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結合"hello world"探討gcc編譯程序的過程
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1. gcc簡介
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gcc/g++是GNU工程的C和C++編譯器。都要用四個步驟中的一個或多個處理輸入文件。
- 預處理(preprocessing)
- 編譯(compilation)
- 匯編(assembly)
- 連接(linking)
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源文件后綴名標識源文件的語言，但是對編譯器來說，后綴名控制著缺省設定。
gcc命令的一般格式為：gcc [選項]?要編譯的文件?[選項] [目標文件]
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gcc的詳細內容，可參考gcc manual。
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2. gcc的輸出選項
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gcc輸出選項，-o后指定輸出文件名，gcc解釋如下。
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| `-o FILE' ?????Place output in file FILE.??This applies regardless to whatever sort of output is being produced, whether it be an executable file, an object file, an assembler file or preprocessed C code. ?????If `-o' is not specified, the default is to put an executable file in `a.out', the object file for `SOURCE.SUFFIX' in `SOURCE.o', its assembler file in `SOURCE.s', a precompiled header file in `SOURCE.SUFFIX.gch', and all preprocessed C source on standard output. ? |
| ????指定輸出文件為FILE，該選項不考慮GCC產生什么輸出，無論是可執行文件、目標文件、匯編文件還是預處理后的C代碼。 ????如果沒有使用`-o'選項，默認的輸出結果是:可執行文件為a.out，source.suffix的目標文件是source.o，匯編文件是source.s，而預處理后的C代碼送往標準輸出。 |
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3. "hello wolrd"的處理過程
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本文以"hello world"程序，探討GCC編譯程序的過程。
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hello.c文件內容如下。
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| #include <stdio.h> ? int main(int argc, char **argv) { ????printf("Hello World! via %x/n", printf); ????return 0; } |
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3.1?預處理(preprocessing)
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在該階段，編譯器將C源代碼中的包含的頭文件如stdio.h編譯進來，可以使用gcc的選項”-E”進行查看預處理結果。
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Gcc manual中對-E選項的解釋如下。
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| `-E' ?????Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of preprocessed source code, which is sent to the standard output. ?????Input files which don't require preprocessing are ignored. |
| ????預處理后停止，不進行編譯。預處理后的代碼送往標準輸出。GCC忽略任何不需要預處理的輸入文件。 |
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格式：gcc -E hello.c -o hello.i
作用：對hello.c進行預處理輸出為hello.i文件
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Hello.c被預處理后的hello.i文件。
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| # 1 "hello.c" # 1 "<built-in>" # 1 "<command line>" # 1 "hello.c" # 1 "/usr/include/stdio.h" 1 3 4 # 28 "/usr/include/stdio.h" 3 4 # 1 "/usr/include/features.h" 1 3 4 # 335 "/usr/include/features.h" 3 4 # 1 "/usr/include/sys/cdefs.h" 1 3 4 # 360 "/usr/include/sys/cdefs.h" 3 4 # 1 "/usr/include/bits/wordsize.h" 1 3 4 # 361 "/usr/include/sys/cdefs.h" 2 3 4 # 336 "/usr/include/features.h" 2 3 4 # 359 "/usr/include/features.h" 3 4 # 1 "/usr/include/gnu/stubs.h" 1 3 4 ? ? # 1 "/usr/include/bits/wordsize.h" 1 3 4 # 5 "/usr/include/gnu/stubs.h" 2 3 4 ? ? # 1 "/usr/include/gnu/stubs-32.h" 1 3 4 # 8 "/usr/include/gnu/stubs.h" 2 3 4 # 360 "/usr/include/features.h" 2 3 4 # 29 "/usr/include/stdio.h" 2 3 4 ? ? # 1 "/usr/lib/gcc/i386-redhat-linux/4.1.2/include/stddef.h" 1 3 4 # 214 "/usr/lib/gcc/i386-redhat-linux/4.1.2/include/stddef.h" 3 4 typedef unsigned int size_t; # 35 "/usr/include/stdio.h" 2 3 4 ? # 1 "/usr/include/bits/types.h" 1 3 4 # 28 "/usr/include/bits/types.h" 3 4 # 1 "/usr/include/bits/wordsize.h" 1 3 4 # 29 "/usr/include/bits/types.h" 2 3 4 ? ? typedef unsigned char __u_char; typedef unsigned short int __u_short; typedef unsigned int __u_int; typedef unsigned long int __u_long; ? ? typedef signed char __int8_t; typedef unsigned char __uint8_t; typedef signed short int __int16_t; typedef unsigned short int __uint16_t; typedef signed int __int32_t; typedef unsigned int __uint32_t; ? ? __extension__ typedef signed long long int __int64_t; __extension__ typedef unsigned long long int __uint64_t; ? ? __extension__ typedef long long int __quad_t; __extension__ typedef unsigned long long int __u_quad_t; # 131 "/usr/include/bits/types.h" 3 4 # 1 "/usr/include/bits/typesizes.h" 1 3 4 # 132 "/usr/include/bits/types.h" 2 3 4 ? ... |
