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Data Segment -- C lang

Posted on 2007-10-08
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What is the difference between

Heap vs Data Segment vs Stack and BSS ?
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Question by:gauravflame
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grg99 earned 1000 total points
ID: 20033250
Well, the question is a little bit too assuming on details, but here goes with the general drift:

"BSS" is where initialized data in a C program is usually kept.

The "Stack" is the most common place where local variables are put, along with function return addresses and parameters.

The "Data segment" is a generic term, often theplace where uninitilized global variables go.

The Heap is from where malloc() space comes from

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by:gauravflame
ID: 20033300
what do you mean by initialized data -- BSS
int a;
or
int a = 10;
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by:grg99
ID: 20034263
I meant "initialized to something other than zero".

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by:peetm
peetm earned 1000 total points
ID: 20035521
In C, something like the data in this would/could be stored in the bss segment:

char * p = "Boo!";

Note that that is not the case however with

char boo[] = "Boo!";

The former has p pointing to a literal, whilst the latter *initializes' the boo character array to the characters in the initializer [which is given in string form].

bss will also contain stuff like the initial value given to a variable, e.g.,

int a = 10; // the 10 *may* be dragged in from the bss segment.

However, it is more likely that the 10 will exist in the generated code, e.g,

mov ax, ah

It's up the the compiler writer - and perhaps the linker writer if the compiler writer has allowed the latter such access.

Also see here for bss stuff:

http://en.wikipedia.org/wiki/.bss

As far as the stack/heap are concerned - well, a disclaimer first ... the C standard includes not a single mention of these terms.

However, the heap is *usually* what C compiler writers will use and refer to as arbitrary memory available to an application vai calls to malloc/calloc etc.

The stack is mainly used for temporary variables, and the return address of functions - as has been pointed out.

e.g., a and b here would be allocated on the stack - if the compiler writer uses such a thing [they could be kept in a linked list on the heap for example.]

int main(void)
{

int a;
int b;

scanf("%d", &a);

// blah blah

return 0;

}
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by:gauravflame
ID: 20036780
clear
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