Redirector objects are used to redirect the output sent to a stream having file descriptor x to another stream having file descriptor y, much like the shell's > operator redirects the standard output to some file.
Redirector objects can also be used to extract the information from a stream having file descriptor x in fact from another stream having file descriptor y, much like the shell's < operator is used to read the information in some file from the standard input.
Redirection using Redirector objects represents a stronger form of redirection than redirection offered by C++ itself, which uses std::streambuf redirection, and which is, because of that, bound to the program's scope. System level redirection, on the other hand, is applied at the system level, allowing the programmer to redirect standard streams when starting a program. For example, the standard error is commonly written to the standard output using an invocation like program 2>&1.
When constructing Redirector objects a file descriptor is required. The file descriptor specified at the constructor is the file descriptor that is used by the program to read information from or to write information to. Another file descriptor is required to set up the redirection: the file descriptor used here is the file descriptor of the stream that actually holds the information which is extracted from the file descriptor that was passed to the Redirector's constructor; or it is the file descriptor of the stream receiving the information which is written to the stream having the file descriptor that was passed to the Redirector's constructor.
When a Redirector object goes out of scope, its file descriptor are left as-is. In particular, note that no close(2) operation is performed on the Redirector's file descriptors. After setting up redirection using the Redirector's member functions and passing the Redirector's file descriptors to code that uses the Redirector's descriptors, the Redirector object could in fact safely be destroyed.
Formally, file descriptors are not defined in C++, but they are available in many types of operating systems. In those systems each `file' has an associated `file descriptor'. A file descriptor is an int, which is an index into the program's file allocation table, maintained by the system. Another type of well-known entities which are file descriptors are sockets.
Well-known filedescriptors (defined in, e.g., unistd.h) having fixed values are
After successfully calling swallow information written to otherFd is in fact written to the constructor's file descriptor. E.g., if the constructor's file descriptor represents a file on disk and otherFd is STDOUT_FILENO then all information sent to the standard output stream is actually sent to the file on disk:
information sent to otherFd -> is received at the constructor's Fd
(e.g., otherFd = STDOUT_FILENO)
Conversely, if the constructor's file descriptor represents a file on
disk and otherFd is STDIN_FILENO then all information extracted from
the standard input stream is actually read from the file on disk.
information extracted from otherFd <- is read from the constructor's Fd
(e.g., otherFd = STDIN_FILENO)
Following swallow both file descriptors are open, and refer to the constructor's file descriptor.
Before setting up the redirection, the original otherFd is closed by close(2). Following swallow both file descriptors can be used, and refer to the constructor's file descriptor. If after calling swallow close(2) is called for one of them, then the other one remains open.
If redirection fails an FBB::Exception object is thrown, whose which() member shows the system's errno value set by the failing dup2(2) function.
After successfully calling through information written to otherFd is in fact written to the constructor's file descriptor. E.g., if the constructor's file descriptor represents a file on disk and otherFd is STDOUT_FILENO then all information sent to the standard output stream is actually sent to the file on disk:
information sent to otherFd -> is received at the constructor's Fd
(e.g., otherFd = STDOUT_FILENO)
Conversely, if the constructor's file descriptor represents a file on
disk and otherFd is STDIN_FILENO then all information extracted from
the standard input stream is actually read from the file on disk.
information extracted from otherFd <- is read from the constructor's Fd
(e.g., otherFd = STDIN_FILENO)
Before setting up the redirection, the original otherFd is closed
by close(2). Following through only otherFd can be used, and
it refers to (i.e., reads or writes) the constructor's file descriptor.
If redirection fails an FBB::Exception object is thrown, whose which() member shows the system's errno value set by the failing dup2(2) function.
#include <iostream>
#include <bobcat/redirector>
using namespace std;
using namespace FBB;
int main()
{
Redirector redirector(Redirector::STDOUT);
redirector.swallow(Redirector::STDERR);
cerr << "This appears at the standard output stream\n"
"use `a.out > /dev/null' to suppress this message" << endl;
}