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3.2?編譯(compilation)
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在這個階段中，Gcc首先要檢查代碼的規范性、是否有語法錯誤等，以確定代碼的實際要做的工作，在檢查無誤后，Gcc把代碼翻譯成匯編語言。可以使用”-S”選項來進行查看，該選項只進行編譯而不進行匯編，生成匯編代碼。
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Gcc manual中對-S選項的解釋如下。
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| `-S' ?????Stop after the stage of?compilation?proper; do not assemble.??The output is in the form of?an assembler code file?for each non-assembler input file specified. ?????By default, the assembler file name for a source file is made by replacing the suffix `.c', `.i', etc., with `.s'. Input files that don't require compilation are ignored. |
| ?????編譯后即停止，不進行匯編。對于每個輸入的非匯編語言文件，輸出文件是匯編語言文件。 ?????缺省情況下，GCC通過用`.o'替換源文件名后綴`.c',`.i'等以產生目標文件名。可以使用-o選項指定選擇其他名字。GCC忽略任何不需要編譯的輸入文件。 |
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格式：gcc –S hello.i –o hello.s
作用：將預處理輸出文件hello.i匯編成hello.s文件
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Hello.i被匯編后的hello.s文件。
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| ??.file????????"hello.c" ??.section????????.rodata .LC0: ??.string????????"Hello World! via %x/n" ??.text .globl main ??.type????????main, @function main: ??leal????????4(%esp), %ecx ??andl????????$-16, %esp ??pushl????????-4(%ecx) ??pushl????????%ebp ??movl????????%esp, %ebp ??pushl????????%ecx ??subl????????$20, %esp ??movl????????$printf, 4(%esp) ??movl????????$.LC0, (%esp) ??call????????printf ??movl????????$0, %eax ??addl????????$20, %esp ??popl????????%ecx ??popl????????%ebp ??leal????????-4(%ecx), %esp ??ret ??.size????????main, .-main ??.ident????????"GCC: (GNU) 4.1.2 20070925 (Red Hat 4.1.2-33)" ??.section????????.note.GNU-stack,"",@progbits |
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3.3?匯編(assembly)
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匯編階段是把編譯階段生成的.s文件轉成二進制目標代碼。
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gcc manual中對選項-C的解釋如下。
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| `-c' ?????Compile or assemble the source files, but do not link. The linking stage simply is not done.??The ultimate output is in the form of an object file for each source file. ?????By default, the object file name for a source file is made by replacing the suffix `.c', `.i', `.s', etc., with `.o'. Unrecognized input files, not requiring compilation or assembly, are ignored. |
| ?????編譯或匯編源文件，但不連接，編譯器輸出對應于源文件的目標文件。 ?????缺省情況下，GCC通過用'.o'替換源文件名后綴`.c', `.i', `.s'等等以產生目標文件名。可以使用-o選項指定選擇其他名字。GCC忽略-c選項后面任何無法識別的不需要編譯或匯編的輸入文件。 |
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gcc manual中對選項-V的解釋如下。
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| `-v' ?????Print (on standard error output) the commands executed to run the stages of compilation.??Also print the version number of the compiler driver program and of the preprocessor and the compiler proper. |
| ????(在標準錯誤輸出)顯示執行編譯階段的命令，同時顯示編譯器驅動程序、預處理器、編譯器的版本號。 |
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格式：gcc?–c hello.s?–o hello.o
作用：將編譯輸出的.s文件匯編成二進制目標代碼.o文件
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我們可以通過objdump程序來查看hello.o的內容。
