pointer to structure from pointer to member

[sieg1974]

Hi,

Given a pointer to a member within a struct, is it possible to
determine the pointer to the structure? If so, how can it be done?

Thanks,

Andre

struct Structure
{
....
int Integer;
....
};

struct Structure * get_pStructure( int *pInteger )
{
....
}

 

[Frederick gotham]

sieg1974 posted:


Maybe something like:

struct Structure
{
...
int Integer;
...
};

struct Structure * get_pStructure( int *pInteger )
{

return (struct Structure*)
((char*)pInteger - offsetof(Structure,Integer));

 

[Frederick Gotham]

return (struct Structure*)
((char*)pInteger - offsetof(Structure,Integer));

Correction:

offsetof(struct Structure,Integer)

 

[shaanxxx]

Correction:

offsetof(struct Structure,Integer)

#include<stdio.h>

#define offsetof(a,b) (&((a *)0 )->b)

typedef struct xxx{
int a;
int b;
int c ;

}ttt;

int main()
{
printf("%d\n", offsetof(struct xxx,a));
printf("%d\n", offsetof(struct xxx,b));
printf("%d\n", offsetof(struct xxx,c));
}

 

[J. J. Farrell]

#include<stdio.h>

#define offsetof(a,b) (&((a *)0 )->b)

typedef struct xxx{
int a;
int b;
int c ;

}ttt;

int main()
{
printf("%d\n", offsetof(struct xxx,a));
printf("%d\n", offsetof(struct xxx,b));
printf("%d\n", offsetof(struct xxx,c));
}

I don't know what you're trying to say in this message, but this
results in undefined behaviour. offsetof() is a Standard macro defined
in <stddef.h>; there's no reason to define a macro of the same name
other than to introduce bugs or reduce portability.

 

[shaanxxx]

Actually i checked with man page, i didnt find there so i thought it is
not in standard(wrong thoughts). I AM NOVICE TO "C" langauge.

 

[Jack Klein]

Actually i checked with man page, i didnt find there so i thought it is
not in standard(wrong thoughts). I AM NOVICE TO "C" langauge.

THEN DON'T ANSWER QUESTIONS HERE UNTIL YOU ARE AN EXPERT.