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// Copyright (c) 2021 Johannes Stoelp
#include <alloc.h>
#include <common.h>
#include <stdint.h>
// Extremely simple and non-thread safe implementation of a dynamic
// memory allocator. Which will greatly suffer under fragmentation as
// we neither use splitting nor coalesce free blocks. It uses
// first-fit and always traverses the block list from the beginning.
//
// Bottom line, this allocator can be optimized in so many ways but it
// doesn't really matter for the purpose of this studies and therefore
// the allocator is implemented in the most naive way.
// Allocation block descriptor.
struct BlockDescriptor {
unsigned mFree;
unsigned mSize;
struct BlockDescriptor* mNext;
};
// Global Allocator.
// Size of available memory to the allocator.
enum { MEMORY_SIZE = 1 * 1024 * 1024 };
// Memory for the allocator (statically reserved in the `.bss` section).
uint8_t gMemory[MEMORY_SIZE];
// Top index into `gMemory` to indicate next free memory.
unsigned gMemoryTop;
// List of allocated blocks (free + used).
struct BlockDescriptor* gHead;
// Request free memory from `gMemory` and advance the `gMemoryTop` index.
static void* brk(unsigned size) {
ERROR_ON(gMemoryTop + size >= MEMORY_SIZE, "Allocator OOM!");
const unsigned old_top = gMemoryTop;
gMemoryTop += size;
return (void*)(gMemory + old_top);
}
// Allocate memory chunk of `size` and return pointer to the chunk.
void* alloc(unsigned size) {
struct BlockDescriptor* current = 0;
// Check if we have a free block in the list of allocated blocks
// that matches the requested size.
current = gHead;
while (current) {
if (current->mFree && current->mSize < size) {
current->mFree = 0;
return (void*)(current + 1);
};
current = current->mNext;
}
// Compute real allocation size: Payload + BlockDescriptor.
unsigned real_size = size + sizeof(struct BlockDescriptor);
// No free block found in the list of blocks, allocate new block.
current = brk(real_size);
// Initialize new block.
current->mFree = 0;
current->mSize = size;
current->mNext = 0;
// Insert new block at the beginning of the list of blocks.
if (gHead != 0) {
current->mNext = gHead;
}
gHead = current;
return (void*)(current + 1);
}
void dealloc(void* ptr) {
// Get descriptor block.
struct BlockDescriptor* current = (struct BlockDescriptor*)ptr - 1;
// Mark block as free.
ERROR_ON(current->mFree, "Tried to de-alloc free block!");
current->mFree = 1;
}
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