libbobcat1-dev_2.21.00-x.tar.gz Template Design Pattern around fork(2)
2005-2012
NAME
FBB::Fork - Implements fork(2) using the Template Design Pattern
SYNOPSIS
#include <bobcat/fork>
Linking option: -lbobcat
DESCRIPTION
FBB::Fork objects may be used to implement the fork(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 must implement
the members childProcess() and parentProcess() as part of the
`Template Method Design Pattern' (see Gamma et al., 1995).
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
FBB
All constructors, members, operators and manipulators, mentioned in this
man-page, are defined in the namespace FBB.
INHERITS FROM
-
CONSTRUCTORS
Fork():
This is the only available constructor.
Note that there is no copy constructor.
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 FBB::Fork. If the forking operation fails, an FBB::Errno exception
is thrown.
PROTECTED 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 exeption
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), pipe(3bobcat),
ipipe(3bobcat) and opipe(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.
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), pipe(3bobcat),
ipipe(3bobcat) and opipe(3bobcat). By default,
parentRedirections() does nothing.
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). Note that the value of pid() is undefined
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.
int waitForChild():
This memebr 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.
EXAMPLE
#include <iostream>
#include <unistd.h>
#include <bobcat/fork>
using namespace std;
using namespace FBB;
class Background: public Fork
{
public:
void childProcess();
void parentProcess();
};
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";
cout << "The child returns value " << waitForChild() << endl;
}
int main()
try
{
Background bg;
bg.fork();
cout << "This is from the parent\n";
return 0;
}
catch(int x)
{
cout << "The child terminates with: " << x << endl;
return x;
}