sbrk() - cast to pointer from integer of different size

Question

I am trying to implement my own heap memory allocator, by allocating memory via sbrk() system call, and managing the return buffer. void *sbrk(intptr_t increment); return value:

From man sbrk:

On success, sbrk() returns the previous program break. (If the break was increased, then this value is a pointer to the start of the newly allocated memory). On error, (void *) -1 is re‐ turned, and errno is set to ENOMEM

#include <gnu/libc-version.h>
#include <stdio.h>
#include <unistd.h>
#include <assert.h>
/*
        puts (gnu_get_libc_version ());
*/

#define VP_ALIGNMENT                (4096)
#define ALIGN(size,alignment)       ( ((size) + (alignment-1)) & ~((alignment-1)) )


static void *extend_heap(int nbytes)
{
        void *sp = sbrk(ALIGN(nbytes, VP_ALIGNMENT));
        if (sp == (void*)-1) {
                perror("fails to retrieve program brk");
        }

        return sp;
}


struct S{
        char m[4096];
};


int main() {
        /* get current brk */
        void *cbrk = sbrk(0);
        printf("current prog brk: %p\n",cbrk);

        /*allocate 4096 bytes and get pointer to the start of the buffer*/ 
        struct S *buf = (struct S*)extend_heap(4000);
        printf("prog brk after allocation: %p\n",sbrk(0));
        
        int c=4096;
        do {
                buf->m[4096-c] = 'a';
        } while(--c);

        c=4096;
        do {
                assert(buf->m[4096-c] == 'a');
        } while(--c);

        return 0;
}

Compile with gcc & gcc flags: cc main.c -pedantic -Wconversion -Werror the program run as expected, but i dont understand why linux allocates a huge amount of memory, 2200hex bytes when i have asked only for 4096 ?

adding one of the gcc flags -ansi or -std=c89 produce compilation errors below:

In function ‘extend_heap’: main.c:15:20: error: initialization of ‘void *’ from ‘int’ makes pointer from integer without a cast [-Werror=int-conversion] 15 | void *sp = sbrk(ALIGN(nbytes, VP_ALIGNMENT)); | ^~~~ main.c: In function ‘main’: main.c:31:22: error: initialization of ‘void *’ from ‘int’ makes pointer from integer without a cast [-Werror=int-conversion] 31 |
void *cbrk = sbrk(0); | ^~~~

sbrk returns void*. why I am getting the error above? My glibc version = 2.31, gcc version 9.3.0

Thanks.