Re: freopen -- the easy way (was: Re: LYNX-DEV 4DOS)

[Laura Eaves]

> Date: Thu, 8 May 1997 10:23:47 -0400
> From: address@hidden (Larry W. Virden, x2487)
>...
> To handle the case of the freopen failing, you might be able to use
> something like
> int oldstderrfd;
> FILE *newstderr;
>
> if ((oldstderrfd = dup(fileno(stderr))) != -1 ) {
>       /* Do your code to do the freopen of stderr, etc.) */
> }
>
> /* Now, when it's time to go back ... */
> if ( (stderr = fdopen(oldstderr,"w+") ) != NULL ) {
>       /* stderr should be back to where it was */
> }

Is fileno defined on non-unix systems?
Also, "stderr = fdopen(..." won't compile as stderr isn't an lvalue.

How about something like the following (I knew I'd blow my lunch hour...
but this seems to work):

#include <stdio.h>

int oldstderrfd;

void restore_stderr(closed)
int closed;
{
    if ( oldstderrfd != -1 ) {
        FILE* newstderr;
        if ( (newstderr = fdopen(oldstderrfd,"w+")) != NULL ) {
            if ( !closed ) fclose(stderr);
            *stderr = *newstderr;
            oldstderrfd = -1;
            /* stderr should be back to where it was */
            /* EXCEPT that the file descriptor might not be 2 */
        }
    }
}

main(argc,argv)
int argc; char **argv;
{
    if ( argc < 2 ) return 0;
    fprintf(stderr,"before freopen: %s, fileno==%d\n",argv[1],fileno(stderr));
    if ((oldstderrfd = dup(fileno(stderr))) != -1 ) {
        /* Do your code to do the freopen of stderr, etc.) */
        if ( freopen(argv[1],"w+",stderr) == NULL )
            restore_stderr(1);
    }
    fprintf(stderr,"after freopen: %s, fileno==%d\n",argv[1],fileno(stderr));
    /* Now, when it's time to go back ... */
    restore_stderr(0);
    fprintf(stderr,"after restoring stderr: %s, 
fileno==%d\n",argv[1],fileno(stderr));
}

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