C语言详解分析进程控制中进程终止的实现

进程退出的形式

进程退出的几种情况

• 正常退出（自愿，代码运行完其结果正确）
• 错误退出（自愿，代码运行完其结果不正确）
• 异常退出（非自愿，代码异常直接终止）
• 被其他进程终止（非自愿）

自愿退出会返回一个退出码，由父进程接收。

在Linux上可以使用命令echo $?显示最近一次退出的进程返回的退出码

  //现有如下代码，源文件名为mycode.c
 # include <stdio.h>
 int main(void)
 {
   printf("i am testing\n");                                                                                                                                     
   return 0;
 }

运行以上代码……

使用命令echo $?显示退出码

正常退出的退出码为0，错误退出的退出码为非0

退出码反应了进程退出的原因。所以有必要知道这些退出码究竟代表着什么。

strerror（）函数的作用就是以字符的形式返回退出码。

//所在头文件
<string.h>
//函数原型
char* strerror(int errNum);

//可以使用下面代码查看退出码对应的信息
 # include <stdio.h>
 # include <string.h>
 int main(void)
 {
   for(int i = 0; i < 135; i++)                                                                                                                                  
   {
     printf("%d: %s\n", i, strerror(i));
   }
   return 0;
 }

Linux系统上一共有134个退出码，退出码“134”就已经不认识它了。

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
134: Unknown error 134

Linux上一切皆为文件，指令也是一个文件，当使用不存在的指令时，就会出现报错信息。

其退出码为2，退出码2代表的含义为： No such file or directory(没有这样的文件或目录)

进程退出的几种方法

正常终止的3种方法：

• main函数返回
• 调用exit
• 调用_exit

最常见的进程终止莫过于main函数返回了！需要注意main函数中返回才是进程退出，而非main函数返回并不能正常终止进程，只是函数返回。

【调用exit】

//所在头文件
 #include <stdlib.h>
 //exit函数原型
 void exit(int status);

exit函数是语言层面的函数，是对系统调用函数_exit的封装。

它可以在main函数中调用，也可以在非main函数中调用，都可以达到终止进程的目的。

C标准还规定了两个宏EXIT_SUCCESS和EXIT_FAILURE可以作为exit的参数，用来指示这个进程是正常退出还是错误退出。

 # include <stdio.h>
 # include <string.h>
 # include <stdlib.h>
 int main(void)
 {
   printf("i am testing\n");
   exit(EXIT_SUCCESS);                                                                                                                                           
   return 0;
 }

 # include <stdio.h>
 # include <string.h>
 # include <stdlib.h>
 int main(void)
 {
   printf("i am testing\n");
   exit(EXIT_FAILURE);     //修改exit函数的参数                                                                                                                                      
   return 0;
 }

【调用_exit函数】

//所在头文件
#include <unistd.h>
//_exit函数原型
void _exit(int status);

_exit是系统调用函数。_exit函数的本质就是调用了系统调用函数_exit。

exit函数封装了系统调用_exit，exit函数还做了其他的事情。

 # include <stdio.h>
 # include <string.h>
 # include <stdlib.h>
 # include <unistd.h>
 int main(void)
 {
   printf("i am testing"); //没有“\n”
   sleep(2);
   exit(EXIT_SUCCESS);
   return 0;                                                                                                                                                     
 }

代码运行的现象：休眠2秒后，在屏幕上打印“i am testing”.

因此，可以看出，exit函数会冲刷缓冲区……

现在换做_exit函数来终止进程。

 # include <stdio.h>
 # include <string.h>
 # include <stdlib.h>
 # include <unistd.h>
 int main(void)
 {
   printf("i am testing"); //没有“\n”
   sleep(2);
   _exit(EXIT_SUCCESS);
   return 0;                                                                                                                                                     
 }

代码运行的现象：休眠两秒后，并没有在屏幕上显示预期的“i am testing”，就终止进程了。

因此可看出，_exit函数并不会冲刷缓冲区……

实际上，_exit系统调用函数只是简单地终止了进程，并不会做其他的工作。而exit函数在调用_exit函数之前还会做一些其他的工作：

• 执行用户定义的清理函数
• 冲刷缓冲区，关闭所有的流等。
• 调用_exit函数

main函数返回，本质上是把返回值作为参数传给exit函数，最后还是调用exit函数。

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