命令行参数
-c
Preprocess, compile, and assemble only (i.e., don't link).
预处理（生成.i，用完删除）、编译、（生成.s，用完删除），汇编（生成.o），不连接（不生成可执行文件）。Btw，貌似很多地方可以用-pipe直接通过管道，不生成临时文件，加快编译
-C
Leave comments in when preprocessing.
预处理时不去除注释（结合其他会留下预处理输出的参数使用，如-E）
-D name[= definition]
Defines the symbol name.
定义一个标识符（相当于程序里面写#define name[= definition]，可以结合#ifndef name实现控制功能）
-e name
Start program execution at name.
（对一个Freestanding的程序，比如自己写的OS）自定义入口函数（常和-ffreestanding -nostartfiles –nostdlib等一起使用）
-E
Preprocess only; output to stdout, unless used with -o.
只预处理，结果输出到标准输出，除非用-o指定输出文件（一般为*.i）
-ffast-math
Permit faster floating-point arithmetic methods at the cost of accuracy or precision.
浮点数运算的时候以精度换速度（第一个f是flag的意思。C99使用比以前更好数值精确度，当然我们也可以不用。。这个参数定义_ _FAST_MATH_ _这个宏，相当于后面要说的6个浮点相关运算参数的并集，which对我来说长得相当古怪！）
-ffinite-math-only
Disregard infinities and NaN ("not a number") values.浮点相关运算参数。我没看懂“Disregard”到底是指丢弃，还是说不检查就接收。。
-ffreestanding
Compile as a freestanding (not hosted) program.
编译一个独立的程序（比如不会去连接crt0.o，“which contains the actual entry point of the executable program”，crt = C Runtime）
-finline-functions, -fno-inline-functions
Enable/disable inline functions.
允许/不允许内联函数
-fno-math-errno
Disable the errno variable for simple math functions.
浮点相关运算参数。浮点数学函数中出错了不使用errno这个全局变量？
-fno-trapping-math
Generate "nonstop" floating-point code.
浮 点相关运算参数。“Generates "nonstop" code, on the assumption that no math exceptions will be raised that can be handled by the user program.”假设没有数学操作产生的异常是用户能处理的，非常拗口。。用国语说就是浮点运算就不产生该有的异常了，反正用户也处理不了
-frounding-math
Don't disregard the rounding-mode features of the floating-point environment (experimental).
浮点相关运算参数-fno-rounding-math的opposite。
-fsignaling-nans
Allow all exceptions raised by signaling NaNs (experimental).
浮点相关运算参数-fno-signaling-nans的opposite。
-fsyntax-only
Don't compile or link; just test input for syntax.
只测试输入语法有效性
-funroll-loops, -fno-unroll-loops
Enable/disable loop optimization.
对循环做优化（比如把一些小循环的跳转改成线性的代码，貌似会使文件增大？）
-funsafe-math-optimizations
Permit optimizations that don't conform to standards and/or don't verify values.
浮点相关运算参数。看名字吧。。。
-fverbose-asm
Include C variable names as comments in assembly language.
在生成的的汇编语言里面将C变量名加到注释里面（结合会留下汇编输出的参数使用，如-S）
-g[ format]
Compile for debugging.
生成包含特定format调试信息的文件（比如-ggdb，结合GDB来调试。当然debug的会比release的大很多）
-I directory[: directory[...]]
Search for "include" files in the specified path.
指定#include时的查找路径，多个路径用冒号隔开
Quote：“The usual search order for include directories is:
1. The directory containing the given source file (for filenames in given in quotation marks in an #include directive).
2. Directories specified by -I options, in command-line order.
3. Directories specified in the environment variables C_INCLUDE_PATH and CPATH.
4. The system's default include directories.”
-I-
Distinguish between -Ipath for #include
#include "myheader.h"
The second group consists of any directories named in an -I option to the right of -I-. These directories are searched for header files named in any #include directive.
Furthermore, if -I- appears on the command line, then the directory containing the source file is no longer automatically searched first for header files.”
-lbasename
Link with library libbasename.so or libbasename.a.
连接以libbasename为名的shared object或者archive
-L directory[: directory[...]]
Search for library files in the specified path.
指定连接时的查找路径，多个路径用冒号隔开
-march= cpu
Intel x86: Generate model-specific code.
为某个平台优化，第一个m意思是machine或者model。对于x86体系来说，-mcpu和-mtune是一样，-march可以为不同cpu产生优化的代码，比如-march=athlon-4（具体的cpu abbr.看gcc manual）
-mcpu= cpu
Sparc, ARM, and RS/6000-PowerPC: Generate model-specific code.
Intel x86: Optimize scheduling for the specified CPU model.
为某个平台优化，对于x86和非x86体系来说，-m系列参数意义有点不一样，见前面-march
Quote：“For several processor types, such as the Sparc, ARM, and RS/6000-PowerPC series, the option -mcpu=cpu generates machine code for the specific CPU type's register set, instruction set, and scheduling behavior. Programs compiled with this option may not run at all on a different model in the same CPU family.”
-mtune= cpu
Optimize scheduling for the specified CPU model.
为某个平台优化，这个比-mcpu要“温和”一点。
Quote：“The option -mtune=cpu is more tolerant. Code generated with -mtune=cpu uses optimized scheduling parameters for the given CPU model, but adheres to the family's common instructions and registers, so that it should still run on a related model.”
-nostartfiles
Don't link startup code.
Quote：“On most systems, GCC also links programs by default with initialization routines in object files named crtbegin.o and crtend.o.”
