Does strcat_s() require the use of realloc()?

Question

Take the example code*:

char *string = (char*)malloc(sizeof(char));
strcat_s(string, strlen(string) + 10 + 1, "characters");

The above code compiles and runs, leading me to believe that memory reallocation is taking place. However when applied on a greater scale (recursively also), I receive memory errors in random places (different each time the program is run).

Could strcat_s() be exceeding boundaries? Is realloc() therefore required to ensure the memory is properly allocated?

Note: It could be that the errors are unrelated, although they have been coincidentally cropping up after applying the code from the example.

*The reason I've only allocated one byte initially, is that contextually I'm working with dynamic sizes, so the size of string will change, but by an unknown amount.

Answer 1

Here you allocate exactly 1 char

 char *string = (char*)malloc(sizeof(char));

so the only string the string can hold is "" (the zero length string)

Then you try to append the string "characters" to string which cannot hold a string other than "" and which is not initialized. Furthermore the result of strlen(string) will be undetermined, because again string is not initialized.

 strcat_s(string, strlen(string) + 10 + 1, "characters");

You probably want this:

char *string = (char*)malloc(sizeof(char) * 100);   // allocate 100 bytes
strcpy(string, "Hello ");
strcat_s(string, 100, "characters");

printf("%s\n", string);  // will display "Hello characters".

Answer 2

The above code compiles and runs, leading me to believe that memory reallocation is taking place.

Just because the program appears to behave as you expect does not mean that it is correct, or even that its behavior is defined at all from C's perspective.

However when applied on a greater scale (recursively also), I receive memory errors in random places (different each time the program is run).

Could strcat_s() be exceeding boundaries?

Yes.

Is realloc() therefore required to ensure the memory is properly allocated?

No.

Neither strcat_s() nor strcat() performs any reallocation. They are not specified to do so, and it would be unsafe for them to do so.

You receive errors because you are using the function incorrectly (even when you don't get errors). It is your responsibility to ensure that the second argument does not exceed the size of the array pointed to by the first, but you are flagrantly disregarding that responsibility. I presume you simply have a serious misunderstanding about what strcat_s() is supposed to do, and what its second parameter means.

The main thing that strcat_s() provides but strcat() doesn't is checking that the specified array bounds are not overrun as a result of the second string being longer than can be accommodated. This relieves you of checking the length of the second string before performing the concatentation, which is advantageous because strcat_s() can do that itself at very low cost, since it must scan that string anyway. strcat_s() has no more ability than any other C operation or function to determine independently how long is the array to which the first argument points. It relies on you to tell it.

If you need to accommodate dynamic adjustment of your array size, then that's your responsibility, as is tracking the current size of the allocation.