string_malloc.c

/*
 * String table allocator
 * A replacement for malloc() for tables of fixed strings
 *
 * Copyright (C) 2015-2022 by Jody Bruchon <jody@jodybruchon.com>
 * Released under The MIT License
 */

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include "string_malloc.h"

/* Size of pages to allocate at once. Must be divisible by uintptr_t.
 * The maximum object size is this page size minus about 16 bytes! */
#ifndef SMA_PAGE_SIZE
#define SMA_PAGE_SIZE 262144
#endif

/* Max freed pointers to remember. Increasing this number allows storing
 * more free objects but can slow down allocations. Don't increase it if
 * the program's total reused freed alloc counter doesn't increase as a
 * result or you're slowing allocs down to no benefit. */
#ifndef SMA_MAX_FREE
#define SMA_MAX_FREE 32
#endif

#ifdef DEBUG
uintmax_t sma_allocs = 0;
uintmax_t sma_free_ignored = 0;
uintmax_t sma_free_good = 0;
uintmax_t sma_free_merged = 0;
uintmax_t sma_free_replaced = 0;
uintmax_t sma_free_reclaimed = 0;
uintmax_t sma_free_scanned = 0;
uintmax_t sma_free_tails = 0;
 #define DBG(a) a
#else
 #define DBG(a)
#endif


/* This is used to bypass string_malloc for debugging */
#ifdef SMA_PASSTHROUGH
void *string_malloc(size_t len) { return malloc(len); }
void string_free(void *ptr) { free(ptr); return; }
void string_malloc_destroy(void) { return; }

#else /* Not SMA_PASSTHROUGH mode */

struct freelist {
	void *addr;
	size_t size;
};

static void *sma_head = NULL;
static uintptr_t *sma_curpage = NULL;
static unsigned int sma_pages = 0;
static struct freelist sma_freelist[SMA_MAX_FREE];
static int sma_freelist_cnt = 0;
static size_t sma_nextfree = sizeof(uintptr_t);


/* Scan the freed chunk list for a suitably sized object */
static inline void *scan_freelist(const size_t size)
{
	size_t *object, *min_p;
	size_t sz, min = 0;
	int i, used = 0, min_i = -1;

	/* Don't bother scanning if the list is empty */
	if (sma_freelist_cnt == 0) return NULL;

	for (i = 0; i < SMA_MAX_FREE; i++) {
		/* Stop scanning once we run out of valid entries */
		if (used == sma_freelist_cnt) return NULL;

		DBG(sma_free_scanned++;)
		object = sma_freelist[i].addr;
		/* Skip empty entries */
		if (object == NULL) continue;

		sz = sma_freelist[i].size;
		used++;

		/* Skip smaller objects */
		if (sz < size) continue;
		/* Object is big enough; record if it's the new minimum */
		if (min == 0 || sz <= min) {
			min = sz;
			min_i = i;
			/* Always stop scanning if exact sized object found */
			if (sz == size) break;
		}
	}

	/* Enhancement TODO: split the free item if it's big enough */

	/* Return smallest object found and delete from free list */
	if (min_i != -1) {
		min_p = sma_freelist[min_i].addr;
		*min_p = sma_freelist[min_i].size;
		sma_freelist[min_i].addr = NULL;
		sma_freelist_cnt--;
		min_p++;
		return (void *)min_p;
	}
	/* Fall through - free list search failed */
	return NULL;
}


/* malloc() a new page for string_malloc to use */
static inline void *string_malloc_page(void)
{
	uintptr_t * restrict pageptr;

	/* Allocate page and set up pointers at page starts */
	pageptr = (uintptr_t *)malloc(SMA_PAGE_SIZE);
	if (pageptr == NULL) return NULL;
	*pageptr = (uintptr_t)NULL;

	/* Link previous page to this page, if applicable */
	if (sma_curpage != NULL) *sma_curpage = (uintptr_t)pageptr;

	/* Update last page pointers and total page counter */
	sma_curpage = pageptr;
	sma_pages++;

	return (void *)pageptr;
}


void *string_malloc(size_t len)
{
	const void * restrict page = (char *)sma_curpage;
	static size_t *address;

	/* Calling with no actual length is invalid */
	if (len < 1) return NULL;

	/* Align objects where possible */
	if (len & (sizeof(uintptr_t) - 1)) {
		len &= ~(sizeof(uintptr_t) - 1);
		len += sizeof(uintptr_t);
	}

	/* Pass-through allocations larger than maximum object size to malloc() */
	if (len > (SMA_PAGE_SIZE - sizeof(uintptr_t) - sizeof(size_t))) {
		/* Allocate the space */
		address = (size_t *)malloc(len + sizeof(size_t));
		if (!address) return NULL;
		/* Prefix object with its size */
		*address = len;
		address++;
		DBG(sma_allocs++;)
		return (void *)address;
	}

