question about memory

simo_mon · 03-20-2009, 05:12 AM

i have some general questions about c and memory

if i go

Code:

char *buff = "timmy";

is that a string literal and do string literals and ints go on the memory stack and mallocs on the heap?

ie

Code:

/*stack*/

int i =0;
char* buff = "timmy";
char *ptr = NULL;

/*heap*/

ptr = malloc(sizeof(char)*50);

is the stuff on the stack in ram ?
is the difference between the stack and the heap is that the stack is ordered and heap is potentially fragmented ( sorry but i gotta know )?

last question

when i call a function in main called loadFile

Code:

int main()
{
   
   FILE *fp;
   int someInt;

   loadFile(fp,"somefile",&someInt);

}

is loadfile now on the stack, and say that loadfile changes the value of fp and someint,

Code:

loadfile(FILE *fp,char *,int * x)
{
   *x=5;
...

}

now fp anf someint are also on the stack and the value at that memory location is
changed?

vart · 03-20-2009, 05:50 AM

Quote:

Originally Posted by simo_mon

Code:

char *buff = "timmy";

should be const char*

Quote:

is that a string literal

yes

Quote:

and do string literals and ints go on the memory stack

variable buff - a pointer - goes to the stack (takes 4 bytes there probably)
string itself is stored elsewhere taking strlen("timmy") + 1 byte there

Quote:

and mallocs on the heap?

pointer is stored on the stack and memory itis pointing on the heap

Quote:

is the stuff on the stack in ram ?

yes

Quote:

is the difference between the stack and the heap is that the stack is ordered and heap is potentially fragmented ( sorry but i gotta know )?

have no idea, why bother? the main difference - stack is limited by compiler settings with values like 1 or 2 M heap is limited by the computer virtual memory size available to one process (2G on XP 32 bit if I'm not mistaken)

Quote:

when i call a function in main called loadFile

Code:

int main()
{
   
   FILE *fp;
   int someInt;

   loadFile(fp,"somefile",&someInt);

}

is loadfile now on the stack, and say that loadfile changes the value of fp and someint,

Code:

loadfile(FILE *fp,char *,int * x)
{
   *x=5;
...

}

now fp anf someint are also on the stack and the value at that memory location is
changed?

the code of the function is never on the stack,

parameters of the function like fp, pointer to "somefile", pointer to someIntfp are placed on the stack before function call

local variables of the function are allocated on the stack as well after the call to it

after function exits - stack pointer returns where it was before function call making all local variables unavailable (values are still there but this space is noted as free and could be used any moment for example by the next function call)

Mortissus · 03-20-2009, 07:11 AM

Quote:

Originally Posted by vart

should be const char*

Why?

Elysia · 03-20-2009, 07:32 AM

Because:
http://apps.sourceforge.net/mediawik..._be_const_char

Mortissus · 03-20-2009, 07:46 AM

So obvious... I am a little rusted.

Thanks ;D

DL1 · 03-20-2009, 08:00 AM

Quote:

Quote:
and do string literals and ints go on the memory stack

variable buff - a pointer - goes to the stack (takes 4 bytes there probably)
string itself is stored elsewhere taking strlen("timmy") + 1 byte there

But if buff were global or static surely it wouldn't go on the stack?

simo_mon · 03-20-2009, 08:00 AM

awesome response_s

thanks very much for answering

its slowly coming together

its actually really helpful to get a handle on what's actually happening in c with memory

and i didn't know that

Code:

char *buffer = "timmy";

is in read only memory

and that

Code:


char buffer[] = "timmy";

lets you write over the memory for the same amount of space...

Elysia · 03-20-2009, 08:02 AM

Quote:

Originally Posted by DL1

But if buff were global or static surely it wouldn't go on the stack?

It wouldn't. The actual pointer is stored in some other space reserved for static and global variables.
Still, the memory allocated with malloc would still be stored on the heap and the pointer only stored in the static and global storage.

Note, however, that is not certain. The C Standard does not mention anything about this. So it's all up to the OS itself.
String literals are very often read-only, but may not be, depending on the OS.
The different storage sections are also different depending on OS.

03-20-2009, 05:12 AM	#1
simo_mon Registered User Join Date: Aug 2008 Posts: 13	question about memory i have some general questions about c and memory if i go Code: char buff = "timmy"; is that a string literal and do string literals and ints go on the memory stack and mallocs on the heap? ie Code: /stack/ int i =0; char buff = "timmy"; char ptr = NULL; /heap/ ptr = malloc(sizeof(char)50); is the stuff on the stack in ram ? is the difference between the stack and the heap is that the stack is ordered and heap is potentially fragmented ( sorry but i gotta know )? last question when i call a function in main called loadFile Code: int main() { FILE fp; int someInt; loadFile(fp,"somefile",&someInt); } is loadfile now on the stack, and say that loadfile changes the value of fp and someint, Code: loadfile(FILE fp,char ,int x) { *x=5; ... } now fp anf someint are also on the stack and the value at that memory location is changed?
simo_mon is offline

03-20-2009, 07:46 AM	#5
Mortissus Registered User Join Date: Dec 2004 Location: Brazil, Porto Alegre Posts: 152	So obvious... I am a little rusted. Thanks ;D
Mortissus is offline

03-20-2009, 08:00 AM	#7
simo_mon Registered User Join Date: Aug 2008 Posts: 13	awesome response_s thanks very much for answering its slowly coming together its actually really helpful to get a handle on what's actually happening in c with memory and i didn't know that Code: char *buffer = "timmy"; is in read only memory and that Code: char buffer[] = "timmy"; lets you write over the memory for the same amount of space...
simo_mon is offline

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