objective-c - 为什么无法从 Objective-C 目标文件中隐藏符号？

Question

我正在编写一个使用 Objective-C 并将 C 链接函数公开给 Unity 的小型动态库(它是一个 native 插件)。

尽管我的编译器设置将Symbols Hidden by Default 设置为Yes，并且我正在使用__attribute__((visibility("default"))) 在我想公开的函数声明上，甚至 static 符号(函数和全局变量)都出现在输出库中。

我重新创建了一个 test Xcode project , 最初包含一个 C 文件

文件.h:

#pragma once

extern __attribute__((visibility("default"))) void not_hidden();

文件.c:

#include "file.h"
#include <stdio.h>

static void hidden()
{
    printf("I'm hidden\n");
}

void not_hidden()
{
    printf("I'm not hidden\n");
    hidden();
}

这就像宣传的那样:

$ nm libHiddenSymbolTest.dylib 
0000000000000f40 T _not_hidden
                 U _printf
                 U dyld_stub_binder

然而，一旦我添加了一个具有类似功能(not_hidden2() 等)的 Objective-C 文件，那么所有的 static 符号同样会被暴露:

$ nm libHiddenSymbolTest.dylib 
                 U _CFStringGetCStringPtr
                 U _NSLog
00000000000054d0 s _OBJC_METACLASS_$___ARCLite__
                 U __Block_copy
                 U __DefaultRuneLocale
0000000000005710 b __ZGVZL18add_image_hook_ARCPK11mach_headerlE7patches
0000000000005830 b __ZGVZL22add_image_hook_swiftV1PK11mach_headerlE7patches
0000000000005640 b __ZGVZL30add_image_hook_autoreleasepoolPK11mach_headerlE7patches
0000000000004160 s __ZL11_class_name
00000000000039a4 t __ZL12cxxConstructP11objc_object
00000000000055d0 d __ZL12demangleLock
000000000000377a t __ZL13demangledNamePKcb
0000000000002ab3 t __ZL13replaceMethodP10objc_classP13objc_selectorPFP11objc_objectS4_S2_zEPS6_
0000000000005748 b __ZL14NSString_class
00000000000035b0 t __ZL14initialize_impP11objc_objectP13objc_selector
0000000000001aa2 t __ZL15__ARCLite__loadv
0000000000003927 t __ZL16scanMangledFieldRPKcS0_S1_Ri
00000000000041a8 s __ZL17_load_method_name
00000000000043d0 s __ZL17_load_method_type
0000000000003461 t __ZL17transcribeMethodsP10objc_classP15glue_class_ro_t
00000000000022b9 t __ZL18add_image_hook_ARCPK11mach_headerl
00000000000035b6 t __ZL18allocateMaybeSwiftP18glue_swift_class_tm
0000000000001f7f t __ZL19patch_lazy_pointersPK11mach_headerP7patch_tm
00000000000034e5 t __ZL19transcribeProtocolsP10objc_classP15glue_class_ro_t
00000000000039d4 t __ZL20fixStringForCoreDataP11objc_object
0000000000003530 t __ZL20transcribePropertiesP10objc_classP15glue_class_ro_t
0000000000002785 t __ZL21__arclite_objc_retainP11objc_object
00000000000025e3 t __ZL21__arclite_object_copyP11objc_objectm
0000000000005740 b __ZL22NSConstantString_class
00000000000027a1 t __ZL22__arclite_objc_releaseP11objc_object
000000000000288f t __ZL22add_image_hook_swiftV1PK11mach_headerl
0000000000003646 t __ZL22copySwiftV1MangledNamePKcb
0000000000005870 b __ZL22original_class_getName
0000000000005848 b __ZL22original_objc_getClass
0000000000005610 b __ZL23NSAutoreleasePool_class
00000000000033dc t __ZL23__arclite_class_getNameP10objc_class
0000000000003230 t __ZL23__arclite_objc_getClassPKc
00000000000054f8 s __ZL24OBJC_CLASS_$___ARCLite__
000000000000248c t __ZL24__arclite_object_setIvarP11objc_objectP9objc_ivarS0_
0000000000005738 b __ZL25NSMutableDictionary_class
0000000000005868 b __ZL25original_objc_getProtocol
0000000000005860 b __ZL25original_objc_lookUpClass
0000000000005878 b __ZL25original_protocol_getName
00000000000027b3 t __ZL26__arclite_objc_autoreleaseP11objc_object
0000000000003384 t __ZL26__arclite_objc_getProtocolPKc
000000000000332f t __ZL26__arclite_objc_lookUpClassPKc
0000000000002797 t __ZL26__arclite_objc_retainBlockP11objc_object
000000000000283f t __ZL26__arclite_objc_storeStrongPP11objc_objectS0_
00000000000033f1 t __ZL26__arclite_protocol_getNameP8Protocol
0000000000005850 b __ZL26original_objc_getMetaClass
00000000000052c8 s __ZL27OBJC_CLASS_RO_$___ARCLite__
0000000000003285 t __ZL27__arclite_objc_getMetaClassPKc
0000000000005758 b __ZL27original_NSKKMS_indexForKey
0000000000002b9a t __ZL28__arclite_NSKKMS_indexForKeyP11objc_objectP13objc_selectorS0_
0000000000002df0 t __ZL28__arclite_objc_readClassPairP10objc_classPK15objc_image_info
0000000000005760 b __ZL28original_NSKKsD_objectForKey
0000000000002bec t __ZL29__arclite_NSKKsD_objectForKeyP11objc_objectP13objc_selectorS0_
0000000000005720 b __ZL29original_NSManagedObject_init
0000000000005718 b __ZL30NSUndoManagerProxy_targetClass
0000000000002ae6 t __ZL30__arclite_NSManagedObject_initP11objc_objectP13objc_selector
0000000000001ecb t __ZL30add_image_hook_autoreleasepoolPK11mach_headerl
000000000000399e t __ZL30arclite_uninitialized_functionv
0000000000005858 b __ZL30original_objc_getRequiredClass
0000000000005280 s __ZL31OBJC_METACLASS_RO_$___ARCLite__
00000000000032da t __ZL31__arclite_objc_getRequiredClassPKc
0000000000005838 b __ZL31original_objc_allocateClassPair
0000000000005840 b __ZL31original_object_getIndexedIvars
0000000000005310 s __ZL32OBJC_$_CLASS_METHODS___ARCLite__
000000000000313d t __ZL32__arclite_objc_allocateClassPairP10objc_classPKcm
00000000000027c5 t __ZL32__arclite_objc_retainAutoreleaseP11objc_object
00000000000031de t __ZL32__arclite_object_getIndexedIvarsP11objc_object
0000000000005770 b __ZL32original_NSKKsD_setObject_forKey
0000000000002c92 t __ZL33__arclite_NSKKsD_setObject_forKeyP11objc_objectP13objc_selectorS0_S0_
