man open 

man 2 write

1、Standard commands （标准命令）
2、System calls （系统调用）
3、Library functions （库函数）
4、Special devices （设备说明）
5、File formats （文件格式）
6、Games and toys （游戏和娱乐）
7、Miscellaneous （杂项）
8、Administrative Commands （管理员命令）
9 其他（Linux特定的）， 用来存放内核例行程序的文档。

gcc  -o 可执行文件  源文件.c(或依赖文件）

gcc -c 源文件.c

gcc -g -o 可执行文件 依赖文件

gcc -O -o 目标文件 依赖文件

       The compiler performs optimization based on the knowledge it has of the program.  Compiling multiple files at once to a single output file modeallows the compiler to use information gained from all of the files when compiling each of them.Not all optimizations are controlled directly by a flag.  Only optimizations that have a flag are listed.-O-O1 Optimize.  Optimizing compilation takes somewhat more time, and a lot more memory for a large function.With -O, the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilationtime.-O turns on the following optimization flags:-fauto-inc-dec -fcprop-registers -fdce -fdefer-pop -fdelayed-branch -fdse -fguess-branch-probability -fif-conversion2 -fif-conversion-finline-small-functions -fipa-pure-const -fipa-reference -fmerge-constants -fsplit-wide-types -ftree-builtin-call-dce -ftree-ccp -ftree-ch-ftree-copyrename -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-fre -ftree-sra -ftree-ter -funit-at-a-time-O also turns on -fomit-frame-pointer on machines where doing so does not interfere with debugging.-O2 Optimize even more.  GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff.  As compared to -O, thisoption increases both compilation time and the performance of the generated code.-O2 turns on all optimization flags specified by -O.  It also turns on the following optimization flags: -fthread-jumps -falign-functions-falign-jumps -falign-loops  -falign-labels -fcaller-saves -fcrossjumping -fcse-follow-jumps  -fcse-skip-blocks -fdelete-null-pointer-checks-fexpensive-optimizations -fgcse  -fgcse-lm -findirect-inlining -foptimize-sibling-calls -fpeephole2 -fregmove -freorder-blocks-freorder-functions -frerun-cse-after-loop -fsched-interblock  -fsched-spec -fschedule-insns  -fschedule-insns2 -fstrict-aliasing-fstrict-overflow -ftree-switch-conversion -ftree-pre -ftree-vrpPlease note the warning under -fgcse about invoking -O2 on programs that use computed gotos.-O3 Optimize yet more.  -O3 turns on all optimizations specified by -O2 and also turns on the -finline-functions, -funswitch-loops,-fpredictive-commoning, -fgcse-after-reload, -ftree-vectorize and -fipa-cp-clone options.-O0 Reduce compilation time and make debugging produce the expected results.  This is the default.-Os Optimize for size.  -Os enables all -O2 optimizations that do not typically increase code size.  It also performs further optimizationsdesigned to reduce code size.-Os disables the following optimization flags: -falign-functions  -falign-jumps  -falign-loops -falign-labels  -freorder-blocks-freorder-blocks-and-partition -fprefetch-loop-arrays  -ftree-vect-loop-versionIf you use multiple -O options, with or without level numbers, the last such option is the one that is effective.

目标文件：依赖文件规则/指令