bsls_linkcoercion.h

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/// @file bsls_linkcoercion.h
///
/// The content of this file has been pre-processed for Doxygen.
///
 
 
// bsls_linkcoercion.h                                                -*-C++-*-
#ifndef INCLUDED_BSLS_LINKCOERCION
#define INCLUDED_BSLS_LINKCOERCION
 
#include <bsls_ident.h>
BSLS_IDENT("$Id: $")
 
/// @defgroup bsls_linkcoercion bsls_linkcoercion
/// @brief  Provide a way to force a link-time dependency into an object.
/// @addtogroup bsl
/// @{
/// @addtogroup bsls
/// @{
/// @addtogroup bsls_linkcoercion
/// @{
///
/// <h1> Outline </h1>
/// * <a href="#bsls_linkcoercion-purpose"> Purpose</a>
/// * <a href="#bsls_linkcoercion-macros"> Macros </a>
/// * <a href="#bsls_linkcoercion-description"> Description </a>
///   * <a href="#bsls_linkcoercion-macro-summary"> Macro Summary </a>
///   * <a href="#bsls_linkcoercion-link-coercion-mechanism"> Link coercion mechanism </a>
///   * <a href="#bsls_linkcoercion-linker-garbage-collection"> Linker garbage collection </a>
///   * <a href="#bsls_linkcoercion-usage"> Usage </a>
///     * <a href="#bsls_linkcoercion-example-1-prevent-linking-debug-and-optimized-translation-units"> Example 1: Prevent Linking Debug and Optimized Translation Units </a>
///
/// # Purpose {#bsls_linkcoercion-purpose}
/// Provide a way to force a link-time dependency into an object.
///
/// # Macros {#bsls_linkcoercion-macros}
///
/// -  BSLS_LINKCOERCION_FORCE_SYMBOL_DEPENDENCY
///
/// # Description {#bsls_linkcoercion-description}
/// This component provides a facility to force a link-time
/// dependency on a symbol of the name supplied to the macro into the
/// translation unit being compiled. This macro can be useful for generating
/// link-time failures if a library version mismatch occurs.
///
/// ## Macro Summary {#bsls_linkcoercion-macro-summary}
///
///
/// This section provides a brief description of the macros defined in this
/// component.
/// @code
/// BSLS_LINKCOERCION_FORCE_SYMBOL_DEPENDENCY(type, refName, referredSymbol);
///     Force a link-time dependency into an object by creating a "symbol" (a
///     static variable with external linkage) having specified 'type'
///     (typically 'const char *'), the specified 'name' (which should be
///     unique), and that will be given the value of the address of the
///     specified 'referredSymbol'.
/// @endcode
/// Notice that this macro is typically used to prevent to binary incompatible
/// object files from being successfully linked together.  This is achieved by
/// calling this macro in a way that `referredSymbol` will have a different
/// symbol name for incompatible builds.
///
/// ## Link coercion mechanism {#bsls_linkcoercion-link-coercion-mechanism}
///
///
/// Link-time coersion is implemented by creating a static variable that
/// references an external symbol defined by the library.  The actual name of
/// this external symbol should be different for different build flavors.  When
/// the executable is linked against the libraries with link-coercion symbols,
/// the linker needs to resolve the symbol name and will fail the link step if
/// the required symbol name is not present in the linked library (i.e. the
/// library was built with a different set of flags that control the coercion
/// symbol creation).  For example, we can create different coercion symbols for
/// code built with c++17 and c++20 flags, effectively preventing translation
/// units compiled with different c++ standard flags from being linked into an
/// working program.
///
/// ## Linker garbage collection {#bsls_linkcoercion-linker-garbage-collection}
///
///
/// Modern linkers may implement link-time garbage collection.  In order to
/// minimize the size of the final executable the linker can remove sections of
/// object files that are not referenced anywhere from the code being linked.
/// Unfortunately, the link coercion symbols are not referenced directly and may
/// be garbage collected and, if removed, lead to a silent failure and
/// successful linkage of a potentially binary incompatible executable.  To
/// avoid such a failure, coercion symbols are placed in the dedicated data
/// section (if supported by the compiler) and this section is marked in-use to
/// avoid it being garbage collected by the linker.
///
/// More information on details on linker garbage collection can be found here:
/// https://maskray.me/blog/2021-02-28-linker-garbage-collection
///
/// ## Usage {#bsls_linkcoercion-usage}
///
///
/// In this section we show intended use of this component.
///
/// ### Example 1: Prevent Linking Debug and Optimized Translation Units {#bsls_linkcoercion-example-1-prevent-linking-debug-and-optimized-translation-units}
///
///
/// In this example we use a link coercion symbol to prevent linking a
/// translation unit built against the header in debug mode, with a translation
/// unit built against the header in optimized mode.
///
/// First we define a macro such that our symbol name will be different for
/// debug and optimized builds of our library:
/// @code
/// mylib_safecompile.h
/// #ifdef NDEBUG
///     #define MYLIB_DEBUG_SYMBOL_NAME mylib_BuiltWithNDebugMode
/// #else
///     #define MYLIB_DEBUG_SYMBOL_NAME mylib_BuiltWithoutNDebugMode
/// #endif
/// @endcode
/// Now we use the `BSLS_LINKCOERCION_FORCE_SYMBOL_DEPENDENCY` macro to require
/// translation units to link against a library having a matching symbol name:
/// @code
/// extern const char* MYLIB_DEBUG_SYMBOL_NAME;
/// BSLS_LINKCOERCION_FORCE_SYMBOL_DEPENDENCY(const char *,
///                                           mylib_coercebasedondebugmode,
///                                           MYLIB_DEBUG_SYMBOL_NAME )
/// @endcode
/// Note that the referred symbol will have a different name depending on
/// whether `NDEBUG` is defined or not.
///
/// Finally, in the corresponding `.cpp` file we must define the appropriate
/// symbol name for the library:
/// @code
///  mylib_safecompile.cpp
///  const char *MYLIB_DEBUG_SYMBOL_NAME = "debug compatibility check: "
///                                         #MYLIB_DEBUG_SYMBOL_NAME;
/// @endcode
/// @}
/** @} */
/** @} */
 
