More rigorous testing and helpful comments

freelist/Makefile

@@ -1,10 +1,8 @@
 CC = gcc
 CFLAGS = -Wall -O2
 
-CFLAGS += -DFREELIST_ALIGN
-
 TARGET = main.elf
-SRC = main.c freelist.c
+SRC = main.c freelist.c hexdump.c
 
 #LDFLAGS =
 

freelist/freelist.c

@@ -1,37 +1,54 @@
 #include "freelist.h"
-#include "stddef.h"
-#include <assert.h>
-#include <stdint.h>
-#include <stdio.h>
-#include <stdlib.h>
-#include <string.h>
 
-struct FreeListBlock {
+struct __attribute__((packed)) FreeListBlock {
-    struct FreeListBlock *next;
+    uint16_t next_offset;
 };
 
-/* Initialize the FreeList */
+/* Align to nearest multiple of align */
-int fl_init(FreeList *fl, uintptr_t start, uintptr_t end, size_t itemsize) {
+static inline size_t align_up_to(size_t n, size_t align) {
-    size_t size = ALIGN(itemsize);
+    return (n + align - 1) & ~(align - 1);
+}
 
-    if (!fl || end <= start)
+// Convert pointer -> 1-based offset (0 means NULL)
-        return EXIT_FAILURE;
+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);
+    for (size_t offset = 1; offset < fl_capacity(fl) + 1; offset++) {
-        block = block->next;
+        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 +58,32 @@ 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)) {
+    if (!fl_is_managed(fl, ptr))
-        return EXIT_FAILURE; /* We cant free memory we do not own */
+        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 +103,13 @@ 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 +122,16 @@ 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;

freelist/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);

freelist/hexdump.c

@@ -0,0 +1,35 @@
+#include <ctype.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <stdio.h>
+
+void hexdump(const void *data, size_t size) {
+    const unsigned char *p = (const unsigned char *)data;
+    size_t               i, j;
+
+    for (i = 0; i < size; i += 16) {
+        // Print offset
+        printf("%08zx  ", i);
+
+        // Print hex bytes
+        for (j = 0; j < 16; j++) {
+            if (i + j < size) {
+                printf("%02X ", p[i + j]);
+            } else {
+                printf("   "); // padding for incomplete lines
+            }
+            if (j == 7)
+                printf(" "); // extra space in middle
+        }
+
+        printf(" |");
+
+        // Print ASCII characters
+        for (j = 0; j < 16 && i + j < size; j++) {
+            unsigned char c = p[i + j];
+            printf("%c", isprint(c) ? c : '.');
+        }
+
+        printf("|\n");
+    }
+}

freelist/hexdump.h

@@ -0,0 +1,8 @@
+#ifndef HEXDUMP_H
+#define HEXDUMP_H
+
+#include <stddef.h>
+
+void hexdump(const void *data, size_t size);
+
+#endif // HEXDUMP_H

freelist/main.c

@@ -1,4 +1,5 @@
 #include "freelist.h"
+#include "hexdump.h"
 #include <assert.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -17,31 +18,63 @@ void printvec(const Vec3 *v) {
     printf("Vec3: (%d, %d, %d)\n", v->x, v->y, v->z);
 }
 
-#define BUFFER_SIZE (4096)
+/* For testing crc poly 0x04C11DB7 */
+uint32_t crc32(const uint8_t *data, size_t length);
+
+#define BUFFER_SIZE (128)
 
 int main() {
     FreeList fl;
-    const char *mem = malloc(BUFFER_SIZE);
 
-    fl_init(&fl, (uintptr_t)mem, (uintptr_t)mem + BUFFER_SIZE, sizeof(Vec3));
+    /* Here we split our malloc'ed memory into three parts, we pass one of
+     * them to our freelist, and keep the other two (head/tail) to check for
+     * overwrites:
+     *
+     *   | Headblock |      Freelist block     | Tailblock |
+     *   |    1x     |            2x           |     1x    |
+     */
+    void *headblock = malloc(BUFFER_SIZE * 2);
+    void *tailblock = headblock + (BUFFER_SIZE + BUFFER_SIZE / 2);
+    void *mem = headblock + (BUFFER_SIZE / 2);
+
+    /* Set these buffers to something known and randomize the buffer we give to the freelist */
+    memset(mem, arc4random(), BUFFER_SIZE);
+    memset(headblock, 0xAB, BUFFER_SIZE / 2);
+    memset(tailblock, 0xF0, BUFFER_SIZE / 2);
+
+    /* Store these for reference at the end of the testing */
+    const uint32_t tail_crc = crc32(tailblock, BUFFER_SIZE / 2);
+    const uint32_t head_crc = crc32(headblock, BUFFER_SIZE / 2);
+
+    /* Display a nice hexdump so you can see the layout */
+    hexdump(headblock, BUFFER_SIZE * 2);
+
+    if (!fl_init(&fl, (uintptr_t)mem, BUFFER_SIZE, sizeof(Vec3))) {
+        printf("Freelist failed to initialize!\n");
+        exit(EXIT_FAILURE);
+    }
+
     const size_t cap = fl_capacity(&fl);
 
