Rerouting stdin and stdout from C
c, redirect, stdio
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);`
Problem
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?