Malloc function (dynamic memory allocation) resulting in an error when it is used globally

#include<stdio.h>
#include<string.h>
char *y;
y=(char *)malloc(40); // gives an error here
int main()
{
    strcpy(y,"hello world");
}

error: conflicting types for 'y'
error: previous declaration of 'y' was here
warning: initialization makes integer from pointer without a cast
error: initializer element is not constant
warning: data definition has no type or storage class
warning: passing arg 1 of `strcpy' makes pointer from integer without cast

Now the real question is, can't we make the dynamic memory allocation globally? Why does it show an error when I use malloc globally? And the code works with no error if I put malloc statement inside the main function or some other function. Why is this so?

#include<stdio.h>
#include<string.h>
char *y;
int main()
{
    y=(char *)malloc(40); 
    strcpy(y,"hello world");
}