Nested Static structure initialization in C

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1.00/5 (2 votes)

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i have a nested structure which contains others structure within it, currently when i move from one page to another my previously initialized structure loses its values. by making it static i am not sure whether all the variables of my structures (even those nested structure variables) maintain there state or not, the non-structure variable of my structure do maintain their value when coming back to the same page(same .c file) where they were initialized.

eg:

C#

struct first {
    struct nestedfirst *nf;
    unsigned count;
    };

CSS

struct nestedfirst  {
    unsigned num;
    };

const struct first *f;

if i declare it as

static struct first *f;

will the nestedfirst structure value will remain available between different file(.c) calls, say i initialized it at file1.c and move to file2.c then comeback to file1.cs will i find its value unchanged if not what should i do to make it happen.

Posted 12-May-15 3:01am

RajneeshSaysHello

Updated 13-May-15 5:05am

Frankie-C

v5

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Comments

Sergey Alexandrovich Kryukov 12-May-15 9:29am

Not clear. What "page"? what looses what value. How about a code sample demonstrating it?
—SA

Andreas Gieriet 12-May-15 9:32am

What do you mean by "page" - there is no such term in C/C++.
Is it C or C++? They may behave differently depending on the context.
Cheers
Andi
PS: What you show in your question is not a nested struct. You show us a struct with two members: one is an int, the other is a pointer to a pointer to some struct.

Rajesh R. Sharma 12-May-15 10:13am

Can u provide a code sample ??

[no name] 12-May-15 11:24am

Only static data is guaranteed to be maintained throughout the application. For example if any pointers in a static structure refer to 'local' variables then those variables cannot be guaranteed if you move from the current function (either up or down). The fact that sometimes the data remains, does not guarantee it.

CPallini 12-May-15 16:05pm

Usually, when coding, you have to know in advance what your are doing.
What do you mean with 'page'?

Albert Holguin 12-May-15 16:49pm

You need to show the code where you are generating and passing the structures. Otherwise it's just a guessing game.

RajneeshSaysHello 13-May-15 1:07am

i updated question please see now

_Asif_ 13-May-15 2:32am

Reading your comments, it seems that you just have started learning c. We would suggest to read a book end to end and then start experimenting.

Frankie-C 13-May-15 11:04am

You don't have nested structures in your example, but pointers to other structures.
Again: explain better what you mean by 'page', what is a page?
If you declare a pointer to something, the pointer remains unchanged, but to the object to which it points can change in any way.

RajneeshSaysHello 14-May-15 0:59am

by page i mean file, fiel1.c , file2.c etc etc. thanks for ans, basically i want to have a static nested structure (with pointer , i will make the object to which pointer points to static too) which i can set inside a function because right now i can only set them during compile time.

3 solutions

Solution 1

You must work with pointers to the top structure. If not the top struct is copyied by value, but all values which are pointers get bogous.

Try to avoid such nesting it isnt good style. Get to a finner and better modularity.

Posted 12-May-15 6:54am

KarstenK

Comments

RajneeshSaysHello 13-May-15 1:08am

you mean static pointers or pointers in general, since i have lots of variables inside my structure with many other nested structure.

Solution 3

Can I suggest that before asking this kind of question you should consult documentation and then you should do a test yourself using your debugger. If you don't know how to use the debugger now is the time to learn.

Your use of the word page is confusing as you actually mean file or translation unit http://en.wikipedia.org/wiki/Translation_unit_%28programming%29[^].

Now if we look here:https://msdn.microsoft.com/en-us/library/s1sb61xd.aspx[^]
we learn:
•When you declare a variable or function at file scope (global and/or namespace scope), the static keyword specifies that the variable or function has internal linkage. When you declare a variable, the variable has static duration and the compiler initializes it to 0 unless you specify another value.

Internal linkage means only visible in the file you declare it and static duration means lives for the life of the program.

So yes if you declare it static it will retain its value throughout the life of the program.

You will need something like:

C++

static struct first *f = new first();
first->count = 12;
first->nf = new nestedfirst();

Posted 16-May-15 4:20am

User 59241

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This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Frankie-C · Accepted Answer · 2015-05-16T02:59:00

Solution 2

If the meaning of page for you is file then maybe I understood what your problem is.
You want to access to same structure from different modules. If so you need to define your structure in only one module, than use a reference to 'external' data in all other modules where you want access same structure. Don't use 'static' qualifier or your struct will not be accessible at all from other modules.
See this example:
1. Create an header file where you will define the protos of your structures:

C++

// header file "header.h" where you define structures

typedef struct _nestedfirst    //define inner structure
{
    unsigned num;
} stNESTEDFIRST;

typedef struct _first
{
    struct _nestedfirst *nf;
    unsigned count;
} stFIRST;

extern stFIRST MyStFirstStructure;

