switch between writing to file and stdout

Question

I want to switch between writing to the file and to the stdout I can't use fprintf, but only printf and freopen something like this:

for(size_t i;i<100;++i)
{
     if(i%2)
     {
        freopen("tmp","w",stdout);
        printf("%d\n",i);
     }
     else
     {
        //return to write to stdout?
        printf("%d\n",i);
     }
}

How can I return to writing to the stdout?

Update

I write cross-platform application and dup can't be used.

Duplicate of http://stackoverflow.com/questions/1673764/freopen-reverting-back-to-original-stream. — Eli Iser, Aug 29 '11 at 10:04
@unkulunkulu Why does it really matter this constraint can't be solved as it integral part of my system that all that I can say. — YAKOVM, Aug 29 '11 at 12:34
@Yakov, because it is weird, we want to know why to be better. — unkulunkulu, Aug 29 '11 at 13:08

score 2 · Accepted Answer · answered Aug 29 '11 at 14:03

2

Never use freopen. It cannot achieve what you want, and it's a very dangerous function. If it fails, the only safe thing you can do is immediately terminate the program or ensure that stdout is never accessed again.

There is a way to do what you want on POSIX systems with dup and dup2. It looks something like this:

fflush(stdout);
int old_stdout = dup(1);
int new_stdout = open("whatever", O_WRDONLY|O_CREAT, 0666);
dup2(new_stdout, 1);
close(new_stdout);
/* use new stdout here */
fflush(stdout);
dup2(old_stdout, 1);
close(old_stdout);

answered Aug 29 '11 at 14:03

R.. GitHub STOP HELPING ICE

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I'm really not sure what you're basing your dire warnings against `freopen` on. got any references? also, http://pubs.opengroup.org/onlinepubs/009695399/functions/freopen.html says `freopen` is commonly used to reattach `stdin`, `stdout` and `stderr` to other files. – Hasturkun Aug 29 '11 at 14:11
See this text in the link you provided: "The original stream shall be closed regardless of whether the subsequent open succeeds." Thus, `stdout` is closed if opening the new file fails, and since it's been closed, any function that uses `stdout` (e.g. `printf`) now uses an invalid `FILE *` and thus invokes undefined behavior. – R.. GitHub STOP HELPING ICE Aug 29 '11 at 14:23
I don't think there's anything stopping you from reassigning these, so you can still handle this gracefully. – Hasturkun Aug 29 '11 at 14:29
Assigning to `stdout` is not allowed. In fact glibc is just about the **only** system that allows it, and while they allow it explicitly in the documentation, assignments to `stdout` are not actually honored by all internal functions (some of them fail to see it due to symbol visibility or copy relocation related bugs...). Per the standards, `stdin`, `stdout`, and `stderr` are macros that expand to expressions of type `FILE *`. There is no reason to expect they should even be lvalues, much less modifiable. – R.. GitHub STOP HELPING ICE Aug 29 '11 at 14:35
Right. I'd temper the warning slightly though, since it should be safe to use `freopen` at program initialization time, where closing one of the standard handles could be handled. – Hasturkun Aug 29 '11 at 14:43

score 0 · Answer 2 · answered Aug 29 '11 at 10:04

0

You need to dup the file descriptors, and reopen to the open descriptor. Why can't you use fprintf? (Is this homework?)

answered Aug 29 '11 at 10:04

tripleee

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switch between writing to file and stdout

2 Answers2