-nostdlib
Don't link with the standard library.
不连接C Stanndard Library
-o file
Direct output to the specified file.
指定输出文件名。
-O0
Turn off all optimization options.
不优化。
-O, -O1
Perform some optimization without taking much time.
优化
-O2
Perform more optimization, including data flow analysis.
再优化
-O3
Perform still more optimization, including inline function compilation.
优化到底
-Os
Optimize for size.
只在文件大小方面做优化
-p
Link in code to output profiling information.
“The -p option adds special functions to your program to output profiling information when you run it. The profiling output is saved in a file called mon.out.”
-pedantic
Output warnings on nonstandard usage.
为非标准用法产生Warning（比如ANSI C不包括但是GUN C扩展的地方，见-std）
-pedantic-errors
Fail on nonstandard usage.
这个比较狠，编写portable代码应该用上
-pg
Link in code to output profiling information for gprof.
为GUN Profiler，gprof输出profiling information，这次输出文件叫gmon.out
-s
Strip symbol tables from executable file.
优化参数。在执行文件中去除符号表
Quote：“This makes the finished program file significantly smaller, and is often used in building a production version.”
-S
Preprocess and translate into assembly language only
预处理，编译，到生成*.s为止，不汇编。
-save-temps
Save intermediate output files.
留下所有编译过程中产生的文件：.i、.s、.o
-shared
Create a shared object file for dynamic linking.
生成.so文件
-static
Don't link to shared object files.
只静态连接，就是说生成的文件不会去连接.so文件
-std=iso9899:1990
-std=c89
-ansi
Support ISO/IEC 9899:1990.
都是一样的。支持ANSI C标准，与标准冲突的扩展被disable（比如GUN的tyepeof操作符，好东西啊。。）
-std=iso9899:199409
Support ISO/IEC 9899:1989 and AMD1.
另一个标准
-std=c99
Support ISO/IEC 9899:1999.
-std=gnu89
Like -ansi, plus GNU extensions (default).
默认-std选项
-std=gnu99
Like -std=c99, plus GNU extensions.
-traditional
Support old-style C. Deprecated; supported only with -E.
注意：“supported only with –E”，只在预处理的时候有用
-trigraphs
Support ISO C trigraphs.
Trigraph | Equivalent |
| ??( | [ |
| ??) | ] |
| ??< | { |
| ??> | } |
| ??= | # |
| ??/ | \ |
| ??! | | |
| ??' | ^ |
| ??- | ~ |
-U name
"Undefine" the symbol name.
和-D对应的
-v
Be verbose: print the options applied at each step of compiling.
--version
Output GCC version and license information.
-w
Disable all warnings.
-Wa, option[, option[...]]
Pass options to assembler command line.
-Wall
Output warnings about a broad range of problems in source code.
-Wl, option[, option[...]]
Pass options to linker command line.
给连接器ld传命令行参数
-Werror
Fail on all warnings.
这个比-pedantic-errors更狠
-Wextra
Output warnings about legal but questionable usage.
这个貌似是最verbose的warning了
-Wtraditional
Warn about differences to old-style C.
-fmerge-constants
Put identical constants in a single location.
优化参数，再补一句：“even across different source files”
-x filetype
Treat subsequent files as being of the specified type.
这 里filetype的选项有“c, c-header, cpp-output, assembler (meaning that the file contains assembly language), assembler-with-cpp, or none.”所有出现在-x之前的文件被当作-x指定的filetype，常用在没用标准扩展名的时候
假设当前目录下有这些源文件：[main.c func.c func.h]，其中main.c要调用func.c中的函数。
【1】生成静态库：
$ gcc -c func.c -o func.o
$ ar r libfunc.a func.o
$ gcc main.c -o main -static -L. -lfunc
$ ./main
【2】生成动态库：
$ gcc -fpic -c func.c -o func.o
$ gcc -shared -o libfunc.so.1.0.0 func.o
$ ln -s libfunc.so.1.0.0 libfunc.so
$ gcc main.c -o main -L. -lfunc
$ export LD_LIBRARY_PATH=$(pwd)
$ ./main
如果将so文件copy到系统lib目录(如/usr/lib)，则最后2步就不用了。
3个小知识：
【1】nm命令：列出目标文件或2进制文件的所有符号。
【2】ldd命令：列出为了使程序正常运行所需要的所有共享库。
【3】/etc/ld.so.conf文件：除了标准目录（/lib和/usr/lib）之外，链接器和加载器搜索共享库时要检查的其他目录，和这个文件相关的一个命令是：ldconfig 。
【4】Gcc选项 -aux-info可以导出被编译源文件所包含的所有函数（或方法）的原型。如：gcc component.c -aux-info prototype.h
Environment Variables
环境变量
CPATH, C_INCLUDE_PATH
Colon-separated list of directories to search for header files, after those indicated by -Idirectory on the command line.
和-I一起，#include的时候用，冒号分隔
COMPILER_PATH
Colon-separated list of directories to search for GCC's own subprogram files.
GCC_EXEC_PREFIX
A prefix for GCC to add to the names of its subprograms when invoking them. May end with a slash.
LIBRARY_PATH
Colon-separated list of directories to search for linker and library files, after directories specified by -Ldirectory on the command line.
和-L一起，连接library的时候用
LD_LIBRARY_PATH
Colon-separated list of directories to search for shared library files. Read not by GCC, but by executables dynamically linked against shared libraries.
找.so的路径，比如/usr/lib/、%SystemRoot%\system32\之类的
TMPDIR
Directory to use for temporary files.
临时文件路径，比如/tmp
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