Redirecting input from file to exec()

Question

I am making simple ANSI C program, that simulates Unix shell. So I am creating child process using fork() and inside child process I am calling exec() to run given (by user) program.

What I need to do is redirect content of file to stdin, so it can be sent to user called program.

Example: cat < file \\user wants run cat and redirect content of that file to it by typing this to my program prompt

I am trying to do so like this:

...child process... int fd = open(path_to_file, O_RDONLY); int read_size = 0; while ((read_size = read(fd, buffer, BUF_SIZE)) != 0) { write(STDIN_FILENO, buffer, read_size); } close(fd); execlp("cat", ...);

Everything goes fine, content of file is written to stdin, but after reading whole file, cat still waiting for an input (I need to tell cat, that input ended), but I cannot figure how :-(?

Any ideas? Thanks a lot!!!

Blagovest Buyukliev · Accepted Answer · 2012-02-28 18:42:24Z

5

Within the child process, redirect the standard input to your open'ed descriptor prior to calling execlp, via the dup2(2) system call:

dup2(fd, 0); execlp("cat", ...);

You don't need the while loop in the parent since cat will read from the newly redirected descriptor by itself.

answered Feb 28, 2012 at 18:42

Blagovest Buyukliev

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3 Comments

Jay Gridley Over a year ago

Great! Thanks a lot man. And could I also manage this command? 'cat < file > out_file' using dup2(...) function?

Blagovest Buyukliev Over a year ago

Yes, you would just need an additional dup2(fd_out, 1).

Blagovest Buyukliev Over a year ago

Are you still calling close prior to execlp?

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Redirecting input from file to exec()

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