retrofitting a .h & .c file to my already working .c program - c

Question

I have a program that I'm doing for class where I need to take the content of one file, reverse it, and write that reversed content to another file. I have written a program that successfully does this (after much googling as I am new to the C programming language). The problem however is that my professor wants us to submit the program in a certain way with a couple supporting .h and .c files (which I understand is good practice). So I was hoping someone could help me understand exactly how I can take my already existing program and make it into one that is to his specifications, which are as follows:
he would like a file named "file_utils.h" that has function signatures and guards for the following two functions
int read_file( char* filename, char **buffer );
int write_file( char* filename, char *buffer, int size);
thus far I have created this file to try and accomplish this.
#ifndef UTILS_H
#define UTILS_H
int read_file(char* filename, char **buffer);
int write_file(char* filename, char *buffer, int size);
#endif
he would like a file named "file_utils.c" that has the implemented code for the previous two functions
he would like a file named "reverse.c" that accepts command arguments, includes a main function, and calls the functions from the previous two files.
now. I understand how this is supposed to work, but as I'm looking at the program I wrote my way I'm unsure how to actually accomplish the same result by adhering to the previously mentioned specifications.
Below is the program that successfully accomplishes the desired functionality
#include<stdlib.h>
#include<stdio.h>
#include<fcntl.h>
#include<string.h>
#include<sys/stat.h>
#include<unistd.h>
int main(int argc, char *argv[]) {
int file1, file2, char_count, x, k;
char buffer;
// if the number of parameters passed are not correct, exit
//
if (argc != 3) {
fprintf(stderr, "usage %s <file1> <file2>", argv[0]);
exit(EXIT_FAILURE);
}
// if the origin file cannot be opened for whatever reason, exit
// S_IRUSR specifies that this file is to be read by only the file owner
//
if ((file1 = open(argv[1], S_IRUSR)) < 0) {
fprintf(stderr, "The origin-file is inaccessible");
exit(EXIT_FAILURE);
}
// if the destination-file cannot be opened for whatever reason, exit
// S_IWUSR specifies that this file is to be written to by only the file owner
//
if ((file2 = creat(argv[2], S_IWUSR)) < 0) {
fprintf(stderr, "The destination-file is inaccessible");
exit(EXIT_FAILURE);
}
// SEEK_END is used to place the read/write pointer at the end of the file
//
char_count = lseek(file1, (off_t) 0, SEEK_END);
printf("origin-file size is %d\n", char_count - 1);
for (k = char_count - 1; k >= 0; k--) {
lseek(file1, (off_t) k, SEEK_SET);
x = read(file1, &buffer, 1);
if (x != 1) {
fprintf(stderr, "can't read 1 byte");
exit(-1);
}
x = write(file2, &buffer, 1);
if (x != 1) {
fprintf(stderr, "can't write 1 byte");
exit(-1);
}
}
write(STDOUT_FILENO, "Reversal & Transfer Complete\n", 5);
close(file1);
close(file2);
return 0;
}
any insight as to how I can accomplish this "re-factoring" of sorts would be much appreciated, thanks!

Answer 1

The assignment demands a different architecture than your program. Unfortunately, this will not be a refactoring but a rewrite.
You have most of the pieces of read_file and write_file already: opening the file, determining its length, error handling. Those can be copy-pasted into the new functions.
But read_file should call malloc and read the file into memory, which is different.
You should create a new function in reverse.c, called by main, to reverse the bytes in a memory buffer.
After that function runs, write_file should attempt to open the file, and only do its error checking at that point.
Your simple program is superior because it validates the output file before any I/O, and it requires less memory. Its behavior satisfies the assignment, but its form does not.