#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <fcntl.h>
#define SIZE 65536

int main(int argc, char *argv[]) {
    int pipefd[2];          // pipefd[0] represents input stream id, pipefd[1] represents output stream id.
    char buffer[SIZE];      // The buffer of main process.
    char childBuffer[SIZE]; // The buffer of sub-process.

    // If the number of arguments is not correct.
    if (argc != 3) {
        perror("Main: main [target] [destination].\n");
        return 1;
    }

    char *srcFile = argv[1]; // Source
    char *dstFile = argv[2]; // Destination

    // Building a pipe. If the returned integer is less than 0,
    if (pipe(pipefd) < 0) {
        printf("An error occured when createing the pipe: %s\n", strerror(errno));
        return 1;
    }
    
    pid_t pid = fork(); // Create a sub-process.

    if (pid == 0) {         // pid == 0 represents it's a sub-process.
        close(pipefd[1]);   // Close pipefd[1]
        ssize_t file_size = read(pipefd[0], childBuffer, sizeof(childBuffer));  // Get the size of child
        close(pipefd[0]);                                       // Close pipefd[0] since it is unable to use.
        int dst_file_fd = open(dstFile, O_CREAT | O_WRONLY);    //  Read a file in writeonly mode. If the file does not exit, create a new one.
        write(dst_file_fd, childBuffer, file_size);             // Write data from buffer into fd.
        close(dst_file_fd);                                     // Close file
    }
    else if (pid > 0) { // pid > 0 represents it's a main process.
        close(pipefd[0]);                                                   // Close pipefd[0]
        int target_file_fd = open(srcFile, O_RDONLY);                       // Open a file in readonly mode.
        ssize_t file_size = read(target_file_fd, buffer, sizeof(buffer));   // Get size of the file
        write(pipefd[1], buffer, file_size);                                // Write data from buffer into pipefd[1]
        close(pipefd[1]);                                                   // Close pipefd[1]
        close(target_file_fd);                                              // Close file
    }
    else { // When error occur
        printf("An error occured when forking child process. %s\n", strerror(errno));
        return 1;
    }
    return 0;
}