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| # objdump -d hello.o ? hello.o:?????file format elf32-i386 ? Disassembly of section .text: ? 00000000 <main>: ???0:???8d 4c 24 04?????????????lea????0x4(%esp),%ecx ???4:???83 e4 f0????????????????and????$0xfffffff0,%esp ???7:???ff 71 fc????????????????pushl??-0x4(%ecx) ???a:???55??????????????????????push???%ebp ???b:???89 e5???????????????????mov????%esp,%ebp ???d:???51??????????????????????push???%ecx ???e:???83 ec 14????????????????sub????$0x14,%esp ??11:???c7 44 24 04 00 00 00????movl???$0x0,0x4(%esp) ??18:???00 ??19:???c7 04 24 00 00 00 00????movl???$0x0,(%esp) ??20:???e8 fc ff ff ff??????????call???21 <main+0x21> ??25:???b8 00 00 00 00??????????mov????$0x0,%eax ??2a:???83 c4 14????????????????add????$0x14,%esp ??2d:???59??????????????????????pop????%ecx ??2e:???5d??????????????????????pop????%ebp ??2f:???8d 61 fc????????????????lea????-0x4(%ecx),%esp ??32:???c3??????????????????????ret??? |
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3.4?鏈接(linking)
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編譯成功后，就進入鏈接。
格式：gcc hello.o -o hello.exe
??或者gcc hello.o -o hello
作用：將編譯輸出的.o文件鏈接為最終可執行文件。
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同樣，我們也可以通過objdump程序來查看鏈接后的輸出hello文件的內容。
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| ? hello:?????file format elf32-i386 ? Disassembly of section .init: ? 08048298 <_init>: ?8048298:????????55???????????????????????????push???%ebp ?8048299:????????89 e5????????????????????????mov????%esp,%ebp ?804829b:????????83 ec 08?????????????????????sub????$0x8,%esp ?804829e:????????e8 71 00 00 00???????????????call???8048314 <call_gmon_start> ?80482a3:????????e8 f8 00 00 00???????????????call???80483a0 <frame_dummy> ?80482a8:????????e8 d3 01 00 00???????????????call???8048480 <__do_global_ctors_aux> ?80482ad:????????c9???????????????????????????leave? ?80482ae:????????c3???????????????????????????ret??? Disassembly of section .plt: ? 080482b0 <__gmon_start__@plt-0x10>: ?80482b0:????????ff 35 40 96 04 08????????????pushl??0x8049640 ?80482b6:????????ff 25 44 96 04 08????????????jmp????*0x8049644 ?80482bc:????????00 00????????????????????????add????%al,(%eax) ... ? 080482c0 <__gmon_start__@plt>: ?80482c0:????????ff 25 48 96 04 08????????????jmp????*0x8049648 ?80482c6:????????68 00 00 00 00???????????????push???$0x0 ?80482cb:????????e9 e0 ff ff ff???????????????jmp????80482b0 <_init+0x18> ? 080482d0 <__libc_start_main@plt>: ?80482d0:????????ff 25 4c 96 04 08????????????jmp????*0x804964c ?80482d6:????????68 08 00 00 00???????????????push???$0x8 ?80482db:????????e9 d0 ff ff ff???????????????jmp????80482b0 <_init+0x18> ? 080482e0 <printf@plt>: ?80482e0:????????ff 25 50 96 04 08????????????jmp????*0x8049650 ?80482e6:????????68 10 00 00 00???????????????push???$0x10 ?80482eb:????????e9 c0 ff ff ff???????????????jmp????80482b0 <_init+0x18> ? ... ? 080483c4 <main>: ?80483c4:????????8d 4c 24 04??????????????????lea????0x4(%esp),%ecx ?80483c8:????????83 e4 f0?????????????????????and????$0xfffffff0,%esp ?80483cb:????????ff 71 fc?????????????????????pushl??-0x4(%ecx) ?80483ce:????????55???????????????????????????push???%ebp ?80483cf:????????89 e5????????????????????????mov????%esp,%ebp ?80483d1:????????51???????????????????????????push???%ecx ?80483d2:????????83 ec 14?????????????????????sub????$0x14,%esp ?80483d5:????????c7 44 24 04 e0 82 04 ????????movl???$0x80482e0,0x4(%esp) ?80483dc:????????08 ?80483dd:????????c7 04 24 d0 84 04 08 ????????movl???$0x80484d0,(%esp) ?80483e4:????????e8 f7 fe ff ff???????????????call???80482e0?<printf@plt> ?80483e9:????????b8 00 00 00 00???????????????mov????$0x0,%eax ?80483ee:????????83 c4 14?????????????????????add????$0x14,%esp ?80483f1:????????59???????????????????????????pop????%ecx ?80483f2:????????5d???????????????????????????pop????%ebp ?80483f3:????????8d 61 fc?????????????????????lea????-0x4(%ecx),%esp ?80483f6:????????c3???????????????????????????ret??? ?80483f7:????????90???????????????????????????nop??? ? ... |
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4. 小結
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編譯程序的4個過程：
預處理：gcc -E hello.c -o hello.i
編譯：gcc –S hello.i –o hello.s
匯編：gcc –c hello.s –o hello.o
鏈接：gcc hello.o -o hello
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轉自:http://blog.csdn.net/livelylittlefish/article/details/5109300
