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Full Discussion: C, UNIX: How to redirect 'stdout' to a file from a C code?
Top Forums Programming C, UNIX: How to redirect 'stdout' to a file from a C code? Post 302983608 by Corona688 on Thursday 13th of October 2016 06:52:51 PM
Old 10-13-2016
freopen DOES NOT redirect standard output! You are only redirecting the C file pointer 'stdout', not the file descriptor it uses.

Basically, you're ripping the floor out from under your program without guaranteeing that the Right Thing has happened to put a new floor back. The 'stdout' file pointer will continue to work, but anything not informed of the change might not -- like cout, and any subprocesses you happen to run. Their output may not go where you expected, or go nowhere at all, or crash. It might work right, if the next file opened happens to land at file descriptor 1 -- or it might not. The behavior is undefined and at the mercy of what libraries and compiler you're using. All that's guaranteed after you do that is that stdio routines like printf() will go where you redirected.

Whatever file descriptor 1 went to before, might not be properly closed after freopen, either. If stdout was an open file descriptor to a terminal preventing your ssh window from closing, such is life.

The "right thing" to do if you really want to redirect "standard output", not just the stdio external variable "stdout", is to ensure that the file you want is opened specifically as file descriptor one, which is what dup2() does in the example you were given. Which is much simpler than it looks once you realize it's redirecting twice: Once for stdout, once for stderr. Re-opening onto file descriptor one with dup2() also guarantees that whatever was there before, is forced to close. This could be especially important if that happened to be a terminal or device file.

Also, your example is concerning. Never mix printf and cout, for starters, especially if you're going to play funny games with redirection.
Last edited by Corona688; 10-13-2016 at 08:04 PM..
 

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