Allocating memory in C Function

Question

Its memory wastage vs cpu utilization question.

If I want to merge 3 strings:

• Approach 1: Should I take all string lengths (strlen) and then allocate.

  char *s = malloc(strlen(s1)+strlen(s2)+strlen(s3)+1);
  

OR

• Approach 2: I should assume 1025 and allocate considering the fact that I know the strings will never go beyond 1025.

  #define MAX 1025
  char *s = malloc(MAX);
  

Please suggest.

Answer 1

Allocate memory for all 3 strings is better.

But if you are 100% (bolded because it is very important) sure that the string never exceed the fixed length, then go ahead. You have to foresee whether you may add things in the future that may exceed the max length, and you have to consider whether user input in any way can overflow the limit.

In case you may not need everything, you can also allocate fixed buffer and truncate the rest of the string if it is too long.

If the string has potential to grow very long (a hundreds of MB), then you shouldn't use this approach. In this case, the solution depends on your application.

Answer 2

Your malloc line looks fine.

The problem with your other alternative, assuming a fixed length, is that you may change your mind later about the fixed length without changing all of the corresponding problems in your code.