8.基础IO(三) - 实践
一.上集回顾
建议先学上篇博客,再向下学习,上篇博客的链接如下:
二.myshell增加重定向功能
1.回顾
上篇博客,我们完成了关于重定向内容的定义和指令的分析
2.执行重定向指令
a.重定向应该交给子进程进行操作
b.程序替换会不会影响我们的重定向?
答案是 不会影响
因为程序替换只会对我们的代码和数据进行操作,不会影响我们的文件描述符表
bool ExecuteCommand() //4.执行命令
{
//子进程执行命令
pid_t id = fork();
if(id < 0)
{
return false;
}
else if(id == 0)
{
//子进程
if(redir == InputRedir)
{
if(filename)
{
int fd = open(filename,O_RDONLY);
if(fd < 0)
{
exit(2);
}
dup2(fd,0);
}
else
{
exit(1);
}
}
else if(redir == OutputRedir)
{
if(filename)
{
int fd = open(filename,O_CREAT | O_WRONLY | O_TRUNC,0666);
if(fd < 0)
{
exit(4);
}
dup2(fd,1);
}
else
{
exit(3);
}
}
else if(redir == AppRedir)
{
if(filename)
{
int fd = open(filename,O_CREAT | O_WRONLY | O_APPEND,0666);
if(fd < 0)
{
exit(6);
}
dup2(fd,1);
}
else
{
exit(5);
}
}
else
{
//没有重定向,Do Nothing!
}
//1.执行命令
execvpe(gargv[0],gargv,genv);
//2.退出
exit(1);
}
int status = 0;
pid_t rid = waitpid(id,&status,0);
if(rid > 0)
{
if(WIFEXITED(status))
{
lastcode = WEXITSTATUS(status);
}
else
{
lastcode = 100;
}
return true;
}
return false;
}
3.修改代码
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
using namespace std;
const int basesize = 1024;
const int argvnum = 64;
const int envnum = 64;
// 全局的命令行参数表
char *gargv[argvnum];
int gargc = 0;
// 全局的变量
int lastcode = 0;
// 我的系统的环境变量
char *genv[envnum];
// 全局的当前shell工作路径
char pwd[basesize];
char pwdenv[basesize];
// 全局变量与重定向有关
#define NoneRedir 0
#define InputRedir 1
#define OutputRedir 2
#define AppRedir 3
int redir = NoneRedir;
char *filename = nullptr;
// " "file.txt
#define TrimSpace(pos) do{\
while(isspace(*pos)){\
pos++;\
}\
}while(0)
string GetUserName()
{
string name = getenv("USER");
return name.empty() ? "None" : name;
}
string GetHostName()
{
string hostname = getenv("HOSTNAME");
return hostname.empty() ? "None" : hostname;
}
string GetPwd()
{
if(nullptr == getcwd(pwd, sizeof(pwd))) return "None";
snprintf(pwdenv, sizeof(pwdenv),"PWD=%s", pwd);
putenv(pwdenv);
return pwd;
}
string LastDir()
{
string curr = GetPwd();
if(curr == "/" || curr == "None") return curr;
size_t pos = curr.rfind("/");
if(pos == std::string::npos) return curr;
return curr.substr(pos+1);
}
string MakeCommandLine()
{
char command_line[basesize];
snprintf(command_line, basesize, "[%s@%s %s]# ",\
GetUserName().c_str(), GetHostName().c_str(), LastDir().c_str());
return command_line;
}
void PrintCommandLine() // 1. 命令行提示符
{
printf("%s", MakeCommandLine().c_str());
fflush(stdout);
}
bool GetCommandLine(char command_buffer[], int size) // 2. 获取用户命令
{
char *result = fgets(command_buffer, size, stdin);
if(!result)
{
return false;
}
command_buffer[strlen(command_buffer)-1] = 0;
if(strlen(command_buffer) == 0) return false;
return true;
}
void ResetCommandline()
{
memset(gargv, 0, sizeof(gargv));
