FBB::Pipe(3bobcat) System Level Communication Pipe FBB::Pipe(3bobcat)

FBB::Pipe - Defines a system level communication pipe

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

FBB::Pipe objects may be used to construct a pipe. Pipe objects offer a simple interface to the reading and writing ends of pipes. Pipe objects are object-wrappers around the pipe(2) system call.

A Pipe object which is created just before a program forks can be used to set up a line of communication between the parent and child process. Information which is written by the child process to its standard output stream can be redirected to the writing end of the pipe (using the writtenBy member). The information appearing at the reading end of the pipe can then be extracted using, e.g., an IFdStream object, initialized with the Pipe’s reading file descriptor, or the reading end of the pipe can be redirected to an existing stream whose file descriptor is known, like cin (which uses the STDIN_FILENO file descriptor).

When a Pipe object goes out of scope, no close(2) operation is performed on the pipe’s file descriptors. After setting up the pipe using the Pipe’s member functions and passing the Pipe’s file descriptors to code that uses the Pipe’s descriptors, the Pipe object might even be destroyed. The using code is responsible for closing the pipe. If the pipe should be closed at destruction time, then a class could be derived from Pipe(3bobcat), whose destructor may then close the pipe. Alternatively, Pope’s close member can be called.

The value -1 indicates that a file descriptor does not refer to a pipe(2) file descriptor.

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

-

Pipe():
The default Pipe constructor constructs a pipe, calling pipe(2).
This constructor throws an Exception exception if the default Pipe constructor did not properly complete. The thrown Exception object’s which() member shows the system’s errno value set by the failing pipe(2) function.
Pipe(Pipe &&tmp):
The move constructor moves the temporary object’s file descriptors to the Pipe object being constructed.
Pipe(int const *fd):
This constructor expects two file descriptors, referring to the read and write file descriptors as returned by pipe(2).
Pipe(bool initialize):
This constructor can be used when the Pipe object should not be associated with an existing pipe. Instead when, initialize == false, it initializes its read and write file descriptors to -1. This constructor may be used by classes that define Pipe data members which can only open their pipes after the object has been constructed. Having constructing a Pipe obj{ false } object it can be associated with an open pipe using obj = Pipe{}. or obj.reset(). When passing the argument true it initializes its pipe (cf. pipe(2)). Copy construction and copy assignment are not defined.

Note that Pipe’s destructor does not close the pipe’s file descriptors. To close the pipes close must be called.

Pipe &operator=(Pipe &&tmp):
The overloaded move assignment operator closes the current pipe and moves tmp’s pipes to the current Pipe object.

void close():
Both file descriptors of the Pipe object are closed;
void closeReadFd():
The file descriptor of the Pipe object that is associated with the reading end of the pipe is closed;
void closeWriteFd():
The file descriptor of the Pipe object that is associated with the writing end of the pipe is closed;
int readFd() const:
Returns the pipe’s file descriptor that is used for reading
void readFrom(int filedescriptor):
Sets up redirection from the internal read filedescriptor to the given filedescriptor: information written to the write-end of the pipe may be retrieved by extracting the information from the stream that is associated with the indicated file descriptor. E.g., after the call readFrom(STDIN_FILENO) information inserted into the write-end of the pipe can be retrieved from cin. The original read file descriptor and the pipe’s write file descriptor are closed.
void readFrom(int const *filedescriptors, size_t n):
Sets up redirection from the internal read filedescriptor to the given filedescriptors: information is read from the Pipe object when reading from any of the n provided filedescriptors. The original read file descriptor and the pipe’s write file descriptor are closed.
int readOnly():
Closes the writing end of the pipe, returns the reading end’s file descriptor. This member can be used, e.g., to construct an IFdStream object to extract the information that is inserted into the write-end of the pipe.
int readOnlyFd():
Same as the previous member, but sets the internally used read file descriptor to -1 (this member can be used to, e.g., pass the read file descriptor to another object which eventually closes the pipe’s reading end).
void reset():
Closes the the current pipe and reopens it with new pipe read and write destriptors.
void reset(int const *fds):
Closes the the current pipe and reopens it with the read and write file destriptors provided by the first two elements of fds.
void swap(Pipe &other):
The current and other Pipe objects are swapped. Following this call the current Pipe objects refer to the other object’s pipe file descriptors and vice versa.
int writeFd() const:
Returns the pipe’s file descriptor that is used for writing.
void writtenBy(int filedescriptor):
Sets up redirection from the internal write filedescriptor to the given filedescriptor: information is written to the Pipe object when writing to the provided filedescriptor. E.g., after the call writtenBy(STDOUT_FILENO) information sent to the standard output stream (by either cout or by a child process (cf. exec(3))) is inserted into the write-end of the pipe. The original write file descriptor and the pipe’s read file descriptor are closed.
void writtenBy(int const *filedescriptors, size_t n):
Sets up redirection from the internal write filedescriptor to the given filedescriptors: information is inserted into the write-end of the Pipe object when writing to each of the n provided filedescriptors. E.g., when passing an array of two int values, respectively equal to STDOUT_FILENO and STDERR_FILENO to this member, all information which is thereafter sent to the standard output or error streams is inserted into the write-end of the pipe. The original write file descriptor and the pipe’s read file descriptor are closed.
int writeOnly():
Closes the reading end of the pipe, returns the writing end’s file descriptor.
int writeOnlyFd():
Same as the previous member, but sets the internally used write file descriptor to -1 (this member can be used to, e.g., pass the write file descriptor to another object which eventually closes the pipe’s writing end).

The RW protected enumeration has the following elements:

READ:
The index in d_fd[] (see below) of the element holding the pipe’s reading file descriptor;
WRITE:
The index in d_fd[] (see below) of the element holding the pipe’s writing file descriptor

void close(RW rw):
When passing argument RW::READ to this member the reading end of the Pipe object’s pipe is closed. When passing argument RW::WRITE the writing end of the Pipe object’s pipe is closed.
int *fd():
Returns a pointer to the two file descriptors (respectively READ and WRITE) currently used by the Pipe object. 

    #include <bobcat/pipe>
    #include <sys/types.h>
    #include <sys/wait.h>
    #include <unistd.h>
    #include <iostream>
    #include <string>
    using namespace std;
    using namespace FBB;
    int main()
    {
        Pipe pipe;                         // construct a pipe
        cout << "Read file descriptor: " << pipe.readFd() << "\n"
                "Write file descriptor: " << pipe.writeFd() << endl;
        int pid = fork();
        if (pid == -1)
            return 1;
        if (pid == 0)                       //child
        {
            pipe.readFrom(STDIN_FILENO);    // read what goes into the pipe
            string s;
            getline(cin, s);
            cout << "CHILD: Got `" << s << "’\n";
            getline(cin, s);
            cout << "CHILD: Got `" << s << "’\n";
            return 0;
        }
        pipe.writtenBy(STDOUT_FILENO);      // write to the pipe via cout
        cout << "first line" << "\n"
                "second line" << ’\n’;
        waitpid(pid, 0, 0);
    }

See also the 2nd example at fork(3bobcat)

bobcat/pipe - defines the class interface

bobcat(7), fork(3bobcat), pipe(2), mkfifo(3)

Note that when a Pipe object goes out of scope, no close(2) operation is performed on the pipe’s ends. If the pipe should be closed by the destructor, derive a class from Pipe(3bobcat), whose destructor performs the required closing-operation.

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;

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

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

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

2005-2023 libbobcat-dev_6.04.00