Use variable in other layer file

Question

I have an assignment that do FAT12 system. Follow the requirement, I need 2 functions to read sector and multi sector. Those functions in HAL layer. I just tested in app (main) layer below: In HAL.c:

#define SIZE_OF_SECTOR  512
static FILE* s_pimage = NULL;

void kmc_open_img_file()
{
    s_pimage = fopen("floppy.img", "rb");
    if(NULL == s_pimage)
    {
        printf("Can not open file!");
    }
}

int32_t kmc_read_sector(uint32_t index, uint8_t *buff)
{
    uint32_t offset = 0;
    int32_t num_of_bytes = 0;

    offset = SIZE_OF_SECTOR * index;
    fseek(s_pimage, SIZE_OF_SECTOR * index, SEEK_SET);
    num_of_bytes = (int32_t)fread(buff, 1, SIZE_OF_SECTOR, s_pimage);
   
    if (num_of_bytes == 0) printf("Fail");
    return num_of_bytes;
}

and in main: (added header file)

int main()
{
    kmc_open_img_file();

    uint8_t *buff = NULL ;
    int32_t a = kmc_read_sector(0, buff);

    printf("%d", a);

    kmc_close_img_file();
    return 0;
}

Result :Fail and 0 byte.

Can anyone help me solve it without extern variable? (this is requirement too)