How to know if a pointer parameter has been passed as static pointer or dynamic pointer, on C ?

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4.00/5 (1 vote)

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I want to know on C language or C++, if I can know within a function if the real parameter of a formal pointer input/output parameter, let's say to a byte array or char array, has been passed using an array of defined size (static pointer) or a pointer created with calloc (dynamic pointer) from the caller. The formal parameter could have been defined as an array or as a pointer.

I need to know that because in the second case I could reallocate the memory to return, on this same parameter, a bigger size string compared to the original allocated space.

I have to do this because I am converting an existing library on C# to C, and at first it has to be exactly as the original, so dont scold at me.

I apologize because I dont know the right names for what I called the types of real parameters and someone can be confused but I don know.

Example >

C++

void FunctionName( char sCadenaHexa[], int SizeOfCadena )
{
  ... How I know how was called .... 
}

Called using "array defined size" or "static pointer" :

C++

char sRealCadena[100] = "something";
FunctionName(  sRealCadena, 100 );

Called using a pointer or "dynamic pointer" :

C++

char *sRealCadena = malloc( 100, sizeof(char) );
FunctionName( sRealCadena, 100 );

Of course I will try to redesign the function adding another parameter as the output string paremeter, but as first approach I have to stick to the current design.

If the thread is repeated Im sorry, but I didnt find it.

Thanx in advance

Posted 4-Mar-13 11:40am

fvalerin

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Sergey Alexandrovich Kryukov 4-Mar-13 18:07pm

I voted 4 just for interesting thinking, but no — wrong way. Please see my answer...
—SA

3 solutions

Solution 1

Never thought about it before - it's a nasty thing to do but I can see your problem.

Does this help? [^]

Posted 4-Mar-13 11:53am

OriginalGriff

Solution 3

Even not rtti would help you, as you can see:

XML

// rtti.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
#include <malloc.h>
#include <stdlib.h>
#include <typeinfo>

int _tmain(int argc, _TCHAR* argv[])
{
    char buffer[256];
    char *p1=buffer;
    char *p2=(char *)malloc(512);
    char *p3=new char[1024];

    //std::type_info
    _tprintf(_T("typename %S\n"),typeid(buffer).name());
    _tprintf(_T("typename %S\n"),typeid(p1).name());
    _tprintf(_T("typename %S\n"),typeid(p2).name());
    _tprintf(_T("typename %S\n"),typeid(p3).name());

    free(p2);
    delete [] p3;
    return 0;
}

Posted 5-Mar-13 21:19pm

jan.mach71

Comments

fvalerin 6-Mar-13 9:52am

This solution only applies to C++, on C I get the error :

This header requires a C++ compiler ...

I know I tagged this question as C or C++, so my mistake. I did it because sometimes stuff on C++ works on C, but this is not the case.

Anyway I will use it when I need this on a C++ project.

Thank you very much for your time and for the try.

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Sergey Alexandrovich Kryukov · Accepted Answer · 2013-03-04T12:07:00

This is a pretty interesting question, but this is not how C or C++ pointers work. There is no a reliable way of knowing if the pointer points to the heap of static region of memory (such as "data segment" in some platforms). In principle, it is possible is some (usually primitive) platforms, but there is no a universal way, and such way cannot exist.

If you think about it, you will see that this is actually good, a right decision. These languages are abstracted from the "types of memory". It allows for implementation of the languages and the applications compiled using those languages to be custom. It also allows to apply the same algorithms to different kinds of pointers. A user can use different types of allocation and deallocation, especially in C++, where the user can define custom new and delete operators. If is very usual to use sub-allocators and other custom memory management facilities.

Instead of finding the method of telling the difference between pointers, you should better design your code properly. If some method returns pointer as a result of allocation, it should be complement by the method used to deallocate memory. The side doing allocation should always deallocate. You should not allow the situations such as when you create a library with some methods are allocating, and the code using the library deallocates. This is a usual design mistakes which totally kills platform compatibility and presents other threats. Avoid it.

—SA