gargc = 0;
// 重定向
redir = NoneRedir;
filename = nullptr;
}
void ParseRedir(char command_buffer[], int len)
{
int end = len - 1;
while(end >= 0)
{
if(command_buffer[end] == '<')
{
redir = InputRedir;
command_buffer[end] = 0;
filename = &command_buffer[end] + 1;
TrimSpace(filename);
break;
}
else if(command_buffer[end] == '>')
{
if(command_buffer[end-1] == '>')
{
redir = AppRedir;
command_buffer[end] = 0;
command_buffer[end-1] = 0;
filename = &command_buffer[end]+1;
TrimSpace(filename);
break;
}
else
{
redir = OutputRedir;
command_buffer[end] = 0;
filename = &command_buffer[end]+1;
TrimSpace(filename);
break;
}
}
else
{
end--;
}
}
}
void ParseCommand(char command_buffer[])
{
// "ls -a -l -n"
const char *sep = " ";
gargv[gargc++] = strtok(command_buffer, sep);
// =是刻意写的
while((bool)(gargv[gargc++] = strtok(nullptr, sep)));
gargc--;
}
void ParseCommandLine(char command_buffer[], int len) // 3. 分析命令
{
ResetCommandline();
ParseRedir(command_buffer, len);
ParseCommand(command_buffer);
}
void debug()
{
printf("argc: %d\n", gargc);
for(int i = 0; gargv[i]; i++)
{
printf("argv[%d]: %s\n", i, gargv[i]);
}
}
void DoRedir()
{
if(redir == InputRedir)
{
if(filename)
{
int fd = open(filename, O_RDONLY);
if(fd < 0)
{
exit(2);
}
dup2(fd, 0);
}
else
{
exit(1);
}
}
else if(redir == OutputRedir)
{
if(filename)
{
int fd = open(filename, O_CREAT | O_WRONLY | O_TRUNC, 0666);
if(fd < 0)
{
exit(4);
}
dup2(fd, 1);
}
else
{
exit(3);
}
}
else if(redir == AppRedir)
{
if(filename)
{
int fd = open(filename, O_CREAT | O_WRONLY | O_APPEND, 0666);
if(fd < 0)
{
exit(6);
}
dup2(fd, 1);
}
else
{
exit(5);
}
}
else
{
// 没有重定向,Do Nothong!
}
}
bool ExecuteCommand() // 4. 执行命令
{
// 让子进程进行执行
pid_t id = fork();
if(id < 0) return false;
if(id == 0)
{
//子进程
DoRedir();
// 1. 执行命令
execvpe(gargv[0], gargv, genv);
// 2. 退出
exit(7);
}
int status = 0;
pid_t rid = waitpid(id, &status, 0);
if(rid > 0)
{
if(WIFEXITED(status))
{
lastcode = WEXITSTATUS(status);
}
else
{
lastcode = 100;
}
return true;
}
return false;
}
void AddEnv(const char *item)
{
int index = 0;
while(genv[index])
{
index++;
}
genv[index] = (char*)malloc(strlen(item)+1);
strncpy(genv[index], item, strlen(item)+1);
genv[++index] = nullptr;
}
// shell自己执行命令，本质是shell调用自己的函数
bool CheckAndExecBuiltCommand()
{
if(strcmp(gargv[0], "cd") == 0)
{
// 内建命令
if(gargc == 2)
{
chdir(gargv[1]);
lastcode = 0;
}
else
{
lastcode = 1;
}
return true;
}
else if(strcmp(gargv[0], "export") == 0)
{
// export也是内建命令
if(gargc == 2)
{
AddEnv(gargv[1]);
lastcode = 0;
}
else
{
lastcode = 2;
}
return true;
}
else if(strcmp(gargv[0], "env") == 0)
{
for(int i = 0; genv[i]; i++)
{
printf("%s\n", genv[i]);
}
lastcode = 0;
return true;
}
else if(strcmp(gargv[0], "echo") == 0)
{
if(gargc == 2)
{
// echo $?
