c - What's the difference between a VLA and dynamic memory allocation via malloc?

Question

I was curious with this:

What is the diference between:

const int MAX_BUF = 1000;
char* Buffer = malloc(MAX_BUF);

and:

char Buffer[MAX_BUF];

Answer 1

• Case 1: In

   char Buffer[MAX_BUF];
  

  Buffer is an array of size MAX_BUF. The allocation technique is called VLA.

• Case 2: In

  const int MAX_BUF = 1000;
  char* Buffer = malloc(MAX_BUF);
  

  Buffer is a pointer which is allocated a memory of size MAX_BUF which is 1000.

and, an array is not the same as a pointer, and C-FAQ has a Very Good collection detailing the reasons.

The major difference, in terms of usability and behaviour are:

1. (1) is on stack, usually ^Note, while (2) is on heap, always.
2. (1) has fixed size once allocated, (2) can be resized.
3. (1) is allocated when the enclosing function is called and has the block scope OTOH, (2) is allocated memory dynamically, at runtime and the returned memory has a lifetime which extends from the allocation until the deallocation.
4. (1) allocated memory need not be managed by programmer, while in (2) all malloc()d memory should be free()d. ^{[Courtesy: Giorgi]}

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^{Note: Wiki}

^{For example, the GNU C Compiler allocates memory for VLAs on the stack.}