwaitForChild\fP should be called to prevent \fIzombies\fP from
occurring\&. Alternatively, the parent process may terminate (e\&.g\&., using
\fBexit\fP(2)) while the child process is still alive\&. This is the normal way to
create a \fIdaemon\fP process\&.

.PP 
.SH "PRIVATE (VIRTUAL) MEMBER FUNCTIONS"
.IP o 
\fBvirtual void childProcess() = 0\fP:
.br 
This member \fImust\fP be implemented by derived classes\&. It defines the
actions that are performed by the child process, following the \fBfork\fP(2)
system call\&. Just before \fIchildProcess\fP is called, \fIchildRedirections\fP
(see below) has been executed\&. The \fIchildProcess()\fP function should
terminate the child process\&. A good way to do this is to throw an exception
which is caught by \fImain()\fP\(cq\&s function try block\&. Terminating a process
using \fBexit\fP(2) is deprecated in \fBC++\fP\&.
.IP 
.IP o 
\fBvirtual void childRedirections()\fP:
.br 
This function \fImay\fP be redefined in derived classes to set up the
redirections that are necessary to communicate with the parent process\&. See
also the classes \fBredirector\fP(3bobcat) and \fBpipe\fP(3bobcat)\&. By default,
\fIchildRedirections\fP does nothing\&.
.IP 
.IP o 
\fBvirtual void parentProcess() = 0\fP:
.br 
This member \fImust\fP be implemented by derived classes\&. It defines the
actions that are performed by the parent process, following the \fBfork\fP(2)
system call\&. Just before \fIparentProcess\fP is called,
\fIparentRedirections\fP (see below) has been executed\&. 
.IP 
When deriving classes from \fIFork\fP their \fIparentProcess\fP
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 \fIexec\&.\&.\fP function, continuously
producing output, interpreted by the parent process) then the parent process
can call \fIendChild\fP to end the child process before ending the
\fIparentProcess\fP function\&.
.IP 
.IP o 
\fBvirtual void parentRedirections()\fP:
.br 
This function \fImay\fP 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 \fBredirector\fP(3bobcat) and \fBpipe\fP(3bobcat)\&. By default,
\fIparentRedirections\fP does nothing\&.

.PP 
.SH "EXAMPLES"
.nf 
#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 << \(dq\&Hello world # \(dq\& << idx << endl;
        sleep(1);
    }
    throw 0;    // caught in main()
}

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

int main()
try
{
    Background bg;

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

.fi 

.PP 
Here\(cq\&s a more extensive example:
.PP 
.nf 
#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, \(dq\&/bin/cat\(dq\&);
    process\&.start();

    // this point is never reached
}

void ChildIO::parentProcess()
{
        // Set up the parent\(cq\&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 << \(dq\&? \(dq\&;
        line\&.clear();
        getline(cin, line);

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

        toChild << line << endl;

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

int main()
try
{
    ChildIO io;

    io\&.fork();

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

.fi 

.PP 
.SH "FILES"
\fIbobcat/fork\fP \- defines the class interface
.PP 
.SH "SEE ALSO"
\fBbobcat\fP(7), \fBcerrextractor\fP(3bobcat), \fBcininserter\fP(3bobcat),
\fBcoutextractor\fP(3bobcat), \fBexec\fP(3), \fBexec\fP(3bobcat), \fBfork\fP(2),
\fBkill\fP(2), \fBpipe\fP(3bobcat), \fBredirector\fP(3bobcat),
\fBstdextractor\fP(3bobcat), \fBwait\fP(2), \fBwaitpid\fP(2)\&.
.PP 
.SH "BUGS"
None Reported\&.
.PP 
.SH "BOBCAT PROJECT FILES"

.PP 
.IP o 
\fIhttps://fbb\-git\&.gitlab\&.io/bobcat/\fP: gitlab project page;
.IP o 
\fIbobcat_6\&.06\&.02\-x\&.dsc\fP: detached signature;
.IP o 
\fIbobcat_6\&.06\&.02\-x\&.tar\&.gz\fP: source archive;
.IP o 
\fIbobcat_6\&.06\&.02\-x_i386\&.changes\fP: change log;
.IP o 
\fIlibbobcat1_6\&.06\&.02\-x_*\&.deb\fP: debian package containing the
libraries;
.IP o 
\fIlibbobcat1\-dev_6\&.06\&.02\-x_*\&.deb\fP: debian package containing the
libraries, headers and manual pages;

.PP 
.SH "BOBCAT"
Bobcat is an acronym of `Brokken\(cq\&s Own Base Classes And Templates\(cq\&\&.
.PP 
.SH "COPYRIGHT"
This is free software, distributed under the terms of the
GNU General Public License (GPL)\&.
.PP 
.SH "AUTHOR"
Frank B\&. Brokken (\fBf\&.b\&.brokken@rug\&.nl\fP)\&.
.PP