How to read past EOF from getc? - c++

Question

I am writing a XOR encryption program which works fine during encryption but during decryption
the
char ca2=fgetc(f);
gets stuck at one point and no decryption takes place after that my best guess about the problem is (the encrypted file contains all sorts of characters ) as soon as fgetc reaches EOF mark which can be present before the actual end of the file it gets stuck there and stop reading the next characters .
is this some kind of limitation of getc() ? here is my rubbish code
int get_file_size(char filename[])
{
FILE *p_file = NULL;
p_file = fopen(filename,"rb");
fseek(p_file,0,SEEK_END);
int size = ftell(p_file);
fclose(p_file);
return size;
}
int endec(char filename[],char psdw[])
{
FILE *f;
int hashed=0,ed=0;
int inphash=inhash(psdw);
inphash=inphash%50;
f=fopen(filename,"r");
if(f==NULL)
printf("failed");
char temps[999999];
long int crs=0,j=0;
int filesz=get_file_size(filename);
printf("file size = %d\n\n",filesz);
while(1){
inphash=inphash+2;
char ca=(char)inphash;
char ca2=fgetc(f);
printf("%c\n",ca2);
if(crs>=filesz)
break;
temps[crs]= ca2 ^ ca;
crs++;
}
fclose(f);
printf("%d",strlen(temps));
FILE *fp;
fp=fopen(filename,"wt");
for(j=0;j<crs;j++){
putc (temps[j] , fp);
printf("%c",temps[j]);
}
fclose(fp);
}

Answer 1

Your problem is right here:
f=fopen(filename,"r");
You open the file for text reading, not for binary. Your file size function gets it right, but your decoder function does not.
The idiomatic way to read a file character by character using the C-style IO routines is like this:
f = fopen(filename, "rb");
if (!f)
// handle error
int c; // NOTE: int, not char!
while ( (c = fgetc(f)) != EOF )
{
// do something with 'c'
}
This idiom does not require you to get the file size as a separate operation. You can rewrite your XOR "encryption" routine with a simple loop of the above form. It will be much clearer and more concise.
Your entire decoder function could be rewritten as follows: (minus the debug code)
int endec(char filename[], char psdw[])
{
int inphash = inhash(psdw) % 50;
char temp[999999]; // really, should be std::vector<char>
FILE *f;
if ( (f = fopen(filename, "rb")) == NULL )
{
printf("opening for read failed\n");
return -1;
}
size_t crs = 0;
int c;
while ( (c = fgetc(f)) != EOF )
{
inphash += 2;
temp[crs++] = (char)(inphash ^ c);
}
fclose(f);
if ( (f = fopen(filename, "wt")) == NULL )
{
printf("opening for write failed\n");
return -1;
}
if (fwrite(temp, crs, 1, f) != crs)
{
printf("short write\n");
fclose(f);
return -1;
}
fclose(f);
return 0;
}
Not stellar error handling, but it is error handling.