操作系统之shell实现（上）

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目录

1. 进程创建

1.1fork函数

​编辑

1.2写时拷贝（Copy-on-write）

1.3 虚拟地址存储 

2. 进程终止

2.1进程退出情况

2.2退出方式

2.3退出码

2.4exit函数VS_exit函数

2.4.1exit函数

 2.4.2_exit函数

​编辑 2.4.3eixt VS_exit​编辑

3. 进程等待

3.1进程等待的必要性

 3.2wait等待

3.3获取子进程status 

4.总结

1. 进程创建

1.1fork函数

fork() 是 Linux 中非常重要的函数，用来从父进程创建一个子进程。父进程返回子进程的PID，子进程返回0。通过 fork() 创建的父子进程可以独立执行各自的代码。

头文件：#include<unistd.h>

返回值：成功父进程返回孩子pid，子进程返回0。失败返回-1。

函数：pid_t frok(void); 

#include<stdio.h>
#include<unistd.h>int main()
{printf("Befor pid is %d\n",getpid());pid_t id=fork();if(id==-1){perror("fork()");exit(1);}printf("After:pid is %d,return id is %d \n",getpid(),id);
}

结果： 

1.2写时拷贝（Copy-on-write）

fork() 使父子进程共享相同的内存内容，直到其中一个进程对内存进行修改，这时才会进行拷贝，避免不必要的资源浪费。 

#include<stdio.h>
#include<unistd.h>
#include<sys/types.h>
#include<stdlib.h>
int main()
{int a=0;printf("Befor pid is %d\n",getpid());pid_t id=fork();if(id==-1){perror("fork()");exit(1);}printf("After:pid is %d,return id is %d \n",getpid(),id);  }

 此时三个地址：a取地址一样

1.3 虚拟地址存储 

尝试在子进程中修改a的地址后，查看a的地址。

#include<stdio.h>
#include<unistd.h>
#include<sys/types.h>
#include<stdlib.h>
int main()
{int a=0;printf("Befor pid is %d,path is %p\n",getpid(),&a);pid_t id=fork();if(id==-1){perror("fork()");exit(1);}if(id==0) {int a=15;printf("After:pid is %d,return id is %d ,path is %p\n",getpid(),id,&a);  }elseprintf("After:pid is %d,return id is %d ,path is %p\n",getpid(),id,&a);}

 三个地址仍然相同？这是为什么呢？

两个虚拟地址相同的原因在于它们位于不同的虚拟页面上，但在各自的虚拟页面内，偏移量相同。因此，尽管它们属于不同的虚拟内存块，最终的虚拟地址值却相同。

2. 进程终止

2.1进程退出情况

• 代码运行完毕，结果正常
• 代码运行完毕，结果不正常
• 进程异常终止

2.2退出方式

如果系统正常终止可以通过$?来查看进程退出码

• 从main函数返回
• 调用exit
• 调用_exit

2.3退出码

每个进程都有一个退出码，0 表示正常退出，非 0 表示出错。

2.4exit函数VS_exit函数

2.4.1exit函数

#include <unistd.h>

#include<stdlib.h>
void exit(int status); 

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
int main()
{printf("exit");exit(1);}

 2.4.2_exit函数

#include <unistd.h>

#include<stdlib.h>
void _exit(int status); 

#include<stdio.h>
#include<unistd.h>
int main()
{printf("_exit");_exit(1);}

 2.4.3eixt VS_exit

3. 进程等待

3.1进程等待的必要性

• 之前讲过，子进程退出，父进程如果不管不顾，就可能造成‘僵尸进程’的问题，进而造成内存
• 泄漏。
• 另外，进程一旦变成僵尸状态，那就刀枪不入，“杀人不眨眼”的kill -9 也无能为力，因为谁也没有办法杀死一个已经死去的进程。
• 最后，父进程派给子进程的任务完成的如何，我们需要知道。如，子进程运行完成，结果对还是不对，或者是否正常退出。
• 父进程通过进程等待的方式，回收子进程资源，获取子进程退出信息

 3.2wait等待

#include <sys/types.h>

#include <sys/wait.h>

pid_t wait(int *status);

返回值：
        成功返回被等待进程pid，失败返回-1。
参数：
        输出型参数，获取子进程退出状态,不关心则可以设置成为NULL

pid_t waitpid(pid_t pid, int *status, int options);

返回值：
        当正常返回的时候waitpid返回收集到的子进程的进程ID；
        如果设置了选项WNOHANG,而调用中waitpid发现没有已退出的子进程可收集,则返回0；
        如果调用中出错,则返回-1,这时errno会被设置成相应的值以指示错误所在；
参数：
        pid：
                Pid=-1,等待任一个子进程。与wait等效。
                Pid>0.等待其进程ID与pid相等的子进程。
        status:

