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Earlier, in section ref(REDIR) streams were hi(redirection) redirected
using the hi(rdbuf)tt(ios::rdbuf) member function. By assigning the
tt(streambuf) of a stream to another stream, both stream objects access the
same tt(streambuf), thus implementing redirection at the level of the
programming language itself.
This may be fine within the context of a bf(C++) program, but once we
leave that context the redirection terminates. The operating system does not
know about tt(streambuf) objects. This situation is encountered, e.g., when a
program uses a ti(system) call to start a subprogram. The example program at
the end of this section uses bf(C++) redirection to redirect the information
inserted into ti(cout) to a file, and then calls
verb( system("echo hello world"))
to echo a well-known line of text. Since tt(echo) writes its information
to the standard output, this would be the program's redirected file if the
operating system would recognize bf(C++)'s redirection.
But redirection doesn't happen. Instead, tt(hello world) still appears at
the program's standard output and the redirected file is left untouched. To
write tt(hello world) to the redirected file redirection must be realized at
the operating system level. Some operating systems (e.g., i(Unix) and
friends) provide system calls like ti(dup) and ti(dup2) to accomplish
this. Examples of the use of these system calls are given in section
ref(PIPE).
Here is the example of the em(failing redirection) at the system level
following bf(C++) redirection using tt(streambuf) redirection:
verbinclude(-a examples/noredir.cc)
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