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 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.
- o
- 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.
- o
- Pipe(Pipe &&tmp):
The move constructor moves the temporary object’s file descriptors to
the Pipe object being constructed.
- o
- Pipe(int const *fd):
This constructor expects two file descriptors, referring to the read and
write file descriptors as returned by pipe(2).
- o
- Pipe(bool initialize):
This constructor can be used when the Pipe object should not be
associated with an existing pipe. Instead it initializes its read and
write file descriptors to -1. This constructor may be used in classes that
define Pipe data members that only after the object is constructed
can open their pipes. Having constructing a Pipe obj(false) object
it can be associated with an open pipe using obj = Pipe{}. When
passing the argument true it calls Pipe() to construct a
pipe.
The move constructor and move assignment operator are available.
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.
- o
- void close():
Both file descriptors of the Pipe object are closed;
- o
- void closeReadFd():
The file descriptor of the Pipe object that is associated with the
reading end of the pipe is closed;
- o
- void closeWriteFd():
The file descriptor of the Pipe object that is associated with the
writing end of the pipe is closed;
- o
- int readFd() const:
Returns the pipe’s file descriptor that is used for reading
- o
- 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.
- o
- 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
(experimental).
- o
- 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.
- o
- 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.
- o
- int writeFd() const:
Returns the pipe’s file descriptor that is used for writing.
- o
- 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.
- o
- 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.
- o
- int writeOnly():
Closes the reading end of the pipe, returns the writing end’s file
descriptor.
The RW protected enumeration has the following
elements:
- o
- READ:
The index in d_fd[] (see below) of the element holding the
pipe’s reading file descriptor;
- o
- WRITE:
The index in d_fd[] (see below) of the element holding the
pipe’s writing file descriptor
- o
- 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.
#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 p; // construct a pipe
cout << "Read file descriptor: " << p.readFd() << "\n"
"Write file descriptor: " << p.writeFd() << endl;
int pid = fork();
if (pid == -1)
return 1;
if (!pid) //child
{
p.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;
}
p.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
Note that when the pipe goes out of scope, no close(2)
operation is performed on the pipe’s ends. If the pipe should be
closed by the desctructor, derive a class from Pipe(3bobcat), whose
destructor performs the required closing-operation.
- o
- https://fbb-git.gitlab.io/bobcat/: gitlab project page;
- o
- bobcat_5.07.00-x.dsc: detached signature;
- o
- bobcat_5.07.00-x.tar.gz: source archive;
- o
- bobcat_5.07.00-x_i386.changes: change log;
- o
- libbobcat1_5.07.00-x_*.deb: debian package containing the
libraries;
- o
- libbobcat1-dev_5.07.00-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).