Thread: Defining STructures

Code:

struct Ethfrm_t
{
	struct FCfrm_t* ptr;
	unsigned char DestEthAdr[6];
	unsigned char SrcEthAdr[6];
	UINT32  VLAN_HdrSave;
	UINT32  reserved;
	UINT32  pad;
	unsigned char Ethbuffer[BUF_SIZE];
}  __attribute__ ((packed));

Ethfrm_t * Ethfrm;
Ethfrm = malloc(sizeof(*Ethfrm));

I'm not really sure how this works, but so if i define a structure with a pointer. Theres somewhere in memory that should have the size of this huge data. Because when i do sizeof(Ethfrm) I get 8 because its the size of the pointer. So if this was the way to create the struct. Is there a way to make a copy of this Struct but using new memory.

Code:

Ethfrm_t * newframe;
newframe = malloc(sizeof(*newframe));
newframe = Ethfrm;

Does this copy all the data in the first struct into the second struct?

Originally Posted by kiros88

Code:

struct Ethfrm_t
{
    struct FCfrm_t* ptr;
    unsigned char DestEthAdr[6];
    unsigned char SrcEthAdr[6];
    UINT32  VLAN_HdrSave;
    UINT32  reserved;
    UINT32  pad;
    unsigned char Ethbuffer[BUF_SIZE];
}  __attribute__ ((packed));

Ethfrm_t * Ethfrm;
Ethfrm = malloc(sizeof(*Ethfrm));

I'm not really sure how this works, but so if i define a structure with a pointer. Theres somewhere in memory that should have the size of this huge data. Because when i do sizeof(Ethfrm) I get 8 because its the size of the pointer. So if this was the way to create the struct. Is there a way to make a copy of this Struct but using new memory.

Code:

Ethfrm_t * newframe;
newframe = malloc(sizeof(*newframe));
newframe = Ethfrm;

Does this copy all the data in the first struct into the second struct?

No, that just reassigns the pointer (losing track of the memory allocated by malloc in the process). You need to dereference the pointers to copy the actual data (eg: *newframe = *Ethfrm).

so does that mean
*newframe = *Ethfrm
will newframe and Ethfrm structs be seperated so i can adjust newframe without affecting Ethfrm.

Okay so I'm going to ask a new question but its related. *new frame and *Ethframe im assuming is different data like the whole new struct different data in memory. Which I'm not sure is right.

But if i did

Code:

while(1){
Ethfrm_t * newframe;
newframe = malloc(sizeof(*newframe));
newframe = Ethfrm;
}

Will this continiuosly make a ton of new frames with new memory or does it just redeclare the same memory space that newframe was the first time it was malloc.

i tried this using int. it didnt work. why is that?

Code:

main(){
    int num1=9, num2, *ptr1, *ptr2;
    
    ptr1 = num1;
    ptr2 = num2;
    
    *ptr2 = *ptr1;
    printf("ptr1 is %d.\nptr2 is %d", ptr1, ptr2);
    system("pause");
}

Code:

main(){
    int num1=9, num2, *ptr1, *ptr2;
    
    ptr1 = &num1;
    ptr2 = &num2;
    
    *ptr2 = *ptr1;
    printf("ptr1 is %d.\nptr2 is %d", *ptr1, *ptr2);
    system("pause");
}

worked! thanks!