How to use redirection in C for file input

Using redirection sends the contents of the input file to stdin, so you need to read from stdin inside your code, so something like (error checking omitted for clarity)

#include <stdio.h>

#define BUFFERSIZE 100

int main (int argc, char *argv[])
{
    char buffer[BUFFERSIZE];
    fgets(buffer, BUFFERSIZE , stdin);
    printf("Read: %s", buffer);
    return 0;
}