Get C XOR return value - c

Question

In the for loop, it prints the correct value 11100001 in each loop, but the main call print char_str is blank.
I hope it can return 11100001 as char.
//xor
char * xorencrypt(char * a, char * b) {
size_t alen = strlen(a);
size_t blen = strlen(b);
char * encrypted = malloc(alen + 1);
int i;
for (i = 0; i < 8; i++) {
encrypted[i] = a[i] ^ b[i];
printf("%s\n", encrypted[i]);
}
encrypted[alen] = '\0';
return encrypted;
}
main {
char * char_str = xorencrypt("11011000", "00111001");
printf("%s\n", char_str);
}

Answer 1

Your code needs a bit of refactoring.
1) You need to include some headers
#include <stdint.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
2) the 'main' function should return an int
int main() {
/* code here */
return 0;
}
3) You should make sure the call to malloc succeeds
char * encrypted = malloc(alen + 1);
assert(encrypted != (char*)0); /* requires #include <assert.h>
4) You should be careful when xor'ing ones and zeros. You are handling chars like integers
you are xoring zeros (value 48 in ascii) with ones (value 49 in ascii)
encrypted[i] = a[i] ^ b[i];
you want something like this instead
int a_i = a[i] - '0';
int b_i = b[i] - '0';
encrypted[i] = (a_i ^ b_i) + '0';

Answer 2

A reworked version of the code that assumes you are dealing exclusively with strings of the binary digits '0' and '1'. If you are dealing with more general strings, you will need a different solution.
#include <assert.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
static
char *xorencrypt(char *a, char *b)
{
size_t alen = strlen(a);
char *encrypted = malloc(alen + 1);
assert(alen == strlen(b));
assert(encrypted != NULL);
for (size_t i = 0; i < alen; i++)
{
assert(a[i] == '0' || a[i] == '1');
assert(b[i] == '0' || b[i] == '1');
encrypted[i] = (a[i] ^ b[i]) + '0';
putchar(encrypted[i]);
}
encrypted[alen] = '\0';
putchar('\n');
return encrypted;
}
int main(void)
{
char *char_str = xorencrypt("11011000", "00111001");
printf("%s\n", char_str);
free(char_str);
return 0;
}
Amongst the changes:
Error check the memory assignment. Using assert() is a bad way to do it in production code, but it does ensure that you check that the memory is allocated.
Check that the strings are the same length.
Remove unused variable blen.
The static is optional; it means the code compiles cleanly under the stringent options I use (which require a prototype declaration of every non-static function before it is defined or used).
The loop index i is of the same type size_t as alen to avoid warnings about comparing signed and unsigned values. I'm using the C99 style 'declare a variable in a for loop' notation.
The upper bound of the loop is based on the measured length of the strings, not a fixed constant.
The original version of the XOR operation either generated a null '\0' or a control-A '\1' for each character.
The revised version of the crucial XOR operation ensures that the result is a printable digit.
The printf("%s\n", encrypted[i]); in the original passed a character to be printed as a string. If your compiler wasn't warning you, turn on the warning options or get a better compiler.
If you'd written printf("%s\n", &encrypted[i]); you would have had problems, potentially, with a string that is not guaranteed to be null terminated inside the loop (though you did null terminate the string after the loop, which was good).
The code in main() frees the allocated memory. It is good practice to ensure there is a free() for every malloc().
I prefer to explicitly return 0 from main(), even though C99 says you don't have to.
This answer was started while the question was active. Then life got in the way.