C - Writing to binary failed (rewrites a file) - c

Question

I'm not sure what I'm doing wrong, trying to write a simple program that encrypts after being ran once renames. When the method is ran twice it is supposed to do the same thing but instead it decrypts the file. The first version worked but it left the old version and made a new version and I wanted a program that I could run once to encrypt and again to decrypt allowing me to change the file extension in the process, for convenience.
When I run the program it crashes on fwrite().
#include <stdio.h>
#include <stdlib.h>
#define KEY '&'
int main(void)
{
FILE *fp; // file pointer
size_t size, test; // file size
char src_file[FILENAME_MAX], dst_file[FILENAME_MAX];
int orig_char, new_char;
int i = 0;
printf("Enter the name of the source file \"file.ext\": ");
scanf("%s", src_file);
if ((fp = fopen(src_file, "rb")) == NULL) { // open file
fprintf(stderr, "Can't open \"%s\"\n", src_file);
exit(EXIT_FAILURE);
}
fseek(fp, 0, SEEK_END); // find the end of file
size = ftell(fp); // file size
fseek(fp, 0, SEEK_SET); // set file position to start
unsigned char buffer[size], *temp = buffer; //buffer
test = fread(buffer, sizeof(buffer[0]), size, fp);
printf("size written: %d, size of file: %d\n", test, size);
if (test != size) {
fprintf(stderr, "Error: operation fwrite failed!\n");
exit(EXIT_FAILURE);
}
fclose(fp);
printf("Enter the name of the destination file \"file.ext\": ");
scanf("%s", dst_file);
if ((fp = fopen(src_file, "wb")) == NULL) {
fprintf(stderr, "Can't open \"%s\"\n", dst_file);
exit(EXIT_FAILURE);
}
puts("Test1");
for (i = 0; (size_t)i < size && orig_char != EOF; i++, temp++) {
orig_char = (int) *temp;
new_char = orig_char ^ KEY;
*temp = new_char;
}
puts("Test3");
test = fwrite(buffer, sizeof(buffer[0]), size, fp);
fclose(fp);
free(buffer);
if ((rename(src_file, dst_file)) != 0)
fprintf(stderr, "Failed to rename file, make sure file doesn't" \
"already exist!\n");
return 0;
}

Answer 1

Here is my final code in case anyone else comes across this problem, I cleaned it up a tad and added a loop feature for offsetting multiple files. I also added some error recovery.
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#define KEY '&'
int main(void)
{
FILE *fp; // file pointer
size_t size, test; // file size
char src_file[FILENAME_MAX], dst_file[FILENAME_MAX];
char ch = 'Y';
int orig_char, new_char;
int i = 0;
while (toupper(ch) != 'N') {
printf("Enter the name of the source file \"file.ext\": ");
scanf("%s", src_file);
while ((fp = fopen(src_file, "rb")) == NULL) { // open file
fprintf(stderr, "Can't open \"%s\"\n", src_file);
printf("Enter the name of the source file \"file.ext\": ");
scanf("%s", src_file);
}
fseek(fp, 0, SEEK_END); // find the end of file
size = ftell(fp); // file size
fseek(fp, 0, SEEK_SET); // set file position to start
unsigned char buffer[size], *temp = buffer; // buffer
// send file to buffer
test = fread(buffer, sizeof(buffer[0]), size, fp);
printf("size written: %d, size of file: %d\n", test, size);
if (test != size) {
fprintf(stderr, "Error: operation fwrite failed!\n");
system("Press any key to continue");
exit(EXIT_FAILURE);
}
fclose(fp);
while ((fp = fopen(src_file, "wb")) == NULL) {
fprintf(stderr, "Can't open \"%s\"\n", src_file);
printf("Enter the name of the source file \"file.ext\": ");
scanf("%s", src_file);
}
// offset buffer data
for (i = 0; (size_t)i < size && orig_char != EOF; i++, temp++) {
orig_char = (int) *temp;
new_char = orig_char ^ KEY;
*temp = new_char;
}
// write buffer to file
test = fwrite(buffer, sizeof(buffer[0]), size, fp);
fclose(fp);
printf("Enter the file's new name \"file.ext\": ");
scanf("%s", dst_file);
while ((rename(src_file, dst_file)) != 0) {
fprintf(stderr, "Failed to rename file, make sure file doesn't already exist!\n");
}
printf("size written: %d, size of file: %d\n", test, size);
printf("File Successfully offset\n\n");
printf("Would you like to continue: ");
scanf(" %c", &ch);
}
return 0;
}