Rewrite everything

Makefile

@@ -6,8 +6,8 @@ BUILD_DIR := build
 BIN_DIR := lib
 TEST_DIR := test
 
-OBJS := $(BUILD_DIR)/arena.o $(BUILD_DIR)/alloc_backend.o
-FPIC_OBJS := $(BUILD_DIR)/fpic/arena.o $(BUILD_DIR)/fpic/alloc_backend.o
+OBJS := $(BUILD_DIR)/arena.o
+FPIC_OBJS := $(BUILD_DIR)/fpic/arena.o
 
 static : $(BIN_DIR)/libarena.a
 

include/alloc_backend.h

@@ -1,9 +0,0 @@
-
-#ifndef ALLOC_BACKEND_H
-#define ALLOC_BACKEND_H
-
-#include <stddef.h>
-
-void* call_alloc_backend(size_t size);
-
-#endif

include/arena.h

@@ -4,16 +4,17 @@
 
 #include <stddef.h> // ptrdiff_t
 
+typedef unsigned char byte_t;
+
 typedef struct arena {
-    void *begin,
-        *next,
-        *prev;
-    size_t cap;
+    size_t next, prev, cap;
+    void* data;
 } __attribute__((aligned(sizeof(void*)))) arena_t;
 
-arena_t* arena_new();
+arena_t arena_new();
 void arena_reset(arena_t* a);
 void* arena_alloc(arena_t* a, size_t len);
+void* arena_calloc(arena_t* a, size_t nmemb, size_t size);
 void* arena_realloc_tail(arena_t* a, size_t len);
 
 #endif

src/alloc_backend.c

@@ -1,29 +0,0 @@
-
-#include "alloc_backend.h"
-
-#define _GNU_SOURCE
-#include <sys/mman.h>
-
-/*
- * Separate function to make switching
- * allocation backend easier
- *
- * must take size_t size as an argument
- * must return NULL on failure
- */
-void* call_alloc_backend(size_t size)
-{
-    void* p = mmap(
-        NULL,
-        size,
-        PROT_READ | PROT_WRITE,
-        MAP_ANONYMOUS | MAP_PRIVATE,
-        -1, /* man mmap(2): "[...], some implementations require fd to be
-               -1 if MAP_ANONYMOUS is specified [...]" */
-        0);
-
-    if (p == MAP_FAILED)
-        return NULL;
-
-    return p;
-}

src/arena.c

@@ -1,77 +1,79 @@
 
 #include "arena.h"
-#include "alloc_backend.h"
 
 #include <errno.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
 #include <unistd.h>
 
-#define ARENA_SIZE ((size_t)(128 * sysconf(_SC_PAGE_SIZE)))
+#include <stdio.h>
 
-typedef unsigned char byte;
+// (sizeof(intptr_t) isn't guaranteed to be the machine word size but on most
+// compilers it is)
+#define WORD_SIZE (sizeof(intptr_t))
 
-/*
- * Allocates and returns new arena
- */
-struct arena* arena_new()
+#define ARENA_SIZE ((size_t)(1 * sysconf(_SC_PAGE_SIZE)))
+
+struct arena arena_new()
 {
-    size_t size = ARENA_SIZE;
+    arena_t a = {
+        .next = 0,
+        .prev = 0,
+        .cap = ARENA_SIZE,
+        .data = malloc(ARENA_SIZE)
+    };
 
-    byte* p = call_alloc_backend(size);
-
-    if (p == NULL)
-        return NULL;
-
-    arena_t* a = (arena_t*)p;
-
-    void* beg = p + sizeof(struct arena);
-    a->begin = beg;
-    a->next = beg;
-    a->prev = beg;
-    a->cap = size - sizeof(struct arena);
+    if (a.data == NULL)
+        exit(errno);
 
     return a;
 }
 
-/*
- * Frees all memory in arena
- */
-void arena_reset(struct arena* a)
+void _arena_realloc_or_panic(arena_t* a, size_t len)
 {
-    a->next = a->begin;
-    a->prev = a->begin;
+    // TODO: gracefully recover.
+    //  although unlikely to be recoverable in most use cases
+    a->data = realloc(a->data, len);
+
+    if (a->data == NULL)
+        exit(errno);
+
+    a->cap = len;
+}
+
+void arena_reset(struct arena* a)
+{
+    a->next = 0;
+    a->prev = 0;
 }
 
-/*
- * Allocate new memory using arena
- */
 void* arena_alloc(struct arena* a, size_t len)
 {
-    void* p = a->next;
+    // align len to machine word size
+    len = (len + WORD_SIZE - 1) & ~(WORD_SIZE - 1);
+    fprintf(stderr, "allocating %zu bytes\n", len);
 
-    if ((byte*)p + len > (byte*)(a->begin) + a->cap) {
-        errno = ENOMEM;
-        return NULL;
-    }
+    a->prev = a->next;
+    a->next += len;
 
-    a->next = (byte*)(a->next) + len;
-    a->prev = p;
+    if (a->next > a->cap)
+        _arena_realloc_or_panic(a, a->cap * 2);
 
+
+    return (byte_t*)(a->data) + a->prev;
+}
+
+void* arena_calloc(struct arena* a, size_t nmemb, size_t size)
+{
+    void* p = arena_alloc(a, nmemb * size);
+    memset((byte_t*)(a->data) + a->prev, 0, size);
     return p;
 }
 
-/*
- * Reallocate last block in arena
- */
 void* arena_realloc_tail(struct arena* a, size_t len)
 {
-    void* p = a->prev;
+    a->next = a->prev;
 
-    if ((byte*)p + len > (byte*)(a->begin) + a->cap) {
-        errno = ENOMEM;
-        return NULL;
-    }
-
-    a->next = (byte*)p + len;
-
-    return a->prev;
+    return arena_alloc(a, len);
 }

src/test_arena.c

@@ -8,25 +8,23 @@
 #include <stdio.h>
 #include <stdlib.h>
 
-static struct arena* default_arena = NULL;
+static struct arena default_arena = { 0 };
 
 int main()
 {
     default_arena = arena_new();
 
-    if (default_arena == NULL) {
-        err(errno, "failed to allocate arena");
-    }
+    printf("\nAttempt to do allocations of 1 byte, cap is %zu, ", default_arena.cap);
 
-    printf("\nAttempt to do `arena->cap` allocations of 1 byte");
-
-    for (size_t i = 0; i < default_arena->cap; i++) {
-        char* c = arena_alloc(default_arena, 1);
+    size_t i;
+    for (i = 0; default_arena.next < default_arena.cap; i++) {
+        char* c = arena_alloc(&default_arena, 1);
         *c = i & 0xff;
         if (c == NULL) {
             err(EXIT_FAILURE, "failed to allocate memory");
         }
     }
+    printf("did %zu allocations", i);
 
     printf("\n    OK");