return unknown size string from argument function in c

Question

I have a function and want to return the unknown size string from function arguments like that:

int encrypt(const char *input, char *output)
{
     output_len = generate_random(); // random output size
     ...
     output = malloc(output_len);
     memcpy(output, data, output_len);
     return 0;
}

when I call the function like below out_buffer is not change and is null

char *out_buffer;
encrypt(in_buffer, out_buffer);

I change my code and use the pointer to pointer like this:

int encrypt(const char *input, char **output)
{
     output_len = generate_random(); // random output size
     ...
     *(output) = malloc(output_len);
     memcpy(output, data, output_len);
     return 0;
}
....
char *out_buffer;
encrypt(in_buffer, &out_buffer);

But it does not work.

Answer 1

You forgot to put a * before output here:

memcpy(*output, data, output_len);
//     ^

The program compiled without errors/warnings because memcpy takes a void pointer and any pointer type is compatible with void* .

Bonus hint

Dont write *(output) but just *output . Though having the parenthesis is not wrong, but just odd.