Label exported from assembly file is not accessible in c

When i run my code in keil simulator the variable __heap_base(which is a label in the startup file)does not contain the base of heap(i found the heap base to be 0x20000000) but it always shows 0x00000000. How to make the __heap_base to hold the correct value so that i can increment and decrement the heap base.

After trying some methods what i found out is that even if i store the address 0x20000000 as an unsigned 32 bit integer in a variable i am unable to access the memory location.

            #include <stdint.h>
            extern uint32_t __heap_base;
            uint32_t memallocate(int size)
            {
                 uint32_t temp=__heap_base;
                 __heap_base=__heap_base+size;
                 return temp;
            }
            int maina()
            {
                int *p=(int*)memallocate(sizeof(int));
                *p=5;
                return 0;
            }

The memory adress 0x20000000 should hold the value 5