Freelist that seems to behave

kern/libkern/freelist.c

@@ -1,37 +1,55 @@
 #include "freelist.h"
-#include "stddef.h"
-#include <assert.h>
-#include <stdint.h>
-#include <stdio.h>
-#include <stdlib.h>
-#include <string.h>
 
-struct FreeListBlock {
-    struct FreeListBlock *next;
+/* Header block used to keep track of all the free blocks */
+struct __attribute__((packed)) FreeListBlock {
+    uint16_t next_offset; /* May be 8, 16, 32, 64, whatever */
 };
 
-/* Initialize the FreeList */
-int fl_init(FreeList *fl, uintptr_t start, uintptr_t end, size_t itemsize) {
-    size_t size = ALIGN(itemsize);
+/* Align to nearest multiple of align */
+static inline size_t align_up_to(size_t n, size_t align) {
+    return (n + align - 1) & ~(align - 1);
+}
 
-    if (!fl || end <= start)
-        return EXIT_FAILURE;
+/* Convert pointer -> 1-based offset (0 means NULL) */
+static inline uint32_t ptr_to_offset(FreeList *fl, void *ptr) {
+    if (!ptr)
+        return 0; // NULL maps to 0
+    uintptr_t diff = (uintptr_t)ptr - fl->start;
+    return (uint32_t)(diff / fl->size) + 1;
+}
+
+/* Convert 1-based offset -> pointer (0 means NULL) */
+static inline void *offset_to_ptr(FreeList *fl, uint32_t offset) {
+    if (offset == 0)
+        return NULL; // 0 = invalid/null
+    return (void *)(fl->start + (uintptr_t)(offset - 1) * fl->size);
+}
+
+/* Initialize the FreeList */
+int fl_init(FreeList *fl, uintptr_t start, size_t size_bytes, size_t itemsize) {
+    /* Fiddle around according to your need, (void *) alignment seems to be enough,
+     * but MAX_ALIGN_T is also an option. Especially for allocator implementation. */
+    size_t size = align_up_to(itemsize, sizeof(void *));
+
+    if (size < sizeof(FreeListBlock) || !fl)
+        return 0;
 
     fl->start = start;
-    fl->end = end;
+    fl->end = start + size_bytes;
     fl->size = size;
     fl->allocated = 0;
 
     FreeListBlock *block = (FreeListBlock *)start;
-    for (size_t i = 0; i < fl_capacity(fl); i++) {
-        block->next = (FreeListBlock *)((void *)block + size);
-        block = block->next;
+
+    for (size_t offset = 1; offset < fl_capacity(fl) + 1; offset++) {
+        block->next_offset = (int32_t)offset;
+        block = offset_to_ptr(fl, offset);
     }
 
-    block->next = NULL;
-
+    block->next_offset = 0; /* Last block */
     fl->free = (FreeListBlock *)start;
-    return EXIT_SUCCESS;
+
+    return 1;
 }
 
 /* Allocate some memory from the FreeList */
@@ -41,26 +59,32 @@ void *fl_alloc(FreeList *fl) {
 
     FreeListBlock *m = fl->free;
 
-    fl->free = fl->free->next;
+    fl->free = offset_to_ptr(fl, fl->free->next_offset); /* May be null, which is fine */
     fl->allocated++;
 
-    memset((void *)m, 0, sizeof(FreeListBlock));
+    /* Wipe it before sending it out, could use memset
+     * here, or even wiping the entire block */
+    m->next_offset = 0;
+
+    /* For reference: */
+    // memset((void *)m, 0, fl->size); /* Wipe entire block */
+    // memset((void *)m, 0, sizeof(FreeListBlock)); /* Wipe only header */
+
     return ((void *)m);
 }
 
 /* Return some memory to the FreeList */
 int fl_free(FreeList *fl, void *ptr) {
-    if (!fl_is_managed(fl, ptr)) {
-        return EXIT_FAILURE; /* We cant free memory we do not own */
-    }
+    if (!fl_is_managed(fl, ptr))
+        return 0; /* We cant free memory we do not own */
 
-    FreeListBlock *block = (FreeListBlock *)(uintptr_t)ptr;
+    FreeListBlock *block = (FreeListBlock *)ptr;
 
-    block->next = fl->free;
+    block->next_offset = ptr_to_offset(fl, fl->free); /* May be null, which is fine */
     fl->free = block;
     fl->allocated--;
 
-    return EXIT_SUCCESS;
+    return 1;
 }
 
 /* Returns how many slots are occupied */
@@ -80,7 +104,13 @@ size_t fl_capacity(FreeList *fl) {
 
 /* Check if a piece of memory is managed by the FreeList */
 int fl_is_managed(FreeList *fl, void *ptr) {
-    return ((uintptr_t)ptr >= fl->start && (uintptr_t)ptr < fl->end);
+    uintptr_t p = (uintptr_t)ptr;
+
+    if (p < fl->start || p >= fl->end) {
+        return 0; // outside pool
+    }
+
+    return ((p - fl->start) % fl->size) == 0; // aligned to block
 }
 
 /* Returns the ratio of metadata versus data as a scalar in range 0..1 */
@@ -93,9 +123,16 @@ size_t fl_check(FreeList *fl) {
     int            avail = 0;
     FreeListBlock *cursor = fl->free;
 
-    while (cursor->next != NULL) {
+    while (cursor) {
         avail++;
-        cursor = cursor->next;
+
+        if (!fl_is_managed(fl, cursor) || avail > fl_capacity(fl))
+            return 0;
+
+        if (cursor->next_offset == 0)
+            break;
+
+        cursor = offset_to_ptr(fl, cursor->next_offset);
     }
 
     return avail;

kern/libkern/freelist.h

@@ -5,18 +5,6 @@
 #include <stdint.h>
 #include <stdlib.h>
 
-/* Align to nearest multiple of align */
-static inline size_t align_up_to(size_t n, size_t align) {
-    return (n + align - 1) & ~(align - 1);
-}
-
-/* Fiddle these around according to your need. Delete or check makefile */
-#ifdef FREELIST_ALIGN
-#define ALIGN(x) (align_up_to(x, sizeof(void *)))
-#else
-#define ALIGN(x) (x)
-#endif
-
 typedef struct FreeListBlock FreeListBlock;
 
 typedef struct {
@@ -27,7 +15,7 @@ typedef struct {
     size_t         allocated;
 } FreeList;
 
-int    fl_init(FreeList *fl, uintptr_t start, uintptr_t end, size_t itemsize);
+int    fl_init(FreeList *fl, uintptr_t start, size_t size_bytes, size_t itemsize);
 int    fl_free(FreeList *fl, void *ptr);
 int    fl_is_managed(FreeList *fl, void *ptr);
 void  *fl_alloc(FreeList *fl);