201 lines
7.9 KiB
Markdown
201 lines
7.9 KiB
Markdown
# GCC 编译连接参数
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## 1. 编译参数
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GCC 编译程序的一般格式为：
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```bash
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gcc -o <target> <source files> [other parameters]
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```
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### 1.1. 常用参数
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* -c：只编译不链接：产生 .o 文件（就是 obj 文件），不产生执行文件
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* -o：指定输出文件名
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* -g：可执行程序包含调试信息
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* -O\<n\>：设置优化级别
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* -static：链接静态库，禁止使用动态库
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* -shared：进行动态库编译，链接动态库
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* -I\<dir\>：在头文件的搜索路径中增加 dir 目录
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* -L\<dir\>：在动态库的搜索路径中增加 dir 目录
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* -l\<name\>：链接静态库（libname.a）或动态库（libname.so）的库文件
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* -fPIC(或 fpic)：生成使用相对地址无关的目标代码
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### 1.2. 编译警告控制参数
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#### 1.2.1. -Werror
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-Werror：将所有的警告当成错误进行处理。
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#### 1.2.2. -Wall
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-Wall：打开 gcc 的所有警告，主要包括以下部分：
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```blk
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-Waddress
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-Warray-bounds=1 (only with -O2)
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-Wbool-compare
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-Wbool-operation
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-Wc++11-compat-Wc++14-compat
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-Wcatch-value (C++ and Objective-C++ only)
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-Wchar-subscripts：警告如果数组下标有char类型。 这是错误的常见原因，因为程序员经常忘记这种类型是在某些机器上签名的。
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-Wcomment
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-Wduplicate-decl-specifier (C and Objective-C only)
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-Wenum-compare (in C/ObjC; this is on by default in C++)
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-Wformat
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-Wformat-security：开启类型不匹配警告。
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-Wint-in-bool-context
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-Wimplicit (C and Objective-C only)
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-Wimplicit-int (C and Objective-C only)
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-Wimplicit-function-declaration (C and Objective-C only)
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-Winit-self (only for C++)
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-Wlogical-not-parentheses
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-Wmain (only for C/ObjC and unless -ffreestanding)
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-Wmaybe-uninitialized
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-Wmemset-elt-size
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-Wmemset-transposed-args
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-Wmisleading-indentation (only for C/C++)
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-Wmissing-attributes
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-Wmissing-braces (only for C/ObjC)
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-Wmultistatement-macros
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-Wnarrowing (only for C++)
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-Wnonnull
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-Wnonnull-compare
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-Wopenmp-simd
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-Wparentheses
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-Wpointer-sign
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-Wreorder
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-Wrestrict
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-Wreturn-type
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-Wsequence-point
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-Wsizeof-pointer-div
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-Wsizeof-pointer-memaccess
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-Wstrict-aliasing
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-Wstrict-overflow=1
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-Wswitch
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-Wtautological-compare
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-Wtrigraphs
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-Wuninitialized
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-Wunknown-pragmas
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-Wunused-function：开启未使用的函数警告。
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-Wunused-label
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-Wunused-value
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-Wunused-variable
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-Wvolatile-register-var
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```
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每个警告参数都有对应的“-Wno-”参数实现相反的控制，例如：
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```blk
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-Wno-unused-parameter：关闭未使用的参数警告。
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-Wno-unused-function：关闭未使用的函数警告。
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-Wno-format-security：关闭类型不匹配警告。
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-Wno-sign-compare：关闭有符号数和无符号数比较警告。
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```
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在同时开启 -Wall 和 -Werror 时将使编译警告变为编译错误，此时语法检查非常严格，如需放开某些限制，可结合“-Wno-”类参数使用，例如：-Wall -Werror -Wno-unused-parameter -Wno-unused-function。
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#### 1.2.3. -Wextra
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-Wextra：启用一些未由-Wall启用的额外警告标志。 （此选项过去称为-W ，旧名称仍然受支持，但更新的名称更具描述性。）
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```blk
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-Wclobbered
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-Wcast-function-type
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-Wempty-body
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-Wignored-qualifiers
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-Wimplicit-fallthrough=3
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-Wmissing-field-initializers
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-Wmissing-parameter-type (C only)
