Mach-O 平台上的库版本控制

Question

当 SOVERSION 目标属性的文档提到在 Mach-O 系统（例如 OS X 和iOS对应的是"compatibility version"，而VERSION对应的是"current version".

在Linux上，它只是类似VERSION 5.4.2和SOVERSION 5的东西，表示该库与5.0及更新版本兼容，VERSION用作Windows 上 <major>.<minor> 形式的 DLL 映像版本（我不确定 SOVERSION 对 Windows 有何影响）。

但是，the docs for the referenced FRAMEWORK target property 中的示例说明了如何在 Mach-O 平台上拥有 VERSION 16.4.0 和 SOVERSION 1.0.0（我对构建框架不感兴趣，只是想知道外国版本控制方案。）

版本控制在 Mach-O 世界中是如何工作的？如果我删除一些功能，我习惯于只修改主要版本，这会破坏兼容性，那么版本 16.4.0 的库怎么可能与库的 1.0.0 版本保持兼容？ "compatible" 是什么意思？

Answer 1

首先，为了解决这个问题，框架只是名为 Something.framework/Something 而不是 libsomething.dylib 的动态库。尽管文件格式完全相同，因此在整个过程中 post 我将简单地将它们称为 dylibs。

现在，让我们从 mach-o/loader.h header（Mach-O 文件格式的 de-facto 权威来源）的摘录开始：

/*
 * Dynamicly linked shared libraries are identified by two things.  The
 * pathname (the name of the library as found for execution), and the
 * compatibility version number.  The pathname must match and the compatibility
 * number in the user of the library must be greater than or equal to the
 * library being used.  The time stamp is used to record the time a library was
 * built and copied into user so it can be use to determined if the library used
 * at runtime is exactly the same as used to built the program.
 */
struct dylib {
    union lc_str  name;         /* library's path name */
    uint32_t timestamp;         /* library's build time stamp */
    uint32_t current_version;       /* library's current version number */
    uint32_t compatibility_version; /* library's compatibility vers number*/
};

/*
 * A dynamically linked shared library (filetype == MH_DYLIB in the mach header)
 * contains a dylib_command (cmd == LC_ID_DYLIB) to identify the library.
 * An object that uses a dynamically linked shared library also contains a
 * dylib_command (cmd == LC_LOAD_DYLIB, LC_LOAD_WEAK_DYLIB, or
 * LC_REEXPORT_DYLIB) for each library it uses.
 */
struct dylib_command {
    uint32_t    cmd;        /* LC_ID_DYLIB, LC_LOAD_{,WEAK_}DYLIB,
                       LC_REEXPORT_DYLIB */
    uint32_t    cmdsize;    /* includes pathname string */
    struct dylib    dylib;      /* the library identification */
};

正如评论中所解释的那样，struct dylib 嵌入在库以及针对它的二进制链接中，两者都包含 current_version 和 compatibility_version 的副本。后者的工作原理在此处进行了解释，但前者不是地址。
相关文档可以在 dyld 手册页上找到（来源是 here，但在 man 之外看起来不太好看）：

DYLD_VERSIONED_FRAMEWORK_PATH
    This  is a colon separated list of directories that contain potential override frame-
    works.  The dynamic linker searches  these  directories  for  frameworks.   For  each
    framework  found  dyld  looks  at  its  LC_ID_DYLIB  and gets the current_version and
    install name.  Dyld then looks for the framework at the install name path.  Whichever
    has the larger current_version value will be used in the process whenever a framework
    with that install name is required.  This is similar  to  DYLD_FRAMEWORK_PATH  except
    instead  of  always overriding, it only overrides is the supplied framework is newer.
    Note: dyld does not check the framework's Info.plist to find its version.  Dyld  only
    checks the -current_version number supplied when the framework was created.

[...]

DYLD_VERSIONED_LIBRARY_PATH
    This is a colon  separated  list  of  directories  that  contain  potential  override
    libraries.  The dynamic linker searches these directories for dynamic libraries.  For
    each library found dyld looks at its LC_ID_DYLIB and  gets  the  current_version  and
    install  name.   Dyld then looks for the library at the install name path.  Whichever
    has the larger current_version value will be used in the  process  whenever  a  dylib
    with  that  install  name  is  required.  This is similar to DYLD_LIBRARY_PATH except
    instead of always overriding, it only overrides is the supplied library is newer.

简而言之：

compatibility_version 用于确定库是否 "new enough" 用于要加载它的二进制文件。
current_version 用于在多个库可用时选择一个库。

至于您对 16.4.0 的当前版本与 1.0.0 的兼容版本感到困惑：从 some sources 来看，Apple 似乎会在有引入的任何类型的功能，并使用次要版本几乎仅修复错误，AFAIK。
所以他们称之为 16.4.0，我可能会称之为 1.16.4。 ;)

Mach-O 平台上的库版本控制

Library versioning on Mach-O platforms

macos

cmake

mach-o