Question

Here is the code of the functions:

int getStream()
    {
           int fd = _dup(fileno(stdout));
           freopen("tmp","w",stdout);
           return fd;
    }


void freeStream(int fd)
{
       _dup2(fd,fileno(stdout));
       close(fd);
}

The code of main program:

printf(“start tets”);
int fd = getStream();
printf(“redirection”);
freeStream(fd);

This is a part of large program which uses llvm
The problem I get after freeStream(fd): LLVM ERROR: IO failure on output stream.
I understand that the problem is with freeStream implementation.
What do you think?
Thanks

Answer 1

As Jason diagnosed, you need to fflush() the stream (stdout) before you close the underlying file (via the _dup2() call).

Another problem is that there is no error checking in the code; you don’t know which, if any, of your function calls are failing. At least while resolving this issue, you need to monitor both freopen() and _dup2() to ensure they work.

It is not clear that on Windows, you can reach behind the back of the <stdio.h> library and change the file descriptor. On Unix, I think it would likely work – not necessarily supported, but probably it would work. But I can easily imagine ways it fails on Windows (without being certain that my imagination is not simply being to vivid).

Ultimately, I am not convinced there’s a portable way to reinstate stdout back to its original output after using freopen() on it.