// echo $PATH
// echo hello
if(gargv[1][0] == '$')
{
if(gargv[1][1] == '?')
{
printf("%d\n", lastcode);
lastcode = 0;
}
}
else
{
printf("%s\n", gargv[1]);
lastcode = 0;
}
}
else
{
lastcode = 3;
}
return true;
}
return false;
}
void InitEnv()
{
extern char **environ;
int index = 0;
while(environ[index])
{
genv[index] = (char*)malloc(strlen(environ[index])+1);
strncpy(genv[index], environ[index], strlen(environ[index])+1);
index++;
}
genv[index] = nullptr;
}
int main()
{
InitEnv();
char command_buffer[basesize];
while(true)
{
PrintCommandLine(); // 1. 命令行提示符
// command_buffer -> output
if( !GetCommandLine(command_buffer, basesize) ) // 2. 获取用户命令
{
continue;
}
ParseCommandLine(command_buffer, strlen(command_buffer)); // 3. 分析命令
if ( CheckAndExecBuiltCommand() )
{
continue;
}
ExecuteCommand(); // 4. 执行命令
}
return 0;
} fd是不需要进行关闭的(close(fd)) --> (子进程创建的)
文件描述符的生命周期随进程(进程退出,fd直接被回收了)
三.系统调用和封装调用
我们要尽量少的使用系统调用(时间耗费多),这也是为什么我们的vector,map等有内存池或者扩容一次(1.5倍 -> 2倍)的原因,这样就能减少系统调用的次数(让程序更高效)
#include
#include
#include
#include
#include
#include
int main()
{
close(1);
int fd1 = open("log1.txt", O_WRONLY | O_CREAT | O_APPEND, 0666);
printf("fd1: %d\n", fd1);
fflush(stdout);
close(fd1);
return 0;
} 但是,如果我们不写close()呢?
#include
#include
#include
#include
#include
#include
int main()
{
close(1);
int fd1 = open("log1.txt", O_WRONLY | O_CREAT | O_APPEND, 0666);
printf("fd1: %d\n", fd1);
//fflush(stdout);
//close(fd1);
return 0;
} 
#include
#include
#include
#include
#include
#include
int main()
{
close(1);
int fd1 = open("log1.txt", O_WRONLY | O_CREAT | O_APPEND, 0666);
printf("fd1: %d\n", fd1);
fprintf(stdout,"helloworld\n");
//fflush(stdout);
//close(fd1);
fclose(stdout);
return 0;
} 
还有一点
由内核缓冲区刷新到外设,是由操作系统决定的(还有强制刷新的系统调用)
四.缓冲区的理解
#include
#include
#include
int main()
{
// C库函数
printf("hello printf\n");
fprintf(stdout, "hello fprintf\n");
const char *message = "hello fwrite\n";
fwrite(message, 1, strlen(message), stdout);
// 系统调用
const char *w = "hello write\n";
write(1, w, strlen(w));
return 0;
} 
#include
#include
#include
int main()
{
// C库函数
printf("hello printf\n");
fprintf(stdout, "hello fprintf\n");
const char *message = "hello fwrite\n";
fwrite(message, 1, strlen(message), stdout);
// 系统调用
const char *w = "hello write\n";
write(1, w, strlen(w));
//创建子进程
fork();
return 0;
} 我们在最后加上一个fork()函数,会出现什么情况呢?
重复打印的,都是C库函数的内容,和系统调用没有关系
父子进程在刷新的时候,发生了写时拷贝
五.模拟实现stdio.h的fopen,fwrite等函数
"my_stdio.h"
#pragma once
#define SIZE 1024
#define FLUSH_NONE 0
#define FLUSH_LINE 1
#define FLUSH_FULL 2
struct IO_FILE
{
int flag; //刷新方式
int fileno; //文件描述符
char outbuffer[SIZE];
int cap;
int size;
// ....
