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03_target.md

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Target

CMake构建规则:

取自cmake --system-information sys_info.txt产生的文件中:

// build rules
CMAKE_C_COMPILE_OBJECT == "<CMAKE_C_COMPILER> <DEFINES> <INCLUDES> <FLAGS> -o <OBJECT> -c <SOURCE>"
CMAKE_C_LINK_EXECUTABLE == "<CMAKE_C_COMPILER> <FLAGS> <CMAKE_C_LINK_FLAGS> <LINK_FLAGS> <OBJECTS> -o <TARGET> <LINK_LIBRARIES>"
CMAKE_C_CREATE_SHARED_LIBRARY == "<CMAKE_C_COMPILER> <LANGUAGE_COMPILE_FLAGS> <CMAKE_SHARED_LIBRARY_CREATE_C_FLAGS> <LINK_FLAGS> -o <TARGET> <SONAME_FLAG> <TARGET_INSTALLNAME_DIR><TARGET_SONAME> <OBJECTS> <LINK_LIBRARIES>"

Makefile项目的两个重要编译流程:

# 1.CMAKE_C_COMPILE_OBJECT 预处理/编译/汇编
# <CMAKE_C_COMPILER> <DEFINES> <INCLUDES> <FLAGS> -o <OBJECT> -c <SOURCE>
gcc -o main.c.o -c main.c

# 2.CMAKE_C_LINK_EXECUTABLE 链接
# <CMAKE_C_COMPILER> <FLAGS> <CMAKE_C_LINK_FLAGS> <LINK_FLAGS> <OBJECTS> -o <TARGET> <LINK_LIBRARIES>
gcc main.c.o -o main

Target目标

everything is a (self-contained) target

build system构建的目标为了产生可执行程序,动态库,静态库...... 这些被成为目标target.

产生这个target需要哪些条件?

  • 源代码文件
  • 头文件搜索路径(默认/相对路径/三方库)
  • 编译器/链接器选项
  • 编译器/链接器特性
  • 链接库

类似面向对象思想,创建target(对象)

构造函数：

  • add_executable() 生成可执行文件
  • add_library() 生成库(动态库/静态库)

成员函数：

  • target_sources() 编译源文件
  • target_include_directories() 头文件路径
  • target_compile_definitions() 编译器传参
  • target_compile_features() 编译特性cxx_lambdas/cxx_range_for/...
  • target_compile_options() 编译选项
  • target_link_libraries() 链接库
  • target_link_options() 链接选项
  • target_link_directories() 链接库的目录
  • get_target_property()
  • set_target_properties()

成员变量

不建议使用的命令

这些是全局的命令.避免一些target的选项,配置冲突,产生一些不符合预期的行为

  • add_compile_options()
  • add_definitions()
  • include_directories()
  • link_directories()
  • link_libraries()

Transitive Usage Requirements

问题:

当项目增长,依赖增多,面临的问题:多层依赖时,(库/编译flag/option)可见性/传递性的问题

大型项目依赖关系图:

image-20210628003657036

分析问题,分析最简单的模型就够了,复杂的都是简单的组合形成的

例子1:库依赖PUBLIC/PRIVATE

# PUBLIC
## c++ -fPIC -shared -Wl,-soname,liba.so -o liba.so a.cxx.o libb.so libc.so
target_link_libraries(b PUBLIC c) 
target_link_libraries(a PUBLIC b)

# PRIVATE/PUBLIC
## c++ -fPIC -shared -Wl,-soname,liba.so -o liba.so a.cxx.o libb.so
target_link_libraries(b PRIVATE c) 
target_link_libraries(a PUBLIC b)

image-20210628104801087

输出编译命令:

c++ -fPIC -shared -Wl,-soname,liba.so -o liba.so a.cxx.o libb.so libc.so
c++ -fPIC -shared -Wl,-soname,liba.so -o liba.so a.cxx.o libb.so

例子2:编程传参INTERFACE

# archive库
add_library(archive archive.cpp)
target_compile_definitions(archive INTERFACE USING_ARCHIVE_LIB)

# serialization库
add_library(serialization serialization.cpp)
target_compile_definitions(serialization INTERFACE USING_SERIALIZATION_LIB)

# archiveExtras库
add_library(archiveExtras extras.cpp)
target_link_libraries(archiveExtras PUBLIC archive)
target_link_libraries(archiveExtras PRIVATE serialization)
## archiveExtras 被编译同时带 -DUSING_ARCHIVE_LIB -DUSING_SERIALIZATION_LIB

# consumer程序
add_executable(consumer consumer.cpp)
## consumer is compiled with -DUSING_ARCHIVE_LIB
target_link_libraries(consumer archiveExtras)

输出编译命令:

c++ -o serialization.o -c serialization.cpp
c++ -o archive.o -c archive.cpp
c++ -DUSING_ARCHIVE_LIB -DUSING_SERIALIZATION_LIB -o extras.o -c extras.cpp

c++ -DUSING_ARCHIVE_LIB -o consumer.o -c consumer.cpp
c++ -o consumer libarchiveExtras.a libarchive.a libserialization.a

