Affine Code Errors - c

Question

So I've been trying to experiement with some decryption algorithms and ciphers and tried to do some of them on my own. At them moment I'm writing in C an affine algorithm which I am trying to decrypt a line from a .txt file which I decrypted myself so here's where I think my problem appears since my code always pops text file corrupted and doesn't proceed to the next task.
file = fopen("encr_affine.txt", "r");
if(file)
{
while ((c = getc(file)) != EOF)
putchar(c);
fclose(file);
}
fgets(str,100,file);
if ( fgets(str,100,file) == NULL )
{
fprintf(stderr, "Text file corrupted\n");
return -1;
}
What is the mistake here? Is it reading the whole line of the txt file or just the first char? And why it doesn't proceed on into the next tasks?
Thank you in advance

Answer 1

It always prints your Text file corrupted message because the file is always closed (or never open) when it gets there.
Look what this first part does:
file = fopen("encr_affine.txt", "r");
if(file)
{
while ((c = getc(file)) != EOF)
putchar(c);
fclose(file);
}
It opens the file, then (if it's really open), it reads the entire file, byte by byte, then prints it all to stdout. It then closes the file.
Then you try to do stuff on a closed file:
fgets(str,100,file);
This would do nothing because the file is closed, and would return NULL. If the file were never opened, it would still return NULL due to an error.
But then you try it again:
if ( fgets(str,100,file) == NULL )
{
fprintf(stderr, "Text file corrupted\n");
return -1;
}
This fgets(str,100,file) always returns NULL because once again the file is closed.
Perhaps what you really want to do is to process the bytes one at a time in your first loop before printing them with putchar().