2. Include this header in all files where you need access to the structure, but in only one instanciate the structure:

C++

// file1.c
 #include <header.h>

stFIRST MyStFirstStructure;    //in file1.c we create the struct

int foo(void)
{
    MyStFirstStructure.nf->num = 0;
    ....
}

C++

// file2.c
 #include <header.h>

int bar(void)
{
    MyStFirstStructure.nf->num += 2;
    ....
}

Please take into account that while the storage attribute applies to the whole structure, meaning that all variables inside the structure get such attribute, in your case in your structure you have declared a pointer to a struct, not a struct!
So the pointer still is permanent and holds indefinitely the value you have set into it, but the nested structure referred by the pointer have its own storage class depending on what you put in its declaration.

Posted 16-May-15 2:59am

Frankie-C

Updated 17-May-15 22:43pm

v5

Comments

[no name] 16-May-15 10:21am

nf not assigned to new struct. This will blow up ---MyStFirstStructure.nf->num = 0;

Frankie-C 16-May-15 10:48am

Of course it's not assigned (and a lot of many many other initialization are missing), this is an just an *example* to explain how to make objects visible between modules, it is not a complete program! :-)
The user is not complaining on crashes or memory exceptions on null pointers, so we *must* assume that he knows how to deal with that.
Please concentrate on the question: how to make available same data aggregate between different modules. ;-)

[no name] 16-May-15 19:07pm

Are we sure that's the question? That's not how the last sentence reads. It seems to be about lifetime and not linkage which is what you are talking about.
I stick to my point about your example. What nf is pointed to is crucial here and to show operations on an unassigned nf could be misleading.

Frankie-C 17-May-15 3:17am

I think this is really a waste of time. The last sentence is:
"will the nestedfirst structure value will remain available between different file(.c) calls, say i initialized it at file1.c and move to file2.c then comeback to file1.cs will i find its value unchanged if not what should i do to make it happen."
Have you seen "say i initialized it at file1.c ..." we have to suppose that he has initialized everything :-).
What nf is pointed to is something that he has managed, if he put a pointer to a struct he has to update that pointer, if he doesn't do that he will have a memory problem, not different data between modules.
In my opinion he has simply declared the same variable in different modules, that leads to different instances, one for each module, each one with its own data.
We are criticizing our answers while no clarification is coming from user. His poor english (even worst than mine :-)) lets open any interpretation.
Even in your solution the use of 'static' on the pointer declaration can be opinablle if that pointer have to be shared between modules it will be not, or will trigger a compiler error if declared 'extern' also... ;-)

[no name] 17-May-15 3:37am

There are 2 or 3 possible interpretations - variable lifetime, linkage or both. You answered one I another. I was going to give yours a 5 when I noticed the matter in your answer I pointed out originally. Thats all. It is a correct answer to one interpretation. I did not pick any issue with you over the OP's question. Its not clear. Yes we know.

Frankie-C 17-May-15 3:52am

Once again because my english is not as good as I would like, please read my first sentence in the answer "maybe I understood what your problem is". I suppose the problem is that, of course there are many other meaning that you outlined. Maybe I'm wrong at all. I don't want to be rude at all :-)
Anyway there is not a sound from the asker, for this reason I said that maybe we are both wasting time, seems he is no more interested... :-D

RajneeshSaysHello 18-May-15 0:57am

it was my day off,,

Frankie-C you are right, i have initialized my structure at one page/file/module now i move to another module/page/file i could access the variable of my structure in another page/file/module but i am not sure about the variables of my nested structure, i have tested it and found and i have to somehow make all the variables to which any pointer variable is being pointed to - 'static'(making pointer variables of my pointer static). the whole situation is being created by nested structure. the pointer self address and the address which it points to remain available but data does not because of its scope and lifetime issues. i could simply pass the reference of pointer but i need to access then again without doing initialization ,, like if user again want to do the task i do not have to waste time doing whole initialization again.

Frankie-C 18-May-15 4:38am

Rajinesh it is very simple: the storage attribute applies to the whole structure, meaning that all variables inside the structure get the attribute. But in your structure you have declared a *pointer* to a struct, not a struct! So the pointer is permanent and holds indefinitely the value you have set into it, but the nested structure referred by the pointer have *its own storage class* as you put in the declaration.
In my example I declared the substructure at module level to make it permanent, if you create it with GC allocation, or as an automatic variable (created on the stack of a procedure) oof course you will fail to access it when, respectively, the GC recollects memory or when you leave the function.
To dynamically create the structures you can follow the example from pwasser uusing the new operator in C++, or you can use the malloc(sizeof(structure)); in C.
If this satisfy your question please accept the answers.

Nested Static structure initialization in C

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