};
typedef struct IO_FILE mFILE;
mFILE *mfopen(const char* filename,const char* mode);
int mfwrite(const void* ptr,int num,mFILE* stream);
void mfflush(mFILE* stream);
void mfclose(mFILE* stream);1.mfopen()的实现
mFILE *mfopen(const char *filename, const char *mode)
{
int fd = -1;
if(strcmp(mode,"r") == 0)
{
fd = open(filename,O_RDONLY);
}
else if(strcmp(mode,"w") == 0)
{
fd = open(filename,O_CREAT|O_ERONLY|O_TRUNC,0666);
}
else if(strcmp(mode,"a") == 0)
{
fd = open(filename,O_CREAT|O_ERONLY|O_APPEND,0666);
}
if(fd < 0)
{
return NULL;
}
mFILE* mf = (mFILE*)malloc(sizeof(mFILE));
if(!mf)
{
close(fd);
return NULL;
}
mf->fileno = fd;
mf->flag = FLUSH_LINE;
mf->size = 0;
mf->cap = SIZE;
return mf;
}
2.mfflush()的实现
void mfflush(mFILE *stream)
{
if(stream->size > 0)
{
write(stream->fileno,stream->outbuffer,stream->size);
stream->size = 0;
}
}3.mfwrite()的实现
int mfwrite(const void *ptr, int num, mFILE *stream)
{
//1.拷贝
memcpy(stream->outbuffer+stream->size,ptr,num);
stream->size += num;
//2.检测是否要刷新
if(stream->flag == FLUSH_LINE && stream->size > 0 && stream->outbuffer[stream->size-1] == '\n')
{
mfflush(stream);
}
return num;
}这里我们只进行对行刷新进行操作,后续可以自己进行补充
4.mclose()的实现
void mfclose(mFILE *stream)
{
if(stream->size > 0)
{
mfflush(stream);
}
close(stream->fileno);
}5.main.c的实现
#include "my_stdio.h"
#include
#include
int main()
{
mFILE* fp = mfopen("./log.txt","a");
if(fp == NULL)
{
return 1;
}
int cnt = 10;
while(cnt)
{
char buffer[64];
snprintf(buffer,sizeof(buffer),"hello message,number is : %d\n",cnt);
cnt--;
mfwrite(buffer,strlen(buffer),fp);
sleep(1);
}
mfclose(fp);
return 0;
} 
#include "my_stdio.h"
#include
#include
int main()
{
mFILE* fp = mfopen("./log.txt","a");
if(fp == NULL)
{
return 1;
}
int cnt = 10;
while(cnt)
{
char buffer[64];
snprintf(buffer,sizeof(buffer),"hello message,number is : %d",cnt);
cnt--;
mfwrite(buffer,strlen(buffer),fp);
sleep(1);
}
mfclose(fp);
return 0;
} 
我们写入的就是在文件缓冲区内部
#include "my_stdio.h"
#include
#include
int main()
{
mFILE* fp = mfopen("./log.txt","a");
if(fp == NULL)
{
return 1;
}
int cnt = 10;
while(cnt)
{
char buffer[64];
snprintf(buffer,sizeof(buffer),"hello message,number is : %d\n",cnt);
mfflush(fp);
cnt--;
mfwrite(buffer,strlen(buffer),fp);
sleep(1);
}
mfclose(fp);
return 0;
} 我们在这里让其强制进行刷新
6.内核数据和磁盘的同步
void mfflush(mFILE *stream)
{
if(stream->size > 0)
{
//写到内核文件的文件缓冲区中!
write(stream->fileno,stream->outbuffer,stream->size);
//刷新到外设
fsync(stream->fileno);
stream->size = 0;
}
}这就叫做数据的持久化操作
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