Wednesday, July 31, 2013

arrayName^ptrVar - headfirstC

How important? So low-level! I feel precise grasp gives us confidence. There are quite a lot of nuances due to this fundamental difference.
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void f(){
 int arr[33] = ...
 int* ptr = ...;

[[headfirst C]] P79 clearly explains that compiler will allocate stack memory for a variable ptr but not for arr. Basically, the "arr" variable doesn't exist in the final executable.

(Assuming a 32bit) If you look at the size of the stack frame, you find 4 bytes for "ptr" and 33*2 bytes for "arr". No allocation for the "arr" variable! Compiler allocates 4 bytes for the ptr VARIABLE "ptr" partly because we can later reseat this pointer. In contrast, the "arr" name is a not reseatable and a permanent alias for the address of array object. Every occurence of "arr" is replaced (by compiler) with the actual address of the array object.

arr = anotherArray;// won't compile.
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On P59, the book points out another key difference between array Name vs ptr Variable

Taking the address of "ptr" gives you the address of the 4-byte ptr VARIABLE. So suppose ptr points to address 0x0197. &ptr may be 0xb762.  In contrast,

&arr == arr // Both expressions evaluate to an address (like 0xb755) of the array OBJECT. There's no separate allocation or separate address for the "arr" variable (non-existent). The "arr" is just a name in source code.

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