SIMPLE SOLUTIONS

FBB::FORK - reference manual online

Implements fork(2) using the Template Design Pattern.

Chapter
2005-2015
FBB::Fork(3bobcat)            Template Design Pattern around fork(2)           FBB::Fork(3bobcat)

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 Algo‐ rithm Design Pattern. The class was designed as a virtual base class for classes imple‐ menting 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: o int WIFEXITED(int status): This macro returns a nonzero value if the child process terminated normally with `exit’ or `_exit’. o 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. o int WIFSIGNALED(int status): This macro returns a nonzero value if the child process terminated because it received a signal that was not handled. o int WTERMSIG(int status): If `WIFSIGNALED’ is true of `status’, this macro returns the signal number of the signal that terminated the child process. o int WCOREDUMP(int status): This macro returns a nonzero value if the child process terminated and produced a core dump. o int WIFSTOPPED(int status): This macro returns a nonzero value if the child process is stopped. o 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 o Fork(): This is the only available constructor. Note that there is no copy constructor.
DESTRUCTOR o virtual ~Fork(): Derived classes may define their own destructor, which is called when the Fork destructor is activated.
MEMBER FUNCTIONS o 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 o 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 child‐ Process() 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++. o 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 redirec‐ tor(3bobcat), pipe(3bobcat), ipipe(3bobcat) and opipe(3bobcat). By default, chil‐ dRedirections does nothing. o 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. o 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 redi‐ rector(3bobcat), pipe(3bobcat), ipipe(3bobcat) and opipe(3bobcat). By default, par‐ entRedirections does nothing. o 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. o 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 pre‐ vent 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. o 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. o int waitForChild(): 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.
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"; 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; } 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 bobcat/fork - defines the class interface
SEE ALSO bobcat(7), ipipe(3bobcat), fork(2), opipe(3bobcat), pipe(3bobcat), redirector(3bobcat), wait(2), waitpid(2)
BUGS None Reported.
DISTRIBUTION FILES o bobcat_4.01.03-x.dsc: detached signature; o bobcat_4.01.03-x.tar.gz: source archive; o bobcat_4.01.03-x_i386.changes: change log; o libbobcat1_4.01.03-x_*.deb: debian package holding the libraries; o libbobcat1-dev_4.01.03-x_*.deb: debian package holding the libraries, headers and manual pages; o http://sourceforge.net/projects/bobcat: public archive location;
BOBCAT Bobcat is an acronym of `Brokken’s Own Base Classes And Templates’.
AUTHOR Frank B. Brokken (@rug.nl).
libbobcat-dev_4.01.03-x.tar.gz 2005-2015 FBB::Fork(3bobcat)
This manual Reference Other manuals
fork(3bobcat) referred by localserversocket(3bobcat) | pipe(3bobcat) | process(3bobcat) | serversocket(3bobcat)
refer to bobcat(7) | _exit(2) | fork(2) | pipe(3bobcat) | redirector(3bobcat) | sleep(1) | wait(2)