How to reuse (rewind) a redirected file in C

Question

I input a text file into my C program by using the < I/O redirection in the terminal:

MyProgram.exe < "data.txt"

And then use getchar() and scanf() in my code to read from the redirected input. However, as this is a redirected file, I can't use a C file pointer to rewind the buffer.

(I want to count the number of lines in a file of an arbitrary size so I can initialize an array, then I want to re-read the file using getchar() and/or scanf()).

How do I achieve a rewind or flush when using redirected input?

Have you actually tried rewinding it? fseek works just on on stdin when stdin is a regular file, and when you redirect like this, stdin is a regular file. — William Pursell, Apr 04 '15 at 01:53
@Lashane: Given that standard input is coming from a disk file, it can be rewound just like any other file stream connected to a disk file. — Jonathan Leffler, Apr 04 '15 at 01:57

score 4 · Accepted Answer · answered Apr 04 '15 at 01:55

However, as this is a redirected file, I cant use a c file pointer to rewind the buffer.

Sure you can! And it's really easy. When a file is being redirected to standard input, stdin is a file, so you can treat it just like any other file:

rewind(stdin);

But keep in mind that this will only work if standard input is being redirected from a file. It won't work if standard input is from the terminal or a pipe (e.g, cat file.txt | myapp).

It is just a bad idea to assume `stdin` is a file. The correct way is not to and store the required history instead. — too honest for this site, Mar 01 '16 at 22:06

How to reuse (rewind) a redirected file in C

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