                输出型参数
                        WIFEXITED(status): 若为正常终止子进程返回的状态，则为真。（查看进程是否是正常退出）
                        WEXITSTATUS(status): 若WIFEXITED非零，提取子进程退出码。（查看进程的退出码）
options:默认为0，表示阻塞等待

                        WNOHANG: 若pid指定的子进程没有结束，则waitpid()函数返回0，不予以等
待。若正常结束，则返回该子进程的ID。

int waitid(idtype_t idtype, id_t id, siginfo_t *infop, int options);

 

3.3获取子进程status

#include<stdio.h>
#include<sys/types.h>
#include<sys/wait.h>
#include<unistd.h>
#include<stdlib.h>
int main()
{int id=0;int cur=5;id=fork();if(id==-1){perror("fork");}if(id==0){while(cur--){printf("My pid is %d,My ppid is %d\n",getpid(),getppid());}sleep(1);exit(1);}int status=0;int idd=waitpid(id,&status,0);printf("idd is %d,status is %d\n",idd,status);}

3.3.1退出码 

为什么退出码是256而不是1？

  printf("idd is %d,status is %d\n",idd,(status>>8));

3.3.2终止信号 

4.总结

 1.Linux下的错误码及描述

0->Success
1->Operation not permitted
2->No such file or directory
3->No such process
4->Interrupted system call
5->Input/output error
6->No such device or address
7->Argument list too long
8->Exec format error
9->Bad file descriptor
10->No child processes
11->Resource temporarily unavailable
12->Cannot allocate memory
13->Permission denied
14->Bad address
15->Block device required
16->Device or resource busy
17->File exists
18->Invalid cross-device link
19->No such device
20->Not a directory
21->Is a directory
22->Invalid argument
23->Too many open files in system
24->Too many open files
25->Inappropriate ioctl for device
26->Text file busy
27->File too large
28->No space left on device
29->Illegal seek
30->Read-only file system
31->Too many links
32->Broken pipe
33->Numerical argument out of domain
34->Numerical result out of range
35->Resource deadlock avoided
36->File name too long
37->No locks available
38->Function not implemented
39->Directory not empty
40->Too many levels of symbolic links
41->Unknown error 41
42->No message of desired type
43->Identifier removed
44->Channel number out of range
45->Level 2 not synchronized
46->Level 3 halted
47->Level 3 reset
48->Link number out of range
49->Protocol driver not attached
50->No CSI structure available
51->Level 2 halted
52->Invalid exchange
53->Invalid request descriptor
54->Exchange full
55->No anode
56->Invalid request code
57->Invalid slot
58->Unknown error 58
59->Bad font file format
60->Device not a stream
61->No data available
62->Timer expired
63->Out of streams resources
64->Machine is not on the network
65->Package not installed
66->Object is remote
67->Link has been severed
68->Advertise error
69->Srmount error
70->Communication error on send
71->Protocol error
72->Multihop attempted
73->RFS specific error
74->Bad message
75->Value too large for defined data type
76->Name not unique on network
77->File descriptor in bad state
78->Remote address changed
79->Can not access a needed shared library
80->Accessing a corrupted shared library
81->.lib section in a.out corrupted
82->Attempting to link in too many shared libraries
83->Cannot exec a shared library directly
84->Invalid or incomplete multibyte or wide character
85->Interrupted system call should be restarted
86->Streams pipe error
87->Too many users
88->Socket operation on non-socket
89->Destination address required
90->Message too long
91->Protocol wrong type for socket
92->Protocol not available
93->Protocol not supported
94->Socket type not supported
95->Operation not supported
96->Protocol family not supported
97->Address family not supported by protocol
98->Address already in use
99->Cannot assign requested address
100->Network is down
101->Network is unreachable
102->Network dropped connection on reset
103->Software caused connection abort
104->Connection reset by peer
105->No buffer space available
106->Transport endpoint is already connected
107->Transport endpoint is not connected
108->Cannot send after transport endpoint shutdown
109->Too many references: cannot splice
110->Connection timed out
111->Connection refused
112->Host is down
113->No route to host
114->Operation already in progress
115->Operation now in progress
116->Stale file handle
117->Structure needs cleaning
118->Not a XENIX named type file
119->No XENIX semaphores available
120->Is a named type file
121->Remote I/O error
122->Disk quota exceeded
123->No medium found
124->Wrong medium type
125->Operation canceled
126->Required key not available
127->Key has expired
128->Key has been revoked
129->Key was rejected by service
130->Owner died
131->State not recoverable
132->Operation not possible due to RF-kill
133->Memory page has hardware error