0000000000001f6d t __ZL33__arclite_objc_autoreleasePoolPopPv
0000000000001f54 t __ZL34__arclite_objc_autoreleasePoolPushv
0000000000005768 b __ZL34original_NSKKsD_removeObjectForKey
0000000000002c3e t __ZL35__arclite_NSKKsD_removeObjectForKeyP11objc_objectP13objc_selectorS0_
0000000000005730 b __ZL35original_NSManagedObject_allocBatch
0000000000002b0c t __ZL36__arclite_NSManagedObject_allocBatchP11objc_objectP13objc_selectorPS0_S0_j
0000000000002434 t __ZL36__arclite_object_setInstanceVariableP11objc_objectPKcPv
0000000000005750 b __ZL36original_NSKKMS_fastIndexForKnownKey
0000000000005880 b __ZL36original_objc_copyClassNamesForImage
0000000000002b48 t __ZL37__arclite_NSKKMS_fastIndexForKnownKeyP11objc_objectP13objc_selectorS0_
00000000000027f0 t __ZL37__arclite_objc_autoreleaseReturnValueP11objc_object
0000000000003409 t __ZL37__arclite_objc_copyClassNamesForImagePKcPj
0000000000005778 b __ZL40original_NSKKsD_addEntriesFromDictionary
0000000000005728 b __ZL40original_NSManagedObject_allocWithEntity
0000000000002cfb t __ZL41__arclite_NSKKsD_addEntriesFromDictionaryP11objc_objectP13objc_selectorP12NSDictionary
0000000000002af9 t __ZL41__arclite_NSManagedObject_allocWithEntityP11objc_objectP13objc_selectorS0_
00000000000021e7 t __ZL42__arclite_NSArray_objectAtIndexedSubscriptP7NSArrayP13objc_selectorm
0000000000002a7a t __ZL42__arclite_NSUndoManagerProxy_isKindOfClassP11objc_objectP13objc_selectorP10objc_class
0000000000002802 t __ZL43__arclite_objc_retainAutoreleaseReturnValueP11objc_object
000000000000282d t __ZL44__arclite_objc_retainAutoreleasedReturnValueP11objc_object
000000000000225b t __ZL46__arclite_NSDictionary_objectForKeyedSubscriptP12NSDictionaryP13objc_selectorP11objc_object
000000000000226d t __ZL47__arclite_NSOrderedSet_objectAtIndexedSubscriptP12NSOrderedSetP13objc_selectorm
00000000000021f9 t __ZL53__arclite_NSMutableArray_setObject_atIndexedSubscriptP14NSMutableArrayP13objc_selectorP11objc_objectm
0000000000002291 t __ZL58__arclite_NSMutableDictionary__setObject_forKeyedSubscriptP19NSMutableDictionaryP13objc_selectorP11objc_objectS4_
000000000000227f t __ZL58__arclite_NSMutableOrderedSet_setObject_atIndexedSubscriptP19NSMutableOrderedSetP13objc_selectorP11objc_objectm
0000000000005888 b __ZL9Demangled
0000000000005650 b __ZZL18add_image_hook_ARCPK11mach_headerlE7patches
0000000000005780 b __ZZL22add_image_hook_swiftV1PK11mach_headerlE7patches
0000000000005620 b __ZZL30add_image_hook_autoreleasepoolPK11mach_headerlE7patches
                 U ___CFConstantStringClassReference
                 U ___stack_chk_fail
                 U ___stack_chk_guard
                 U __dyld_register_func_for_add_image
0000000000005260 s __non_lazy_classes
                 U __objc_empty_cache
                 U __objc_empty_vtable
                 U _asprintf
                 U _bzero
                 U _calloc
                 U _class_addMethod
                 U _class_addProperty
                 U _class_addProtocol
                 U _class_getInstanceMethod
                 U _class_getInstanceSize
                 U _class_getInstanceVariable
                 U _class_getIvarLayout
                 U _class_getName
                 U _class_getSuperclass
                 U _class_isMetaClass
                 U _class_replaceMethod
                 U _class_respondsToSelector
                 U _free
                 U _hash_create
                 U _hash_search
0000000000001a30 t _hidden
0000000000001a70 t _hidden2
                 U _ivar_getName
                 U _ivar_getOffset
                 U _kCFCoreFoundationVersionNumber
                 U _malloc
                 U _memcmp
                 U _memcpy
                 U _method_setImplementation
0000000000001a00 T _not_hidden
0000000000001a50 T _not_hidden2
                 U _objc_allocateClassPair
                 U _objc_autoreleasePoolPush
                 U _objc_collectingEnabled
                 U _objc_constructInstance
                 U _objc_copyClassNamesForImage
                 U _objc_getClass
                 U _objc_getMetaClass
                 U _objc_getProtocol
                 U _objc_getRequiredClass
                 U _objc_initializeClassPair
                 U _objc_lookUpClass
                 U _objc_msgSend
                 U _objc_readClassPair
                 U _objc_registerClassPair
                 U _objc_retain
0000000000001a87 T _objc_retainedObject
0000000000001a90 T _objc_unretainedObject
0000000000001a99 T _objc_unretainedPointer
                 U _object_getClass
                 U _object_getIndexedIvars
                 U _object_getIvar
                 U _object_setIvar
                 U _printf
                 U _property_copyAttributeList
                 U _protocol_getMethodDescription
                 U _protocol_getName
                 U _pthread_mutex_lock
                 U _pthread_mutex_unlock
                 U _sel_getUid
                 U _strcmp
                 U _strlen
                 U _strncmp
                 U dyld_stub_binder

