how to simulate keyboard input for a program in C

Question

I have a binary of a program that waits for an input using scanf. I need to write a C code that will be able to simulate keyboard input. i.e. close the stdin pointer for that binary and instead repoint it to a file. I used this code

int main()
{
        FILE *fin;
        int result;
        char string[80];
        close(0);

        fin = fopen("text", "r");

        if(NULL == fin)
        {
                printf("Unable to open file.");
                return 0;
        }
        dup(fin);
        return 0;
}

But i found that each program has its own stdin pointer. Is there a way for me to simulate keyboard input for one binary from another C program ?

Answer 1

It's not a program you're looking for, it's capabilities of your shell:

 program < input.txt

This way you push input.txt as stdin to program.

If you want a program, then you can just run a program and reroute it's output to the other programs input.

Answer 2

You can use a simple pipe when calling the programs as a basic measure.

Another way would be to make your keyboard simulator act as a launcher. It should start the program you are testing with stdin as your simulator's stdout.

You can do the latter by forking your program and use the parent process to create a pipe and the child process to reopen stdin (even stdout) using a file descriptor to that pipe and exec the program you want to supply the input to.

See this for pipes and this for file descriptors (as creating a pipe will give you only a file descriptor).

P.S.: Sorry about not being able to provide code at the moment, but I'm not anywhere near a UNIX workstation and a bit too rusty on UNIX OS programming to write reliable code from memory.