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;
}