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How to compile in c language (detailed example)

May 05, 2019 am 10:28 AM
c language compile

gcc命令其实依次执行了四步操作:1.预处理(Preprocessing), 2.编译(Compilation), 3.汇编(Assemble), 4.链接(Linking)。

How to compile in c language (detailed example)

为了下面步骤讲解的方便,我们需要一个稍微复杂一点的例子。假设我们自己定义了一个头文件mymath.h,实现一些自己的数学函数,并把具体实现放在mymath.c当中。然后写一个test.c程序使用这些函数。程序目录结构如下:

├── test.c
└── inc
    ├── mymath.h
    └── mymath.c
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程序代码如下:

// test.c
#include <stdio.h>
#include "mymath.h"// 自定义头文件
int main(){
    int a = 2;
    int b = 3;
    int sum = add(a, b); 
    printf("a=%d, b=%d, a+b=%d\n", a, b, sum);
}
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头文件定义:

// mymath.h
#ifndef MYMATH_H
#define MYMATH_H
int add(int a, int b);
int sum(int a, int b);
#endif
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头文件实现:

// mymath.c
int add(int a, int b){
    return a+b;
}
int sub(int a, int b){
    return a-b;
}
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1.预处理(Preprocessing)

预处理用于将所有的#include头文件以及宏定义替换成其真正的内容,预处理之后得到的仍然是文本文件,但文件体积会大很多。gcc的预处理是预处理器cpp来完成的,你可以通过如下命令对test.c进行预处理:

gcc -E -I./inc test.c -o test.i

或者直接调用cpp命令

$ cpp test.c -I./inc -o test.i

上述命令中-E是让编译器在预处理之后就退出,不进行后续编译过程;-I指定头文件目录,这里指定的是我们自定义的头文件目录;-o指定输出文件名。

经过预处理之后代码体积会大很多:

X 文件名 文件大小 代码行数

预处理前 test.c 146B 9

预处理后 test.i 17691B 857

预处理之后的程序还是文本,可以用文本编辑器打开。

2.编译(Compilation)

这里的编译不是指程序从源文件到二进制程序的全部过程,而是指将经过预处理之后的程序转换成特定汇编代码(assembly code)的过程。编译的指定如下:

$ gcc -S -I./inc test.c -o test.s

上述命令中-S让编译器在编译之后停止,不进行后续过程。编译过程完成后,将生成程序的汇编代码test.s,这也是文本文件,内容如下:

// test.c汇编之后的结果test.s
    .file   "test.c"
    .section    .rodata
.LC0:
    .string "a=%d, b=%d, a+b=%d\n"
    .text
    .globl  main
    .type   main, @function
main:
.LFB0:
    .cfi_startproc
    pushl   %ebp
    .cfi_def_cfa_offset 8
    .cfi_offset 5, -8
    movl    %esp, %ebp
    .cfi_def_cfa_register 5
    andl    $-16, %esp
    subl    $32, %esp
    movl    $2, 20(%esp)
    movl    $3, 24(%esp)
    movl    24(%esp), %eax
    movl    %eax, 4(%esp)
    movl    20(%esp), %eax
    movl    %eax, (%esp)
    call    add 
    movl    %eax, 28(%esp)
    movl    28(%esp), %eax
    movl    %eax, 12(%esp)
    movl    24(%esp), %eax
    movl    %eax, 8(%esp)
    movl    20(%esp), %eax
    movl    %eax, 4(%esp)
    movl    $.LC0, (%esp)
    call    printf
    leave
    .cfi_restore 5
    .cfi_def_cfa 4, 4
    ret 
    .cfi_endproc
.LFE0:
    .size   main, .-main
    .ident  "GCC: (Ubuntu 4.8.2-19ubuntu1) 4.8.2"
    .section    .note.GNU-stack,"",@progbits
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3.汇编(Assemble)

汇编过程将上一步的汇编代码转换成机器码(machine code),这一步产生的文件叫做目标文件,是二进制格式。gcc汇编过程通过as命令完成:

$ as test.s -o test.o

等价于:

gcc -c test.s -o test.o

这一步会为每一个源文件产生一个目标文件。因此mymath.c也需要产生一个mymath.o文件

4.链接(Linking)

链接过程将多个目标文以及所需的库文件(.so等)链接成最终的可执行文件(executable file)。

命令大致如下:

$ ld -o test.out test.o inc/mymath.o ...libraries...

结语

经过以上分析,我们发现编译过程并不像想象的那么简单,而是要经过预处理、编译、汇编、链接。尽管我们平时使用gcc命令的时候没有关心中间结果,但每次程序的编译都少不了这几个步骤。也不用为上述繁琐过程而烦恼,因为你仍然可以:

$ gcc hello.c # 编译

$ ./a.out # 执行

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