c - Stack pointer difference for char pointer and array

Question

I have a char array as below:

 char buffer[100]

And another char pointer as below:

 char *buffer
 buffer = malloc(100)

When I use GDB to check out the stack pointer, they are actually different. Why?

Answer 1

That is because the char buffer[100] will be allocated on the stack, which will occupy 100 bytes of storage. Therefore the stack pointer esp/rsp will point to a lower memory (taking stack grows downwards)

 +-    +------------+   <-- ebp
 |     |            |
 b     +------------+
 u     |            |
 f     +------------+
 f     |            |       holds 100 elements of buffer array       
 e     +------------+
 r          .
            .
 a          .
 r     +------------+
 r     |            |
 +-    +------------+  <-- esp

And in the case of char *buffer only one char * type object's memory (sizeof (char *)) will be allocated on the stack. When you do buffer = malloc (100) the base address of a memory block with 100 bytes guaranteed will be returned. This allocated memory is generally taken from the heap. Therefore now buffer holds the base address of the just allocated memory block. So, in this case because the memory is from the heap, and the stack only holds the char * type object, therefore the stack pointer is on higher location (taking stack grown downwards)

    +------------+   <-- ebp
    |   0xabcd   |             buffer , char * type
    +-----+------+   <-- esp
          | 
          |
          |             0xabcd 0xabce
          |             +-----+-----+-----+       +-----+-----+
          +------------>|     |     |     | . . . |     |     | 
                        +-----+-----+-----+       +-----+-----+
                                     0xabcf . . .

                        |                                     |
                        +------ 100 bytes mem block in heap --+ 

Also note Richard J. Ross III's comment.