How to allocate dynamic memory in a dynamic structure variable? - c

Question

I want to allocate some memory to one of the members of a dynamic structure, dynamically. But I don't know what do I do and also, i don't know how to use them after allocating.
This is my code work :
#include <stdio.h>
#include <conio.h>
#include <stdlib.h>
int main()
{
struct Sample_Structure {
float *Number_ptr; // This is the pointer to allocate some memory, dynamically.
int Num1;
int Num2;
} *struct_ptr;
struct_ptr = (struct Sample_Structure*) malloc (sizeof (struct Sample_Structure));
if (!struct_ptr)
{
printf("An error occurred while allocating memory.\n");
exit(1);
}
// I don't know what do I write here to allocate some memory to the pointer, "Number_ptr" in the structure
printf("Enter your number 1 and 2 : ");
scanf("%d%d", &struct_ptr->Num1, &struct_ptr->Num2);
system("cls");
printf("Your numbers are %d and %d\n", Struct_ptr->Num1, Struct_ptr->Num2);
return 0;
}
I wrote some comments for you to know where did I want to allocate some memory to the pointer of the structure, "Sample_Structure". Generally, can we do that? If yes, how?

Answer 1

you can allocate memory the for it , the same way you allocated for *struct_ptr
like this
struct_ptr->Number_ptr = (float *) malloc(sizeof(float));.
and when you want to scan data for it you shouldn't use & for it:
scanf("%d%d%f", &struct_ptr->Num1, &struct_ptr->Num2, struct_ptr->Number_ptr);
and in printf you should use * for it:
printf("Your numbers are %d and %d\nand %f\n", struct_ptr->Num1, struct_ptr->Num2, *struct_ptr->Number_ptr);
also you should this capital S to lowercase letter in this printf:
printf("Your numbers are %d and %d\n", struct_ptr->Num1, struct_ptr->Num2);
also note that you don't need to cast for malloc and it's not good to that , unless you're using c++ compiler.
int main()
{
struct Sample_Structure {
float* Number_ptr;
int Num1;
int Num2;
} *struct_ptr;
struct_ptr = (struct Sample_Structure *)malloc(sizeof(struct Sample_Structure));
if (!struct_ptr)
{
printf("An error occurred while allocating memory.\n");
exit(1);
}
struct_ptr->Number_ptr = (float *) malloc(sizeof(float));
printf("Enter your number 1 and 2 : ");
scanf("%d%d%f", &struct_ptr->Num1, &struct_ptr->Num2, struct_ptr->Number_ptr);
system("cls");
printf("Your numbers are %d and %d\nand %f\n", struct_ptr->Num1, struct_ptr->Num2, *struct_ptr->Number_ptr);
return 0;
}

Answer 2

For example, you want to allocate memory for 10 floats for Number_ptr:
struct_ptr->Number_ptr = malloc( sizeof(float) * 10 );
Correction:
Change this line
printf("Your numbers are %d and %d\n", Struct_ptr->Num1, Struct_ptr->Num2);
^ ^
to
printf("Your numbers are %d and %d\n", struct_ptr->Num1, struct_ptr->Num2);
And, you're leaking memory. You need to deallocate memory using free() after use. Make sure for every allocation e.g. malloc(), there is corresponding deallocation e.g. free().
Also, validate if the memory is allocated or not.
Here's a small example for your reference (live):
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#define MAX 10
typedef struct S
{
int* ilist;
} S;
int main()
{
S s;
s.ilist = malloc( MAX * sizeof(*s.ilist) ); // allocation
if ( s.ilist == NULL )
{
printf("Unable to allocate memory!\n");
return -1;
}
// An assertion may also be used like this (Header: <assert.h>):
// assert( s.ilist != NULL );
for ( int i = 0; i < MAX; ++i )
{
s.ilist[i] = i + 1; // store elements
}
for ( int i = 0; i < MAX; ++i )
{
printf("%d ", s.ilist[i]); // read elements
}
free( s.ilist ); // deallocation
return 0;
}
Output:
1 2 3 4 5 6 7 8 9 10