Rerouting stdin and stdout from C


I want to reopen the stdin and stdout (and perhaps stderr while I'm at it) filehandles, so that future calls to printf() or putchar() or puts() will go to a file, and future calls to getc() and such will come from a file.

1) I don't want to permanently lose standard input/output/error. I may want to reuse them later in the program.

2) I don't want to open new filehandles because these filehandles would have to be either passed around a lot or global (shudder).

3) I don't want to use any open() or fork() or other system-dependent functions if I can't help it.

So basically, does it work to do this:

stdin = fopen("newin", "r");

And, if it does, how can I get the original value of stdin back? Do I have to store it in a FILE * and just get it back later?

Best Solution

Why use freopen()? The C89 specification has the answer in one of the endnotes for the section on <stdio.h>:

116. The primary use of the freopen function is to change the file associated with a standard text stream (stderr, stdin, or stdout), as those identifiers need not be modifiable lvalues to which the value returned by the fopen function may be assigned.

freopen is commonly misused, e.g. stdin = freopen("newin", "r", stdin);. This is no more portable than fclose(stdin); stdin = fopen("newin", "r");. Both expressions attempt to assign to stdin, which is not guaranteed to be assignable.

The right way to use freopen is to omit the assignment: freopen("newin", "r", stdin);

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