What is the difference between calling Input function for m1, m2 variable of structure. Also what is the use of malloc in struct

Question

struct marks{
int n;
int *arr;
};
Input(struct marks *s) {
Body.....
}
main()
{
marks *m1=malloc(sizeof(marks));
marks *m2;
Input(m1) ;
Input(&m2) ;
return 0;
}

Answer 1

Thanks for clarifying my doubt

Answer 2

I was asking regarding C language only not C++

Answer 3

Okay I understand. One more think can you please say me what's the difference between this two line of code
marks *m1= malloc(sizeof(marks));
And marks *m1;
Which one when to use.

Answer 4

Martin Taylor

"In C the correct syntax is
struct marks *m1 = (struct marks*) malloc(sizeof(struct marks));"

Syntactically correct, idiomatic C-style C++ - but quite distasteful in pure C.

In C, the idiomatic way is to never cast the return type of malloc(), calloc() or realloc() where void * is automatically type-promoted. If you later change the variable type, you have to trace through the source just to change the pointless cast.
On a C89 compiler (still used for portability reasons) where stdlib.h may have been forgotten, the cast may hide compiler warnings for what's clearly a bug.

struct marks *m1 = malloc(sizeof(*m1));

is the idiomatic way (in C) and clearly much better since it's type-agnostic. Now 'm1' could be anything from struct, char, or a float pointer and you don't have to touch the allocation code again.

- Just a nitpick.