/** @addtogroup bsl
 * @{
 */
/** @addtogroup bsls
 * @{
 */
/** @addtogroup bsls_linkcoercion
 * @{
 */
 
#include <bsls_platform.h>
 
 
 
#if defined(BSLS_PLATFORM_OS_LINUX)
    // Note: we cannot detect the version of the binutils here, so we
    // tie those specific attributes to the version of the compilers that
    // should support them.
    // Please see the "Linker garbage collection" section for details.
    #if  (defined(BSLS_PLATFORM_CMP_GNU)                                      \
             && BSLS_PLATFORM_CMP_VERSION >= 110000)                          \
      || (defined(BSLS_PLATFORM_CMP_CLANG)                                    \
             && BSLS_PLATFORM_CMP_VERSION >= 130000)
        #define BSLS_LINKCOERCION_FORCE_SYMBOL_DEPENDENCY(type,               \
                                                          refName,            \
                                                          referredSymbol)     \
        _Pragma("GCC diagnostic push")                                        \
        _Pragma("GCC diagnostic ignored \"-Wattributes\"")                    \
        static type *dummyAcCeSs##refName() {                                 \
            return &referredSymbol;                                           \
        }                                                                     \
        static type *refName __attribute__((retain,used,section("coercion"))) \
                                         = dummyAcCeSs##refName();            \
        _Pragma("GCC diagnostic pop")
    #else
        #define BSLS_LINKCOERCION_FORCE_SYMBOL_DEPENDENCY(type,               \
                                                          refName,            \
                                                          referredSymbol)     \
        static type *dummyAcCeSs##refName() {                                 \
            return &referredSymbol;                                           \
        }                                                                     \
        static type *refName __attribute__((used)) = dummyAcCeSs##refName();
    #endif
 
#elif defined(BSLS_PLATFORM_OS_DARWIN)
    #define BSLS_LINKCOERCION_FORCE_SYMBOL_DEPENDENCY(type,                   \
                                                      refName,                \
                                                      referredSymbol)         \
    static type *dummyAcCeSs##refName() {                                     \
        return &referredSymbol;                                               \
    }                                                                         \
    static type *refName __attribute__((used)) = dummyAcCeSs##refName();
#elif defined(BSLS_PLATFORM_CMP_IBM)
    #define BSLS_LINKCOERCION_FORCE_SYMBOL_DEPENDENCY(type,                   \
                                                      refName,                \
                                                      referredSymbol)         \
    static type *refName = &referredSymbol;
#elif defined(BSLS_PLATFORM_CMP_SUN)
    #define BSLS_LINKCOERCION_FORCE_SYMBOL_DEPENDENCY(type,                   \
                                                      refName,                \
                                                      referredSymbol)         \
    type refName() __attribute__((weak));                                     \
    type refName() { return referredSymbol; }
#else
    #define BSLS_LINKCOERCION_FORCE_SYMBOL_DEPENDENCY(type,                   \
                                                      refName,                \
                                                      referredSymbol)         \
    namespace {                                                               \
        extern type *const refName = &referredSymbol;                         \
    }
#endif
 
 
 
#endif
 
// ----------------------------------------------------------------------------
// Copyright 2013 Bloomberg Finance L.P.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// ----------------------------- END-OF-FILE ----------------------------------
 
/** @} */
/** @} */
/** @} */