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-Wold-style-declaration (C only)
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-Woverride-init
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-Wsign-compare (C only)：开启有符号数和无符号数比较警告。
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-Wtype-limits
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-Wuninitialized
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-Wshift-negative-value (in C++03 and in C99 and newer)
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-Wunused-parameter (only with -Wunused or -Wall)：开启未使用的参数警告。
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-Wunused-but-set-parameter (only with -Wunused or -Wall)
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```
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选项 -Wextra 还会打印以下情况的警告消息：
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指针与整数零与< ， <= ， >或>= 。
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（仅限C ++）枚举器和非枚举器都出现在条件表达式中。
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（仅限C ++）不明确的虚拟基础。
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（仅限C ++）为已声明为register的数组下标。
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（仅限C ++）取得已声明register的变量的地址。
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（仅限C ++）基类不在派生类的复制构造函数中初始化。
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### 1.3. -fomit-frame-pointer
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GCC 会在汇编时加入一些堆栈跟踪信息，以便调试系统能够获取堆栈及调用信息如 call trace 等，称为“stack frame pointer(SFP)”，通过 -fomit-frame-pointer 参数可关闭之。
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## 2. 连接参数
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### 2.1. rpath/rpath-link
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* -rpath-link：用于“链接”的时候的，例如你显式指定的需要 FOO.so，但是 FOO.so 本身是需要 BAR.so 的，后者你并没有指定，而是 FOO.so 引用到它，这个时候，会先从 -rpath-link 给的路径里找。
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* -rpath: 用于“运行”的时候。运行的时候，如果要找 FOO.so 文件，会从这个选项里指定的地方去找。对于交叉编译，交叉编译链接器需已经配置 --with-sysroot 选项才能起作用。也就是说，-rpath 指定的路径会被记录在生成的可执行程序中，用于运行时查找需要加载的动态库。
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举例如下：
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如果 prog 依赖 libfoobar.so，而 libfoobar.so libfoo.so 和 libbar.so，按如下方式编译 program 将产生编译错误：
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```bash
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$ gcc -c -Wall main.c
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$ gcc -o prog main.o -L. -lfoobar
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/usr/bin/ld: warning: libfoo.so, needed by ./libfoobar.so, not found (try using -rpath or -rpath-link)
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/usr/bin/ld: warning: libbar.so, needed by ./libfoobar.so, not found (try using -rpath or -rpath-link)
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./libfoobar.so: undefined reference to `bar'
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./libfoobar.so: undefined reference to `foo'
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collect2: error: ld returned 1 exit status
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```
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此时可在编译 prog 时增加 -rpath/-rpath-link 参数：
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```bash
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gcc -o prog main.o -L. -lfoobar -Wl,-rpath=$(pwd)
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```
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即可。此时能够正常编译和运行 prog，因为相关信息已经被写入到 libfoobar.so 文件中：
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```bash
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$ readelf -d prog
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Dynamic section at offset 0xe08 contains 26 entries:
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Tag Type Name/Value
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0x0000000000000001 (NEEDED) Shared library: [libfoobar.so]
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0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
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0x000000000000000f (RPATH) Library rpath: [/home/imk/develop/so/scrap]
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... ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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...
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```
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-rpath-link 只用于链接时查找。运行可执行程序时，-rpath-link 指定的路径不再有效，而 -rpath 指定的路径还有效。
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The linker uses the following search paths to locate required shared libraries:
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1. Any directories specified by -rpath-link options.
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2. Any directories specified by -rpath options. The difference between -rpath and -rpath-link is that directories specified by -rpath options are included in the executable and used at runtime, whereas the -rpath-link option is only effective at link time. Searching -rpath in this way is only supported by native linkers and cross linkers which have been configured with the --with-sysroot option.
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3. On an ELF system, for native linkers, if the -rpath and -rpath-link options were not used, search the contents of the environment variable "LD_RUN_PATH".
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4. On SunOS, if the -rpath option was not used, search any directories specified using -L options.
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5. For a native linker, the search the contents of the environment variable "LD_LIBRARY_PATH".
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6. For a native ELF linker, the directories in "DT_RUNPATH" or "DT_RPATH" of a shared library are searched for shared libraries needed by it. The "DT_RPATH" entries are ignored if "DT_RUNPATH" entries exist.
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7. The default directories, normally /lib and /usr/lib.
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8. For a native linker on an ELF system, if the file /etc/ld.so.conf exists, the list of directories found in that file.
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If the required shared library is not found, the linker will issue a warning and continue with the link.
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在 Makefile 中设置 LD_RUN_PATH/LD_LIBRARY_PATH 的话，需要将环境变量的设置写到具体的编译命令一行中，因为 Makefile 规则中的每一行都在其自己的 shell 实例中运行，如：
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```Makefile
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CC=LD_LIBRARY_PATH=/my/library/path gcc
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foo:
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CC $< -o $@
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```
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## 3. 外部参考资料
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1. [gcc警告选项汇总](https://blog.csdn.net/qq_17308321/article/details/79979514)
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