String manipulation in C?

Question

I have a C string of unknown length, it is itself a concatenation of other strings and looks like this:

S = ABCD

Where B is an encrypted value while C and D are hash values.

A is the only human readable thing and can take 1 of 4 possible values, and hence can have 4 different length.

Once I compared A to its 4 possible values, I have thus obtained its length.

I also know that both C and D are hash values and that they must be 20bytes long (20 chars).

How can I retrieve B without modifying S?

Answer 1

Try this, with code borrowed from Ratchetr:



#insert

#insert



char *getB(char *S,

size_t lenA /*some value you already know how to compute*/

) {

size_t lenS = strlen(S),

lenC = 20,

lenD = 20,

lenB = lenS - (lenA + lenB + lenC);

char *B=malloc(lenB+1);

strncpy(B, S+lenA, lenB);

return B;

}



Don't forget to call free(B) when you no longer need it.



@I_got_reported: The title makes it clear that he means C strings, the only strings available in C, unless you write your own.

Answer 2

You didn't say what TYPE of string you were using, be it c-string or real strings. If you are using c-strings, the array is just that, an array; you can manipulate it just like any other array, just have a function return the second value of the array.



That may work on the string class/object, but I'm not sure, you'd have to do some research on that or try it yourself. It may work, it may not.

Answer 3

Assuming it is a valid C string (a nul terminated array of char), then:



lenS = strlen(S);

lenA = ;

lenC = 20;

lenD = 20;



Now you just solve for lenB:



lenB = lenS - (lenA + lenB + lenC);

Answer 4

Jesus christ, you will not find the answer to that here. Seriously, Yahoo answers is great for some stuff, but for this kind of thing take your issue to stackoverflow.com