Returning char array in C function

Question

I've quite a few problems with a very small C program I've been writing. Looked through all the stack overflow articles I could find but haven't had much success. The program is suppose to use a very simple XOR "encryption" on a plain text string. Both input string and key are 6 chars long. I'm new to C and pointers. I think i'm failing to grasp some the fundamentals of the language.

#include <stdio.h>
#include <string.h>

#define LENGTH 7
#define KEY "secret"

char * encryptDecrypt(char *plaintext);

int main(int argc, char **argv)
{
    if(argc > 1)
    {
        char *plainText = argv[1];
        printf("Encrypting plaintext: %s (%i)\n", plainText, strlen(plainText));
        char *cipherText = encryptDecrypt(plainText);
        printf("Encrypted: %s (%i)\n", cipherText, strlen(cipherText));
        char *decryptedText = encryptDecrypt(cipherText);
        printf("Decrypted: %s (%i)\n", decryptedText, strlen(decryptedText));
    }
    return 0;
}

char * encryptDecrypt(char *text)
{
    char result[LENGTH];
    for (int i = 0; i < LENGTH-1; i++)
    {
        result[i] = (char)(text[i] ^ KEY[i]);
    }
    char *resultPtr = &result;
    return resultPtr;
}

Running the program with arg "foobar" outputs:

Encrypting plaintext: foobar (6)

Encrypted: ╠╠╠╠╠╠╠╠T¨ (19)

Decrypted: ╠╠╠╠╠╠╠╠T¨ (19)

Problems:

1. Printing the pointer to the result array is different when used in the encryptDecrypt function and after it has been returned
2. Using XOR on the cipher text isn't reverting it back to the original plain text (although because whatever i'm printing is wrong, this part may be alright)
3. The string length of the encrypted/decrypted text is 19 chars long? How is that possible if the original string was 6 chars?

Answer 1

 char *resultPtr = &result;
 return resultPtr;

You can't do this. result doesn't exist when function ends, you can't return result.

Modify your function like this:

void encryptDecrypt(char *text, char *result)
{
  for (int i = 0; i < LENGTH - 1; i++)
  { 
    result[i] = (char)(text[i] ^ KEY[i]);
  }
}

create an array at the caller site, and pass it as result. e.g.

char result[LENGTH] = {0}; 
encryptDecrypt(plainText, result);

---

NOTE: Actually using %s for printing encrypted data is not best idea, because for example as a result of XOR you may get a null byte in between the text, which will be considered as a null terminator for your string, and printf won't show rest of the string. Consider something like this for printing cipher text.

Your format specifier for strlen is also wrong, use %zu insted of %i, or you will trigger undefined behaviour.

Answer 2

you cant return a pointer to a local variable. You have to either

• pass in the address of a buffer
• malloc space and return it (the caller then has to release it)
• strdup the local variable and return that (this is really just malloc wrapped up for you)

The reason you can't return a pointer to local variables is because they are destroyed when you exit the function

Answer 3

Your encryptDecrypt is returning the address of a local variable. That memory is invalid once the function returns, so attempting to use it is undefined behavior.

You should use malloc to dynamically allocate the memory for the result of this function. Also, you should pass in the size of the string so you know how much space to allocate.

Printing the encrypted string as a string is also undefined behavior, because what you have is not a string but an array of characters which is not null terminated.

Also, be sure to use the %zu format specifier when printing the result of sizeof.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define LENGTH 7
#define KEY "secret"

char * encryptDecrypt(char *text, int len);

int main(int argc, char **argv)
{
    if(argc > 1)
    {
        char *plainText = argv[1];
        // use the string length + 1 to include the null terminator
        int len = strlen(plainText)+1;
        printf("Encrypting plaintext: %s (%zu)\n", plainText, strlen(plainText));
        char *cipherText = encryptDecrypt(plainText,len);
        //printf("Encrypted: %s (%zu)\n", cipherText, strlen(cipherText));
        char *decryptedText = encryptDecrypt(cipherText,len);
        printf("Decrypted: %s (%zu)\n", decryptedText, strlen(decryptedText));
        // clean up the allocated memory
        free(cipherText);
        free(decryptedText);
    }
    return 0;
}

char * encryptDecrypt(char *text, int len)
{
    char *result = malloc(len);   // allocate memory for the result
    for (int i = 0; i < len; i++)
    {
        // if the text is longer that the key, wrap around on the key
        result[i] = (char)(text[i] ^ KEY[i%LENGTH]);
    }
    return result;    // return the allocated buffer
}

EDIT:

Fixed the indexing on KEY to prevent overruning it.

Answer 4

The result character array in encryptDecrypt function is a local variable to this function only and it's content will remain in the memory as long as encryptDecrypt function is executing. Once the function execution finishes array result content may or may not be present in the memory.(If the content is destroyed then it will contain some garbage value).

Array name itself points to the first element of the array.
So char *resultPtr = &result; is sending the address of pointer pointing to first element. Instead you should write char *resultPtr = &result[0]; or char *resultPtr = result;

To make your code working, replace
char result[LENGTH];
with

 char * result = (char*)malloc(sizeof(LENGTH));
 result[LENGTH-1] = '\0';      // To make result string null terminated

malloc function is used for dynamic memory allocation, now the content present in result array will stay in the memory(even after the execution of the function is over) until it is explicitly deallocated from the memory using free function.

NOTE: Even though result string after encryption is null terminated but we can not use printf("%s",result); to print it because encryption method uses XOR which even can make other characters of string as null terrminator. To print such a string you have to create your own function to print first 6 bytes of the result array .