Merge pull request #2470 from Shopify/at-fix-mmap

Fix unavailable MAP_ANONYMOUS

Author: soutaro

include/rbs/util/rbs_allocator.h

@@ -3,15 +3,32 @@
 
 #include <stddef.h>
 
-#ifndef alignof
-#if defined(__GNUC__) || defined(__clang__)
-#define alignof(type) __alignof__(type)
+/* Include stdalign.h for C11 and later for alignof support */
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L
+#include <stdalign.h>
+#endif
+
+/*
+ * Define a portable alignment macro that works across all supported environments
+ */
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L
+/* C11 or later - use _Alignof directly (always available in C11+) */
+#define rbs_alignof(type) _Alignof(type)
+#elif defined(__cplusplus) && __cplusplus >= 201103L
+/* C++11 or later has alignof keyword */
+#define rbs_alignof(type) alignof(type)
+#elif defined(__GNUC__) || defined(__clang__)
+/* GCC and Clang provide __alignof__ */
+#define rbs_alignof(type) __alignof__(type)
 #elif defined(_MSC_VER)
-#define alignof(type) __alignof(type)
+/* MSVC provides __alignof */
+#define rbs_alignof(type) __alignof(type)
 #else
-// Fallback using offset trick
-#define alignof(type) offsetof(struct { char c; type member; }, member)
-#endif
+/* Fallback using offset trick for other compilers */
+#define rbs_alignof(type) offsetof( \
+    struct { char c; type member; },                     \
+    member                          \
+)
 #endif
 
 struct rbs_allocator;
@@ -26,13 +43,13 @@ void *rbs_allocator_calloc_impl(rbs_allocator_t *, size_t count, size_t size, si
 void *rbs_allocator_realloc_impl(rbs_allocator_t *, void *ptr, size_t old_size, size_t new_size, size_t alignment);
 
 // Use this when allocating memory for a single instance of a type.
-#define rbs_allocator_alloc(allocator, type) ((type *) rbs_allocator_malloc_impl((allocator), sizeof(type), alignof(type)))
+#define rbs_allocator_alloc(allocator, type) ((type *) rbs_allocator_malloc_impl((allocator), sizeof(type), rbs_alignof(type)))
 // Use this when allocating memory that will be immediately written to in full.
 // Such as allocating strings
-#define rbs_allocator_alloc_many(allocator, count, type) ((type *) rbs_allocator_malloc_many_impl((allocator), (count), sizeof(type), alignof(type)))
+#define rbs_allocator_alloc_many(allocator, count, type) ((type *) rbs_allocator_malloc_many_impl((allocator), (count), sizeof(type), rbs_alignof(type)))
 // Use this when allocating memory that will NOT be immediately written to in full.
 // Such as allocating buffers
-#define rbs_allocator_calloc(allocator, count, type) ((type *) rbs_allocator_calloc_impl((allocator), (count), sizeof(type), alignof(type)))
-#define rbs_allocator_realloc(allocator, ptr, old_size, new_size, type) ((type *) rbs_allocator_realloc_impl((allocator), (ptr), (old_size), (new_size), alignof(type)))
+#define rbs_allocator_calloc(allocator, count, type) ((type *) rbs_allocator_calloc_impl((allocator), (count), sizeof(type), rbs_alignof(type)))
+#define rbs_allocator_realloc(allocator, ptr, old_size, new_size, type) ((type *) rbs_allocator_realloc_impl((allocator), (ptr), (old_size), (new_size), rbs_alignof(type)))
 
 #endif

src/location.c

@@ -8,7 +8,7 @@
 void rbs_loc_alloc_children(rbs_allocator_t *allocator, rbs_location_t *loc, size_t capacity) {
     rbs_assert(capacity <= sizeof(rbs_loc_entry_bitmap) * 8, "Capacity %zu is too large. Max is %zu", capacity, sizeof(rbs_loc_entry_bitmap) * 8);
 
-    loc->children = rbs_allocator_malloc_impl(allocator, RBS_LOC_CHILDREN_SIZE(capacity), alignof(rbs_loc_children));
+    loc->children = rbs_allocator_malloc_impl(allocator, RBS_LOC_CHILDREN_SIZE(capacity), rbs_alignof(rbs_loc_children));
 
     loc->children->len = 0;
     loc->children->required_p = 0;

src/util/rbs_allocator.c

@@ -20,17 +20,38 @@
 #include <unistd.h>
 #include <sys/types.h>
 #include <sys/mman.h>
-#endif
-
-#if defined(__APPLE__) || defined(__FreeBSD__) || defined(__OpenBSD__) || defined(__sun)
-#define MAP_ANONYMOUS MAP_ANON
+#include <fcntl.h>
 #endif
 
 struct rbs_allocator {
     uintptr_t heap_ptr;
     uintptr_t size;
 };
 
+static void *portable_mmap_anon(size_t size) {
+#ifdef _WIN32
+    /* Windows doesn't use this function - VirtualAlloc is used instead */
+    return NULL;
+#else
+    void *ptr;
+
+#if defined(MAP_ANONYMOUS)
+    ptr = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+#elif defined(MAP_ANON)
+    ptr = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANON, -1, 0);
+#else
+    /* Fallback to /dev/zero for systems without anonymous mapping */
+    int fd = open("/dev/zero", O_RDWR);
+    rbs_assert(fd != -1, "open('/dev/zero') failed");
+
+    ptr = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
+    close(fd); /* Can close fd after mapping */
+#endif
+
+    return ptr;
+#endif
+}
+
 static size_t get_system_page_size(void) {
 #ifdef _WIN32
     SYSTEM_INFO si;
@@ -48,7 +69,7 @@ static void *map_memory(size_t size) {
     LPVOID result = VirtualAlloc(NULL, size, MEM_RESERVE | MEM_COMMIT, PAGE_READWRITE);
     rbs_assert(result != NULL, "VirtualAlloc failed");
 #else
-    void *result = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+    void *result = portable_mmap_anon(size);
     rbs_assert(result != MAP_FAILED, "mmap failed");
 #endif
     return result;