Redirecting file contents into another program in C

Question

Making a basic shell and I need to handle file redirection. Specifically I am stumped on how I can feed data into a program like

sort

for example from a file.

Something on the command line would look like this:

$ sort < test.txt

How do I accomplish this kind of functionality in my own mini shell made in C?

I thought that maybe

sort

read from

stdin

but I guess I can't really write the contents of a file to

stdin

due to the read only aspect so I'm wondering how I actually get this data into something like

sort

for example? I am currently using forks and then using

execvp()

to execute programs.

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Answer 1

You need to use freopen.

It works similarly to fopen. The first argument is the filename, the second argument is the mode, and the third argument is the file pointer to redirect. The return value is the new file pointer. So if you want to redirect stdin:

FILE *fp = freopen("input.txt", "r", stdin);

It returns a NULL pointer on failure, just like fopen.

Redirecting file contents into another program in C

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