	/* Initialize on first use */
	if (sma_pages == 0) {
		/* Initialize the freed object list */
		for (int i = 0; i < SMA_MAX_FREE; i++) sma_freelist[i].addr = NULL;
		/* Allocate first page and set up for first allocation */
		sma_head = string_malloc_page();
		if (sma_head == NULL) return NULL;
		sma_nextfree = sizeof(uintptr_t);
		page = sma_head;
	}

	/* Allocate objects from the free list first */
	address = (size_t *)scan_freelist(len);
	if (address != NULL) {
		DBG(sma_free_reclaimed++;)
		return (void *)address;
	}

	/* Allocate new page if this object won't fit */
	if ((sma_nextfree + len + sizeof(size_t)) > SMA_PAGE_SIZE) {
		size_t sz;
		size_t *tailaddr;

		/* See if page tail has usable remaining capacity */
		sz = sma_nextfree + sizeof(size_t) + sizeof(uintptr_t);

		/* Try to add page tail to free list rather than waste it */
		if (sz <= SMA_PAGE_SIZE) {
			sz = SMA_PAGE_SIZE - sma_nextfree - sizeof(size_t);
			tailaddr = (size_t *)((uintptr_t)page + sma_nextfree);
			*tailaddr = (size_t)sz;
			tailaddr++;
			string_free(tailaddr);
			DBG(sma_free_tails++;)
		}

		page = string_malloc_page();
		if (!page) return NULL;

		sma_nextfree = sizeof(uintptr_t);
	}

	/* Allocate the space */
	address = (size_t *)((uintptr_t)page + sma_nextfree);
	/* Prefix object with its size */
	*address = len;
	address++;
	sma_nextfree += len + sizeof(size_t);

	DBG(sma_allocs++;)
	return (void *)address;
}


/* Free an object, adding to free list if possible */
void string_free(void * const addr)
{
	int freefull = 0;
	struct freelist *emptyslot = NULL;
	static uintptr_t before, after;
	static size_t *sizeptr;
	static size_t size;

	/* Do nothing on NULL address */
	if (addr == NULL) goto sf_failed;

	/* Get address to real start of object and the object size */
	sizeptr = (size_t *)addr - 1;
	size = *(size_t *)sizeptr;
	if (size == 0) goto sf_double_free;

	/* Mark the freed object to catch double free attempts */
	*(size_t *)sizeptr = 0;

	/* Calculate after-block pointer for merge checks */
	after = (uintptr_t)addr + size;

	/* If free list is full, try to replace a smaller object */
	if (sma_freelist_cnt == SMA_MAX_FREE) freefull = 1;

	/* Attempt to merge into other free objects */
	for (int i = 0; i < SMA_MAX_FREE; i++) {
		/* Record first empty slot */
		if (emptyslot == NULL && sma_freelist[i].addr == NULL) {
			emptyslot = &(sma_freelist[i]);
//			break;
		} else if (freefull != 0 && sma_freelist[i].size < size) {
			/* Replace object if list is full and new one is bigger */
			emptyslot = &(sma_freelist[i]);
			DBG(sma_free_replaced++;)
			break;
		} else if ((uintptr_t)(sma_freelist[i].addr) == after) {
			/* Merge with a block after this one */
			sma_freelist[i].addr = sizeptr;
			sma_freelist[i].size += (size + sizeof(size_t *));
			DBG(sma_free_good++;)
			DBG(sma_free_merged++;)
			return;
		} else {
			before = (uintptr_t)addr + size;
			if (before == (uintptr_t)(sma_freelist[i].addr)) {
				/* Merge with a block before this one */
				sma_freelist[i].size += (size + sizeof(size_t *));
				DBG(sma_free_good++;)
				DBG(sma_free_merged++;)
			}
		}
	}

	/* Merges failed; add to empty slot (if any found) */
	if (emptyslot != NULL) {
		if (emptyslot->addr == NULL) sma_freelist_cnt++;
		emptyslot->addr = sizeptr;
		emptyslot->size = size;
		DBG(sma_free_good++;)
		return;
	}

	/* Fall through */
sf_failed:
	DBG(sma_free_ignored++;)
	return;

sf_double_free:
	fprintf(stderr, "string_malloc: ERROR: attempt to string_free() already freed object at %p\n", addr);
	return;
}

/* Destroy all allocated pages */
void string_malloc_destroy(void)
{
	uintptr_t *cur;
	uintptr_t *next;

	cur = sma_head;
	if (sma_head == NULL) return;
	while (sma_pages > 0) {
		next = (uintptr_t *)*cur;
		free(cur);
		cur = next;
		sma_pages--;
	}
	sma_head = NULL;
	return;
}

#endif /* SMA_PASSTHROUGH */