Where is global variables like $_GLOBAL , $_POST etc stored?

Question

When i attended an interview, the interviewer asked me this question. Which memory they are using heap , stack etc. I googled it but i didn't get any clear answer.

Answer 1

Well, since you tagged C, I'll start with that.

In the C runtime, global variables are stored in one of two places; the data segment or the BSS segment. The way you determine which one a particular variable belongs to is whether or not it is initialized.

Initialized global (and static) variables go inside the data segment.

Uninitialized global (and static) variables go inside the BSS segment.

Visually, the entire runtime looks like this:

 _______
|  Text |
|_______|
|  Data |   <-- Initialized globals / statics
|_______|
|  BSS  |   <-- Uninitialized globals / statics (basically a bunch of 0s)
|_______|
|       |
| Stack |
|_______|
|       |
|  Heap |
|_______|

Unlike variables on the stack and the heap, which are created at runtime, global variables exist as part of your program's executable image file (a.out, foobar.exe).

Answer 2

The values of $_POST internally are created inside php_auto_globals_create_post() and made available via PG(http_globals)[TRACK_VARS_POST], which is just a way to reference http_globals.

The definition of aforementioned http_globals tells us that it's an array of zval * elements, one for each $_POST, $_GET, $_COOKIE, etc. (arrays are also stored inside a zval container).

Allocating a zval is done via ALLOC_ZVAL(), which calls the following functions:

1. _emalloc()
2. _malloc()

The malloc() function allocates memory on the heap, so therefore the answer is heap.