Add comments and error checking to A9

Assignment9/src/main.c

@@ -7,16 +7,17 @@
 #include "png.h"
 
 int main(int argc, char *argv[]) {
+	// Error handling
 	if (argc != 2) {
 		printf("Usage: %s <png file>\n", argv[0]);
-		exit(1);
+		exit(EXIT_FAILURE);
 	}
 	if (strlen(argv[1]) > PATH_MAX) {
 		printf("Path too long\n");
-		exit(1);
+		exit(EXIT_FAILURE);
 	}
+	// Read file
 	char *path = argv[1];
-
 	char *png_buffer = load_file(get_fd(path));
 	if (!is_png(png_buffer)) {
 		printf("It's not a PNG file\n");
@@ -26,6 +27,8 @@ int main(int argc, char *argv[]) {
 	}
 	png_chunk **chunks = get_png_chunks(png_buffer);
 	free(png_buffer);
+	// Done with buffer, as chunks are a structured way to access the data
+	// Iterate over chunks to display info and "decrypt" IDAT chunks
 	int size = get_number_of_chunks(chunks);
 	for (int i = 0; i < size; i++) {
 		// Check if header is IDAT or IEND
@@ -41,11 +44,7 @@ int main(int argc, char *argv[]) {
 		}
 		printf("Chunk size is:%d\n", chunks[i]->length);
 	}
-	// Uncomment to not overwrite original file
-	//char new_path[PATH_MAX];
-	//getcwd(new_path, sizeof(new_path));
-	//strcat(new_path, "/output.png");
-	//write_png_chunks(new_path, chunks);
+	// Write file and free memory
 	write_png_chunks(path, chunks);
 	destroy_chunks(chunks);
 	return EXIT_SUCCESS;

Assignment9/src/png.c

@@ -39,12 +39,12 @@ int get_fd(char *path) {
 	FILE *fp = fopen(path, "rb");
 	if (fp == NULL) {
 		perror("fopen");
-		exit(1);
+		exit(EXIT_FAILURE);
 	}
 	int fd = fileno(fp);
 	if (fd == -1) {
 		perror("fileno");
-		exit(1);
+		exit(EXIT_FAILURE);
 	}
 	return fd;
 }
@@ -53,7 +53,7 @@ FILE *get_fp(int fd) {
 	FILE *fp = fdopen(fd, "rb");
 	if (fp == NULL) {
 		perror("fdopen");
-		exit(1);
+		exit(EXIT_FAILURE);
 	}
 	return fp;
 }
@@ -66,7 +66,7 @@ char *load_file(int fd) {
 	char *buffer = malloc(size);
 	if (buffer == NULL) {
 		perror("malloc");
-		exit(1);
+		exit(EXIT_FAILURE);
 	}
 	fread(buffer, 1, size, fp);
 	fclose(fp);
@@ -81,15 +81,16 @@ png_chunk *get_png_chunk(char *buffer, unsigned int offset) {
 	png_chunk *chunk = malloc(sizeof(png_chunk));
 	if (chunk == NULL) {
 		perror("malloc");
-		exit(1);
+		exit(EXIT_FAILURE);
 	}
 	memcpy(&chunk->length, buffer + offset + 0, 4);
+	// Convert length to host byte order because PNG lengths is in network byte order/Big Endian
 	chunk->length = ntohl(chunk->length);
 	memcpy(&chunk->type, buffer + offset + 4, 4);
 	chunk->data = malloc(chunk->length);
 	if (chunk->data == NULL) {
 		perror("malloc");
-		exit(1);
+		exit(EXIT_FAILURE);
 	}
 	memcpy(chunk->data, buffer + offset + 8, chunk->length);
 	memcpy(&chunk->crc, buffer + offset + 8 + chunk->length, 4);
@@ -100,9 +101,11 @@ png_chunk **get_png_chunks(char *buffer) {
 	png_chunk **chunks = malloc(sizeof(png_chunk *));
 	if (chunks == NULL) {
 		perror("malloc");
-		exit(1);
+		exit(EXIT_FAILURE);
 	}
+	// Running offset of the buffer
 	unsigned int offset = PNG_SIGNATURE_SIZE;
+	// Index of the current chunk
 	int i = 0;
 	while (true) {
 		chunks[i] = get_png_chunk(buffer, offset);
@@ -112,11 +115,14 @@ png_chunk **get_png_chunks(char *buffer) {
 			break;
 		}
 		i++;
-		chunks = realloc(chunks, sizeof(png_chunk *) * (i + 1));
-		if (chunks == NULL) {
+		// Add one because realloc is 1-indexed when multiplying by sizeof
+		png_chunk **err_check = realloc(chunks, sizeof(png_chunk *) * (i + 1));
+		if (err_check == NULL) {
+			destroy_chunks(chunks);
 			perror("realloc");
-			exit(1);
+			exit(EXIT_FAILURE);
 		}
+		chunks = err_check;
 	}
 	return chunks;
 }
@@ -126,6 +132,7 @@ int get_number_of_chunks(png_chunk **chunks) {
 	while (memcmp(chunks[i]->type, "IEND", 4) != 0) {
 		i++;
 	}
+	// Add one for the IEND chunk
 	return ++i;
 }
 
@@ -148,14 +155,16 @@ void write_png_chunks(char *path, png_chunk **chunks) {
 	FILE *fp = fopen(path, "wb");
 	if (fp == NULL) {
 		perror("fopen");
-		exit(1);
+		exit(EXIT_FAILURE);
 	}
 	fwrite(PNG_SIGNATURE, PNG_SIGNATURE_SIZE, 1, fp);
 	unsigned int size = get_number_of_chunks(chunks);
 	for (int i = 0; i < size; i++) {
+		// Convert length to network byte order for writing
 		chunks[i]->length = htonl(chunks[i]->length);
 		fwrite(&chunks[i]->length, 4, 1, fp);
 		fwrite(chunks[i]->type, 4, 1, fp);
+		// Convert back so we can accurately use it to write the length of the data
 		chunks[i]->length = ntohl(chunks[i]->length);
 		fwrite(chunks[i]->data, chunks[i]->length, 1, fp);
 		fwrite(&chunks[i]->crc, 4, 1, fp);

Assignment9/src/png.h

@@ -30,15 +30,14 @@
 
 #pragma once
 
+// Needed for uint types
 #include <arpa/inet.h>
-#include <stdio.h>
-#include <stdlib.h>
-#include <string.h>
 
 // http://www.libpng.org/pub/png/spec/1.2/PNG-Rationale.html#R.PNG-file-signature
 #define PNG_SIGNATURE "\211PNG\r\n\032\n"
 #define PNG_SIGNATURE_SIZE 8
 
+// The key used to "encrypt/decrypt" the PNG chunks
 #define KEY 42
 
 typedef struct png_chunk {