如何在 Mac 上使用 Xcode 在 C 中设置 I/O 重定向？

Question

I'm writing a program and I need to set the program up to get input from a text file and save the output in a text file. I'm using Xcode and I'm on a mac. I know I need to enter some redirection commands in the console but I'm not exactly sure how to do it. For example text files feel free to use input.txt and output.txt.

Thank you for the help.

Answer 1

On the terminal (command line) you can do

command < input.txt > output.txt

or

cat input.txt | command > output.txt

Edit:

Ok, thanks a lot. Just curious, when I type that in the terminal, 
how does it know to go to Xcode?

The only connection between running the command on the terminal and Xcode is that you build the program in Xcode. That produces an executable that can be run either by Xcode or from the shell (terminal). Running the program on the terminal has absolutely no effect on Xcode.

There might well be a way to redirect I/O from within Xcode (perhaps that is what you originally wanted) but I am unaware of it (don't use Xcode).

Answer 2

It seems difficult if not impossible to tell Xcode you wish to redirect standard input, output or error for your compiled command-line program. The Program > Scheme > Edit Scheme menu only helps out with application "Arguments Passes On Launch" [Cmd+<] which is something different than the redirection operators < and > in a shell. There seems no connection with a Terminal and Xcode, which also explains that your cannot debug a program launched from such external shell.

As a basic workaround without dependencies, several sites suggest you would bake this redirection into your program's code. This can even be done conditionally for Apple macOS like:

#ifdef __APPLE__
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
    freopen("error.txt", "w", stderr);
#endif

A slightly more elaborate example would leverage the "Environment Variables" as can be set on the very same Product Scheme tab and having program code like:

#include <stdlib.h>

#ifdef __APPLE__
        if (getenv("STDIN")) {
            if (!freopen(getenv("STDIN"), "r", stdin)) {
                NSLog(@"Unable to redirect stdin to '%s'\n", getenv("STDIN"));
            }
        } else {
            NSLog(@"Unknown environment variable STDIN so assuming stdin\n");
        }
        if (getenv("STDOUT")) {
            if (!freopen(getenv("STDOUT"), "w", stdout)) {
                NSLog(@"Unable to redirect stdout to '%s'\n", getenv("STDOUT"));
            }
        } else {
            NSLog(@"Unknown environment variable STDOUT so assuming stdout\n");
        }
#endif