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头文件包含，写法#include<文件名>,
这个就是C语言程序的入口，所有的C程序都是从main开始执行，一个C的源程序必须有一个main函数，也只能有一个main函数
//注释一行
/* */代表块注释，可以注释多行代码
代表一个代码单元
C语言规定，所有的变量和函数必须先声明，然后才能使用.
可以使用大小写字母，下划线，数字，但第一个字母必须是字母或者下划线
字母区分大小写
变量名最好用英文，而且要有所含义，通过变量的名称就能猜测变量的意思。
在C语言当中任何函数遇到return代表这个函数停止，当main函数遇到return，代表整个程序退出
return代表函数的返回值，如果返回类型是void，可以直接写return,而不需要返回任何值
-o代表指定输出文件名
-E代表预编译
预编译处理include的本质就是简单的将include中的文件替换到c文件中
如果include包含的头文件在系统目录下，那么就用#include <>,如果包含的文件在当前目录下，那么用#inlclude“”
-S代表汇编
-c代表编译
Linux C 学习
1，编写一个helloworld 程序
vim hello.c
#include "stdio.h"
int main(){
printf("Hello World!\n\n");
return 0;
}
一步到位编译执行
chunli@pc0003:/tmp/C$ gcc helloworld.C chunli@pc0003:/tmp/C$ ./a.out Hello World!
C编译过程
chunli@pc0003:/tmp/C$ gcc --help
Usage: gcc [options] file...
Options:
-pass-exit-codes Exit withhighest error code from a phase
--help Display this information
--target-help Display target specific commandline options
--help={common|optimizers|params|target|warnings|[^]{joined|separate|undocumented}}[,...]
Display specific types of command line options
(Use '-v --help' todisplay command line options of sub-processes)
--version Display compiler version information
-dumpspecs Display all of the built in specstrings
-dumpversion Display the version of thecompiler
-dumpmachine Display the compiler's targetprocessor
-print-search-dirs Display the directories in thecompiler's search path
-print-libgcc-file-name Displaythe name of the compiler's companion library
-print-file-name=<lib> Display the full path to library <lib>
-print-prog-name=<prog> Display the full path to compiler component <prog>
-print-multiarch Displaythe target's normalized GNU triplet, used as
a component in the library path
-print-multi-directory Displaythe root directory for versions of libgcc
-print-multi-lib Display the mapping between commandline options and
multiple library search directories
-print-multi-os-directory Display the relative path to OS libraries
-print-sysroot Display the target librariesdirectory
-print-sysroot-headers-suffix Display the sysroot suffix used to findheaders
-Wa,<options> Pass comma-separated <options> on to the assembler
-Wp,<options> Pass comma-separated <options> on to the preprocessor
-Wl,<options> Pass comma-separated <options> on to the linker
-Xassembler<arg> Pass <arg> on tothe assembler
-Xpreprocessor<arg> Pass <arg> on tothe preprocessor
-Xlinker<arg> Pass <arg> onto the linker
-save-temps Do not delete intermediate files
-save-temps=<arg> Donot delete intermediate files
-no-canonical-prefixes Do notcanonicalize paths when building relative
prefixes to other gcc components
-pipe Use pipes rather thanintermediate files
-time Time the execution of eachsubprocess
-specs=<file> Override built-in specs with the contents of <file>
-std=<standard> Assume that the input sources are for <standard>
--sysroot=<directory> Use<directory> as the root directory for headers
and libraries
-B<directory> Add<directory> to the compiler's search paths
-v Display the programsinvoked by the compiler
-### Like -v but options quotedand commands not executed
-E Preprocess only; do notcompile, assemble or link
-S Compile only; do notassemble or link
-c Compile and assemble,but do not link
-o <file> Place the output into<file>
-pie Create a positionindependent executable
-shared Create a shared library
-x <language> Specify the language of thefollowing input files
Permissible languages include: c c++ assembler none
'none' means revert to the default behavior of
guessing the language based on the file's extension
Options starting with -g, -f, -m, -O, -W, or --param areautomatically
passed on to thevarious sub-processes invoked by gcc. Inorder to pass
other options on tothese processes the -W<letter> options must be used.
For bug reporting instructions, please see:
<file:///usr/share/doc/gcc-4.8/README.Bugs>.
源C代码程序 hello.c
第一步：预编译，把include文件的内容原封不动的放到源代码中 gcc -o hello.i hello.c
第二步：汇编，把预编译的结果变成汇编代码
第三步：编译，把汇编的结果变成二进制文件
第四步: 链接，把编译的二进制文件与系统库连接起来
chunli@pc0003:/tmp/C$ gcc -o hello.i -E hello.c chunli@pc0003:/tmp/C$ gcc -o hello.s -S hello.c chunli@pc0003:/tmp/C$ gcc -o hello.o -c hello.s chunli@pc0003:/tmp/C$ gcc -o hello hello.o chunli@pc0003:/tmp/C$ ./hello Hello World!
查看链接的库
chunli@pc0003:/tmp/C$ ldd hello linux-vdso.so.1=> (0x00007fff217f8000) libc.so.6 =>/lib/x86_64-linux-gnu/libc.so.6 (0x00007f5340914000) /lib64/ld-linux-x86-64.so.2(0x00005654d9706000)
调用系统的程序
vim hello.c
#include "stdio.h"
#include "stdlib.h"
int main(){
system("cat hello.c");
printf("Hello World!\n\n");
return 0;
}
编译 gcc hello.c
执行 ./a.out
输出：
#include "stdio.h"
#include "stdlib.h"
int main(){
system("cat hello.c");
printf("Hello World!\n\n");
return 0;
}
Hello World!
向屏幕输出的其他方式：
chunli@pc0003:/tmp/C$ cat my_printf.c
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main()
{
int i = 99;
printf("i=%d \n",i/0x10);
fwrite("abc\n",1,2,stdout);
//write("abc\n",4,STDOUT_FILENO,"abc");
return ;
}
C 语言版计算器
#include <stdio.h>
#include <stdlib.h>
int main(int argc,char *args[])
{
if(argc <3 )
printf("请输入两个整数！\n");
else
{
int a = atoi(args[1]);
int b = atoi(args[2]);
int c = a+b;
printf("两个数的和是 %d \n",c);
}
return 0;
}
使用方法
chunli@pc0003:/tmp/C$ !gcc gcc calc.c chunli@pc0003:/tmp/C$ ./a.out 3 3 两个数的和是 6 chunli@pc0003:/t
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