Pointer to string changes its value unexpectedly

Question

I have noted that, when writing a string in an array allocated with malloc(), its value changes. To be clear, here is the code that replicates this \"error\":

Answer 1

The string literal "something" represents a null-terminated series of characters that live in some read-only region (most likely .text if you build/link/run on a GNU/Linux system).

Initially, a_p points to somewhere in the memory region within which malloc is allocating buffers. You can freely write to the location to which a_p points, free() it, etc. When you perform the assignment a_p = "something";, you are not writing characters to the location to which a_p points, but rather changing that pointer to point to the beginning of that string. Now, you're pointing into (read-only) memory that wasn't allocated by malloc so its corresponding free function isn't able to do anything meaningful.

Use strcpy (or better yet for safety, strncpy) instead to copy characters to the destination, a_p.