Write Block Footer

Task

Implement the function write_block_footer

• Given a pointer to a free block_header, write s its footer (containing the block size) to the last 8 bytes of the block
• Remember: ONLY FREE blocks have footers. The footer is a struct block_footer containing just the size
• The footer should be placed at the very end of the block (last 8 bytes)
• Don’t modify the struct definitions at the top of the file

Function Signature

// Given a pointer to a free block_header, write the footer to the last 8 bytes of the block. The footer contains only the block size (no status bits).
void write_block_footer(struct block_header* block);

---

Testing

Compiling

Use gcc to compile the program and use ./ to run the program

$ gcc write_block_footer.c -o write_block_footer
$ ./write_block_footer

Example

To test the function, create a struct block_header block with a specific size, call write_block_footer, then check if the footer was written correctly.

struct block_header block;
block.size_status = 0x0000000000000080;  // 128 bytes, free
write_block_footer(&block);
// Footer should be at address: block + 128 - 8
// Footer should contain: 0x0000000000000080

Test Case 1

Input: size_status = 0x0000000000000040
Expected: Footer at offset 56 (64-8) contains 0x40

Test Case 2

Input: size_status = 0x0000000000000100
Expected: Footer at offset 248 (256-8) contains 0x100

NOTE

• Extract the block size using: size = block->size_status & SIZE_MASK
• Calculate footer address: (struct block_footer*)((char*)block + size - sizeof(struct block_footer))
• The footer’s size field should contain the block size (without status bits)

---

Code

#include <stddef.h>
#include <stdint.h>
#include <stdlib.h>
 
#define SIZE_MASK (~0xF)
 
struct block_header
{
    size_t size_status;
};
 
struct block_footer
{
    size_t size;
};
 
void write_block_footer(struct block_header *block)
{
    size_t size = block->size_status & SIZE_MASK;
    struct block_footer* footer_addr = (struct block_footer*)((char*) block + size - sizeof(struct block_footer));
    footer_addr->size = size;
}