PRIVATE/INTERFACE/PUBLIC规则

  • PRIVATE:只给自己用,不给依赖者用
  • INTERFACE:自己不用,给依赖者用(当依赖于纯头文件库时,在 add_library() 中使 用INTERFACE就非常有有用了)
  • PUBLIC:PRIVATE + INTERFACE自己和依赖者都可以用
  • $<BUILD_INTERFACE>: Used by consumers from this project or use the build directory
  • $<INSTALL_INTERFACE>: Used by consumers after this target has been installed

CMake中的参数传递

如何通过cmake命令去影响编译哪些代码

  1. cmake命令参数 --> CMakeLists.txt
  2. CMakeLists.txt --> C/CXX代码中

1. cmake命令参数传递到CMakeLists.txt文件中

$ cmake --help

Options
  -D <var>[:<type>]=<value>    = Create or update a cmake cache entry. # cmake -D CMAKE_BUILD_TYPE=Debug ..

2. CMakeLists.txt --> C/CXX代码中

示例: 通过cmake命令行去控制代码中的日志级别

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "log.h"
#include "config.h"

// cmake -S . -B build -D LOG_DEBUG_ENABLE=ON # 控制当前log级别
int main(int argc, char* argv[]) {
#ifdef LOG_DEBUG_ENABLE
    log_set_level(LOG_DEBUG);
#else
    log_set_level(LOG_WARN);
#endif

    log_trace("trace");
    log_debug("debug");
    log_info("info");
    log_warn("warn");
    log_error("error");
    return 0;
}

流程: cmake命令行 --> CMakeLists.txt --> 生成头文件 --> C/CXX代码

configfile命令:通过input文件生成output(头文件), 并且可以把cmake中的变量写入到output中;input文件一般用in后缀,表示input

cmake_minimum_required(VERSION 3.12 FATAL_ERROR)

project(04arguments)

add_library(log STATIC
    log.c
)

add_executable(arguments
    main.c
)

# configure_file(<input> <output>)
configure_file(
    ${CMAKE_CURRENT_SOURCE_DIR}/config.h.in
    ${CMAKE_CURRENT_SOURCE_DIR}/config.h
)

target_link_libraries(arguments PRIVATE log)
target_include_directories(arguments PRIVATE ${log_SOURCE_DIR})

cmake中的变量写入到output中?

关键字语法:

  • #cmakedefine语法
  • #cmakedefine01语法
// #cmakedefine语法
#cmakedefine LOG_DEBUG_ENABLE
// 不存在: /* #undef LOG_DEBUG_ENABLE */
// 存在: #define LOG_DEBUG_ENABLE

#cmakedefine LOG_DEBUG_ENABLE ${LOG_DEBUG_ENABLE}
// 不存在: /* #undef LOG_DEBUG_ENABLE */
// 存在: #define LOG_DEBUG_ENABLE ON

// #cmakedefine01语法
#cmakedefine01 LOG_DEBUG_ENABLE
// 不存在: #define LOG_DEBUG_ENABLE 0
// 存在: #define LOG_DEBUG_ENABLE 1

config.h.in文件

#cmakedefine LOG_DEBUG_ENABLE

编译命令:

normal:
	cmake -S . -B build
	cmake --build build
	./build/arguments

debug:
	cmake -S . -B build -D LOG_DEBUG_ENABLE=ON
	cmake --build build
	./build/arguments

[add_library](dfile:///Users/shibin/Library/Application Support/Dash/DocSets/CMake/CMake.docset/Contents/Resources/Documents/cmake.org/cmake/help/v3.20/command/add_library.html#add-library)

  • [Normal Libraries](dfile:///Users/shibin/Library/Application Support/Dash/DocSets/CMake/CMake.docset/Contents/Resources/Documents/cmake.org/cmake/help/v3.20/command/add_library.html#normal-libraries)
  • [Object Libraries](dfile:///Users/shibin/Library/Application Support/Dash/DocSets/CMake/CMake.docset/Contents/Resources/Documents/cmake.org/cmake/help/v3.20/command/add_library.html#object-libraries)
  • [Interface Libraries](dfile:///Users/shibin/Library/Application Support/Dash/DocSets/CMake/CMake.docset/Contents/Resources/Documents/cmake.org/cmake/help/v3.20/command/add_library.html#interface-libraries)
  • [Imported Libraries](dfile:///Users/shibin/Library/Application Support/Dash/DocSets/CMake/CMake.docset/Contents/Resources/Documents/cmake.org/cmake/help/v3.20/command/add_library.html#imported-libraries)
  • [Alias Libraries](dfile:///Users/shibin/Library/Application Support/Dash/DocSets/CMake/CMake.docset/Contents/Resources/Documents/cmake.org/cmake/help/v3.20/command/add_library.html#alias-libraries)

生成器表达式

todo

  • Generator expressions use the $<> syntax.

  • Not evaluated by command interpreter.

    It is just a string with $<>.

  • Evaluated during build system generation.

  • Not supported in all commands (obviously).

生成器表达式是CMake 在生成时(即配置之后)构造, 用于生成特定于配置的构建输出

https://cmake.org/cmake/help/latest/manual/cmake-generator-expressions.7.html