stdout redirect fails with wprintf()

Question

The following code:

#include <stdio.h>

main()
{
    int    fd;
    fpos_t pos;

    printf("stdout, ");

    fflush(stdout);
    fgetpos(stdout, &pos);
    fd = dup(fileno(stdout));
    freopen("stdout.out", "w", stdout);

    f();

    fflush(stdout);
    dup2(fd, fileno(stdout));
    close(fd);
    clearerr(stdout);
    fsetpos(stdout, &pos);        /* for C9X */

    printf("stdout again\n");
}

f()
{
    printf("stdout in f()");
}

Works fine to redirect stdout and then put it back. However, changing f() to:

f()
{
    wprintf(L"stdout in f()");
}

Will not allow the stdout to be restored. Anyone know why this may be the case?

[matt test] uname -r
3.4.6-2.fc17.x86_64

Could [`fwide`](http://en.cppreference.com/w/c/io/fwide) be relevant here? — Shahbaz, Aug 01 '12 at 14:21
Actually, why are you messing with file descriptors at all? You can do everything with `fopen` and `freopen`, and you only need `fdopen` once at the end to restore the original output. — Kerrek SB, Aug 01 '12 at 14:29
What even if you run the executable as `./main > blah.log`. Can you guarantee that the `stdout` will be put back to `blah.log` — Matt Clarkson, Aug 01 '12 at 14:33
Perhaps more interesting; the final printf fails, but sets errno to zero and does not set the error indicator for the stream. — William Pursell, Aug 01 '12 at 14:41
Yeah, I've just added error checking and there are some weird things going on. — Matt Clarkson, Aug 01 '12 at 14:42
Things work very differently if you close STDOUT_FILENO after the first dup, before the freopen. — William Pursell, Aug 01 '12 at 14:43

score 2 · Accepted Answer · answered Aug 01 '12 at 14:44

2

The problem is due to the orientation of the stdout. Before calling wprintf() call fwide() with mode=0 and check if it returns a +ve value. If it does, you can use wprintf() safely. And after you are done using wprintf(), close the stdout and re-open it to use normal printf() again. Or, continue using wprintf() and wide character functions on the existing stdout, thereafter. Once the stdout has an orientation(byte or wide), it cannot be changed and persists until it is closed.

answered Aug 01 '12 at 14:44

askmish

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You are correct about the orientation of `stdout`. It seems you cannot mix `wprintf` and `printf` calls in a program. – Matt Clarkson Aug 01 '12 at 14:55
I'm not sure how to close and reopen stdout to point to the same place it was in. – Matt Clarkson Aug 01 '12 at 15:15
Take a look at: http://stackoverflow.com/questions/9084099/re-opening-stdout-and-stdin-file-descriptors-after-closing-them and also at the manual for reopen() system call. – askmish Aug 02 '12 at 06:59

stdout redirect fails with wprintf()

1 Answers1