Thread: Where does structure variable store and what does it store

I am trying to figure out Where does structure variable store and what does it store using printf statement

I have written this code

Code:

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


typedef struct{
	
	int var1;
	int var2;
	
} svar;


int main ()
{
	
	svar *spvar = malloc(sizeof(svar));
	spvar -> var1 = 1;
	spvar -> var2 = 1;
	
	printf("spvar  store at memory location -> %p \n", (void*)spvar); 
    printf("spvar -> var1 store at memory location -> %p \n", (void*)spvar -> var1); 
	printf("spvar -> var2 store at memory location -> %p \n", (void*)spvar -> var2); 
	
	printf("spvar = %d \n", (void*)spvar); 
    printf("spvar -> value = %d \n", (void*)spvar -> var1); 
	printf("spvar -> var2 = %d \n", (void*)spvar -> var2); 
	
	
	return 0;
}

spvar store at memory location -> 00C61598
spvar -> var1 store at memory location -> 00000001
spvar -> var2 store at memory location -> 00000001
spvar = 12981656
spvar -> value = 1
spvar -> var2 = 1

I think the two memory location spvar -> var1 and spvar -> var2 are not getting right

Originally Posted by Salem

You're printing values, not addresses.

Like this:
printf("spvar -> var1 store at memory location -> %p \n", (void*)&spvar -> var1);

Code:

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


typedef struct{
	
	int var1;
	int var2;
	
} svar;


int main ()
{
	
	svar *spvar = malloc(sizeof(svar));
	spvar -> var1 = 1;
	spvar -> var2 = 1;
	
	printf("spvar  store at memory location -> %p \n", (void*)spvar); 
    printf("spvar -> var1 store at memory location -> %p \n", (void*)&spvar -> var1); 
	printf("spvar -> var2 store at memory location -> %p \n", (void*)&spvar -> var2); 


	printf("spvar = %d \n", (void*)spvar); 
    printf("spvar -> value = %d \n", (void*)spvar -> var1); 
	printf("spvar -> var2 = %d \n", (void*)spvar -> var2); 
	
	
	return 0;
}

spvar store at memory location -> 00C41598
spvar -> var1 store at memory location -> 00C41598
spvar -> var2 store at memory location -> 00C4159C
spvar = 12850584
spvar -> value = 1
spvar -> var2 = 1

I just want to confirm that spvar variable store at memory location 00C41598 and spvar variable hold value 12850584

Please give confirm if this is correct

Originally Posted by Rahul11

[CODE]

spvar store at memory location -> 00C41598
spvar -> var1 store at memory location -> 00C41598
spvar -> var2 store at memory location -> 00C4159C
spvar = 12850584
spvar -> value = 1
spvar -> var2 = 1

I just want to confirm that spvar variable store at memory location 00C41598 and spvar variable hold value 12850584

Please give confirm if this is correct

It looks accurate to me based on the fact that var1 is stored in 4bytes of memory.

0xc4159c - 0x0c41598 = 4

Four bytes is makes sense as the size of an int.

0xc41598 = 12850584

I want to modify the value of structure member without modifying actual address. I have written function but It doesn't works as expected

Code:

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

typedef struct{
    
    int var1;
    int var2;
    
} svar;

void foo ( svar *spvar , int fvar1, int fvar2 )
{
    spvar = malloc(sizeof(svar));
    spvar -> var1 = fvar1;
    spvar -> var2 = fvar2;
    
    printf("value %d stoare at memory location %p (spvar->var1) \n", (void*)spvar -> var1, (void*)&spvar -> var1 );      
    printf("value %d store at memory location %p (spvar->var2) \n", (void*)spvar -> var2, (void*)&spvar -> var2);
    
}


int main ()
{
    svar *spvar = NULL;    
    foo (spvar , 1, 2 );
    printf("\n");    
    foo (spvar , 3, 4);
    printf("\n");    
    foo (spvar , 3, 4);
    return 0;
}

value 1 stoare at memory location 00A01598 (spvar->var1)
value 2 store at memory location 00A0159C (spvar->var2)

value 3 stoare at memory location 00A015C8 (spvar->var1)
value 4 store at memory location 00A015CC (spvar->var2)

value 3 stoare at memory location 00A03328 (spvar->var1)
value 4 store at memory location 00A0332C (spvar->var2)

You are allocating the memory for the struct three times, and throwing away the memory each time returning from foo()! Allocate the memory BEFORE calling foo(), and pass in the ONE instance of the struct object into foo()!

Check the return value from malloc() to insure the allocation worked correctly!

You only have to cast the pointer if you are displaying the address using %p, and NEVER cast an int to a (void *)!

A good up to date book on the C Language would have explained this and more!

Code:

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

typedef struct{

    int var1;
    int var2;

} svar;

void foo ( svar *spvar , int fvar1, int fvar2 )
{
    spvar -> var1 = fvar1;
    spvar -> var2 = fvar2;

    // Cast pointers to (void *), but NOT ints
    printf("value %d store at memory location %p (spvar->var1) \n", spvar -> var1, (void*) &spvar -> var1 );
    printf("value %d store at memory location %p (spvar->var2) \n\n", spvar -> var2, (void*) &spvar -> var2);
}


int main ()
{
    svar *spvar = NULL;

    // Allocate the memory BEFORE calling function foo()
    spvar = malloc(sizeof(svar));
    if(spvar == NULL)
    {
       printf("Memory allocation failed!\n");
       return 1;
    }

    foo (spvar , 1, 2 );

    foo (spvar , 3, 4);

    foo (spvar , 3, 4);

    free(spvar);  // Free the memory

    return 0;
}