请注意，我已将各种 strip symbols 设置保留为默认设置，这对于 C 文件符号似乎没问题，如第一个 nm 输出所示，以上。

谁能解释为什么会这样，或许还能建议一种方法来隐藏我不想暴露的 Objective-C 符号？

更新:我在 OSX 10.11.0 上使用 Xcode 7.0.1。

更新 #2(更多实验):

1. 在项目中包含 .m 文件会导致 Xcode 在编译 .c 文件期间停止传递 -fvisibility=hidden，从而忽略 Symbols Hidden by Default build设置。重复;在添加 .m 之前，它被传递给了编译器。
2. 将导出的符号文件设置为包含_not_hidden 和_not_hidden2 的文件没有任何效果。

在我看来，忽略默认隐藏的符号是问题的根本原因，所以我尝试显式设置 __attribute__((visibility("hidden"/"default")) ) (通过预处理器宏)在每个函数和变量上，它也没有任何效果。所以这似乎与链接器有关。

Answer 1

您仍然在 dylib 中看到一堆符号的原因有多种:

<强>1。您已设置(或未设置)的剥离模式无法获得其所有功能。

(尝试在build设置中调整 strip 样式，通常删除 debugging symbols 没问题，但是，我为此将其提升为 Non-Global Symbols目的，这可能是你所能接受的。如果你决定去除 all symbols，你的 dylib 可能不会编译，并且它基本上会导致它在那时无法使用)

<强>2。一些 Objective-C 符号是绝对必需的。

(这些包括任何 objc 类型，以及其他类型，例如 class、property、object，等等，因为它们都支持运行时，是的，现在仍然有很多)

<强>3。您可能正在查看 dylib 的调试版本，而不是发布版本。

(确保您查看的是 dylib 的release 版本。那将是 Product > Archive > Export。然后应该很好地去除大部分实际未使用的符号。

---

尽管只是使用我在上面显示的设置 resulted in approximately 100+ fewer symbols，但您可能需要稍微调整一下 strip 设置。比当前显示在您的转储中。将符号包含在文件中确实不错，事实上，如果出现问题，它会更有帮助。不过，如果您想删除不相关或未使用的符号，这些符号只会让事情变得困惑，这是可以理解的。