Objective

Write arena_alloc and arena_reset, a bump allocator that serves 8-aligned slices out of one 4096-byte static pool and never calls malloc.

Steps

• arena_alloc(n) returns the address of n usable bytes cut out of a static pool of exactly 4096 bytes that you declare in arena_alloc.c. There is no heap here: malloc is not in the allowed list.
• Every address you hand out is a multiple of 8. A granted request consumes n rounded up to the next multiple of 8 and not one byte more, so a 1-byte block is followed by 7 bytes of padding that no later block reuses.
• Declare the pool so its own first byte is already aligned on 8, an array of 8-byte words or an _Alignas(8) array of unsigned char. A plain static char pool[4096] guarantees no alignment, and a base you have to round up costs you the bytes you need to honour a request for the whole capacity.
• Blocks never overlap, and no block runs past the end of the pool. Every byte of a granted block sits inside the 4096.
• Return NULL when the rounded request is larger than the space still free. A refused request consumes nothing, so a request that does fit still succeeds right after it.
• arena_alloc(0) returns NULL and consumes nothing.
• Reject a request larger than the capacity before you round it up. (n + 7) & ~(size_t)7 wraps to 0 when n is SIZE_MAX, and a need of zero bytes fits in any arena.
• arena_reset() puts the cursor back on the first byte of the pool. Nothing is freed in the free sense, no memory goes back to the system, but the whole 4096 bytes become available again and every pointer handed out before the reset is dead.
• Grademe never prints the addresses you return, they move from one run to the next. Each test computes an invariant and prints one line per block or per pair of blocks: align:ok when the address is a multiple of 8, inpool:ok when the whole block sits inside the pool, overlap:none when two blocks share no byte, same:yes when two addresses are equal, bytes:ok when a block still holds what was written into it, granted:N for the number of requests served before the pool ran out, and null when arena_alloc answered NULL. The label at the start of a line (a, b, full, a|b) says which block, or which pair, the line is about.
• A failure reads as the opposite token: align:bad, inpool:bad, overlap:yes, same:no, bytes:bad, or overlap:unknown when one of the two pointers came back NULL.
• To find the pool without printing an address, every test first asks a fresh arena for the whole 4096 bytes, notes where that block starts and ends, then calls arena_reset() before the real scenario. An arena_reset that does not reopen the pool therefore turns every test red, not only the ones named after it.

Objective

Write arena_alloc and arena_reset, a bump allocator that serves 8-aligned slices out of one 4096-byte static pool and never calls malloc.

Steps

• arena_alloc(n) returns the address of n usable bytes cut out of a static pool of exactly 4096 bytes that you declare in arena_alloc.c. There is no heap here: malloc is not in the allowed list.
• Every address you hand out is a multiple of 8. A granted request consumes n rounded up to the next multiple of 8 and not one byte more, so a 1-byte block is followed by 7 bytes of padding that no later block reuses.
• Declare the pool so its own first byte is already aligned on 8, an array of 8-byte words or an _Alignas(8) array of unsigned char. A plain static char pool[4096] guarantees no alignment, and a base you have to round up costs you the bytes you need to honour a request for the whole capacity.
• Blocks never overlap, and no block runs past the end of the pool. Every byte of a granted block sits inside the 4096.
• Return NULL when the rounded request is larger than the space still free. A refused request consumes nothing, so a request that does fit still succeeds right after it.
• arena_alloc(0) returns NULL and consumes nothing.
• Reject a request larger than the capacity before you round it up. (n + 7) & ~(size_t)7 wraps to 0 when n is SIZE_MAX, and a need of zero bytes fits in any arena.
• arena_reset() puts the cursor back on the first byte of the pool. Nothing is freed in the free sense, no memory goes back to the system, but the whole 4096 bytes become available again and every pointer handed out before the reset is dead.
• Grademe never prints the addresses you return, they move from one run to the next. Each test computes an invariant and prints one line per block or per pair of blocks: align:ok when the address is a multiple of 8, inpool:ok when the whole block sits inside the pool, overlap:none when two blocks share no byte, same:yes when two addresses are equal, bytes:ok when a block still holds what was written into it, granted:N for the number of requests served before the pool ran out, and null when arena_alloc answered NULL. The label at the start of a line (a, b, full, a|b) says which block, or which pair, the line is about.
• A failure reads as the opposite token: align:bad, inpool:bad, overlap:yes, same:no, bytes:bad, or overlap:unknown when one of the two pointers came back NULL.
• To find the pool without printing an address, every test first asks a fresh arena for the whole 4096 bytes, notes where that block starts and ends, then calls arena_reset() before the real scenario. An arena_reset that does not reopen the pool therefore turns every test red, not only the ones named after it.