     printf("Item size: %lu\n", sizeof(Vec3));
+    printf("FreeList Capacity: %lu\n", cap);
     printf("Buffer size: %d\n", BUFFER_SIZE);
+    printf("Freelist Blocksize: %lu\n", fl.size);
     printf("Space utilization internal: %.2f%%\n", 100.0 * fl_utilization(&fl, sizeof(Vec3)));
     printf("Space utilization external: %.2f%%\n", 100.0 * ((float)fl.size * cap) / BUFFER_SIZE);
 
     assert(fl_available(&fl) == cap);
+    assert(fl_check(&fl) == cap);
 
     Vec3 *a = fl_alloc(&fl);
     Vec3 *b = fl_alloc(&fl);
     Vec3 *c = fl_alloc(&fl);
 
-    memset(a, 0x23, sizeof(Vec3));
+    memset(a, 0xCA, sizeof(Vec3));
-    memset(b, 0x24, sizeof(Vec3));
+    memset(b, 0xCB, sizeof(Vec3));
-    memset(c, 0x25, sizeof(Vec3));
+    memset(c, 0xCC, sizeof(Vec3));
 
-    *a = vec3_new(12, 13, 435);
+    *a = vec3_new(0xAAAA, 0xAAAA, 0xAAAA);
     *b = vec3_new(192, 199, 435);
     *c = vec3_new(432, 11, 435);
 
@@ -54,9 +87,9 @@ int main() {
 
     printf("Available: %zu of %zu\n", fl_available(&fl), fl_capacity(&fl));
 
-    assert(fl_free(&fl, a) == EXIT_SUCCESS);
+    assert(fl_free(&fl, a));
-    assert(fl_free(&fl, b) == EXIT_SUCCESS);
+    assert(fl_free(&fl, b));
-    assert(fl_free(&fl, c) == EXIT_SUCCESS);
+    assert(fl_free(&fl, c));
 
     printf("Available: %zu of %zu\n", fl_available(&fl), fl_capacity(&fl));
 
@@ -72,6 +105,10 @@ int main() {
         ptr_buf[i] = (uintptr_t)fl_alloc(&fl);
     }
 
+    /* Write some data to the last element to catch out of bounds writes */
+    size_t last_elem = fl_capacity(&fl) - 1;
+    memset((void *)ptr_buf[last_elem], 0x42, sizeof(Vec3));
+
     assert(0 == fl_available(&fl));
     assert(fl_allocated(&fl) == fl_capacity(&fl));
     assert(fl_check(&fl) == 0);
@@ -81,10 +118,39 @@ int main() {
         fl_free(&fl, (void *)ptr_buf[i]);
     }
 
+    free(ptr_buf);
+
+    /* All slots are free here */
     assert(cap == fl_available(&fl));
     assert(fl_allocated(&fl) == 0);
     assert(fl_check(&fl) == cap);
 
+    /* Check so our head and tail blocks are intact */
+    assert(crc32(headblock, BUFFER_SIZE / 2) == head_crc);
+    assert(crc32(tailblock, BUFFER_SIZE / 2) == tail_crc);
+
+    /* Hexdump the heap, free the block and declare victory */
+    hexdump(headblock, BUFFER_SIZE * 2);
+    free(headblock);
     printf("All tests passed!\n");
     return 0;
 }
+
+uint32_t crc32(const uint8_t *data, size_t length) {
+    uint32_t crc = 0xFFFFFFFF;
+    uint32_t poly = 0x04C11DB7;
+
+    for (size_t i = 0; i < length; i++) {
+        crc ^= ((uint32_t)data[i]) << 24;
+
+        for (int j = 0; j < 8; j++) {
+            if (crc & 0x80000000) {
+                crc = (crc << 1) ^ poly;
+            } else {
+                crc <<= 1;
+            }
+        }
+    }
+
+    return crc ^ 0xFFFFFFFF;
+}