malloc for union inside structure fails.

Hi Guru's,
I have some problem while allocating memory to a union inside structure. Below is the code i am using

ON SYSTEM1:
This works fine

ON SYSTEM2:
compiler complains saying "need structure or union type" while allocating MYSTRUCT1.
if i change,
shreyas[0].UnionAttr.struct1 = (MYSTRUCT1 *) malloc (sizeof(MYSTRUCT1)
to
shreyas[0].UnionAttr->struct1 = (MYSTRUCT1 *) malloc (sizeof(MYSTRUCT1)

This compiler on SYSTEM2 is happy. but second way does not look correct to me and compiler on system 1 complains about it.

Can someone pls help know which is the correct way to allocate memory?
If first one is correct then what should i look in for to avoid this error?
Could this be an issue with compiler on SYSTEM2?

NOTE: if i use second method on SYSTEM2 code segfaults during malloc.

Please help ASAP, i am stuck because of this for long time.

#include <stdio.h>
#include <string.h>
#include <stdlib.h>


typedef struct mystruct1 {
int a;
int b;
} MYSTRUCT1;

typedef struct mystruct2 {
int a;
int b;
} MYSTRUCT2;

typedef union myunion {
MYSTRUCT1 *struct1;
MYSTRUCT2 *struct2;
} MYUNION;

typedef struct mystruct {
int a;
MYUNION UnionAttr;
} MYSTRUCT;

int main()
{
int flag = 1;
int i;
// Allocate memory
MYSTRUCT *shreyas;
if ((shreyas = (MYSTRUCT *) calloc (2,sizeof(MYSTRUCT))) == NULL)
fprintf(stderr, "Error allocating mystruct\n");
// Allocate to union elements
if ((shreyas[0].UnionAttr.struct1 = (MYSTRUCT1 *) malloc (sizeof(MYSTRUCT1))) == NULL) {
fprintf(stderr, "Error allocating mystruct1\n");
free(shreyas);
}

if ((shreyas[1].UnionAttr.struct2 = (MYSTRUCT2 *) malloc (sizeof(MYSTRUCT2))) == NULL) {
fprintf(stderr, "Error allocating mystruct2\n");
free(shreyas);
}
// Access elements
shreyas[0].UnionAttr.struct1->a = 10;
shreyas[1].UnionAttr.struct2->a = 20;
shreyas[0].UnionAttr.struct1->b = 100;
shreyas[1].UnionAttr.struct2->b = 200;

printf("Struct 1 elements a = %d b = %d\n", shreyas[0].UnionAttr.struct1->a, shreyas[0].UnionAttr.struct1->b);
printf("Struct 2 elements a = %d b = %d\n", shreyas[1].UnionAttr.struct2->a, shreyas[1].UnionAttr.struct2->b);
printf("Success\n");
return;


}

VS2010 compiles the posted code (except that main() must return a value).

Your code is so C-ish. Why not graduate to C++? Rather than a union of pointers you could use inheritance (two (or more) structs/classes derived from a common base struct/class). And you could use new/delete for allocations.

Quote:

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Originally Posted by shreyaswaghmare

Hi Guru's,
I have some problem while allocating memory to a union inside structure.

---

If your problem is a compilation issue, it would help if you posted the name and version of the compiler you're using. The issue as you've described is not a run-time issue with malloc(), it is an issue with source code management.

Secondly, if it is the same compiler, then obviously your code is different on the two systems. A compiler just doesn't give out errors because it is run on another computer -- the code must be different in some way on the machines, or the compilers are different (different compiler brand and/or version).

Regards,

Paul McKenzie

I would throw away the compiler you are using for system2!! You don't say what compilers you are using but your code works fine for me using Visual Studio 2003 onwards. Note that if the call to calloc fails, then your program doesn't terminate as it probably should but just outputs an error to stderr and trys to continue with all sorts of bad memory problems!!! Also if the malloc call for MYSTRUCT2 fails then you are not freeing the memory already allocated for MYSTRUCT1. Personally when I use pointers I nearly always typedef a pointer definition :cool:so that I don't have a load of * cluttering up my code. As below

Code:

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#include <stdio.h>
#include <string.h>
#include <stdlib.h>

typedef struct mystruct1 {
        int a;
        int b;
} MYSTRUCT1, *PMYSTRUCT1;

typedef struct mystruct2 {
        int a;
        int b;
} MYSTRUCT2, *PMYSTRUCT2;

typedef union myunion {
        PMYSTRUCT1 struct1;
        PMYSTRUCT2 struct2;
} MYUNION;

typedef struct mystruct {
        int          a;
        MYUNION UnionAttr;
} MYSTRUCT, *PMYSTRUCT;

int main()
{
PMYSTRUCT shreyas;

        //Allocate memory
        if ((shreyas = (PMYSTRUCT)calloc(2, sizeof(MYSTRUCT))) == NULL) {
                fprintf(stderr, "Error allocating mystruct\n");
                return (1);
        }

        // Allocate to union elements
        if ((shreyas[0].UnionAttr.struct1 = (PMYSTRUCT1)malloc(sizeof(MYSTRUCT1))) == NULL) {
                fprintf(stderr, "Error allocating mystruct1\n");
                free(shreyas);
                return (2);
        }

        if ((shreyas[1].UnionAttr.struct2 = (PMYSTRUCT2)malloc(sizeof(MYSTRUCT2))) == NULL) {
                fprintf(stderr, "Error allocating mystruct2\n");
                free(shreyas[0].UnionAttr.struct1);
                free(shreyas);
                return (3);
        }

        // Access elements
        shreyas[0].UnionAttr.struct1->a = 10;
        shreyas[1].UnionAttr.struct2->a = 20;
        shreyas[0].UnionAttr.struct1->b = 100;
        shreyas[1].UnionAttr.struct2->b = 200;

        printf("Struct 1 elements a = %d b = %d\n", shreyas[0].UnionAttr.struct1->a, shreyas[0].UnionAttr.struct1->b);
        printf("Struct 2 elements a = %d b = %d\n", shreyas[1].UnionAttr.struct2->a, shreyas[1].UnionAttr.struct2->b);
        printf("Success\n");

        return (0);
}

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