進程退出的形式

進程退出的幾種情況

• 正常退出（自愿，代碼運行完其結果正確）
• 錯誤退出（自愿，代碼運行完其結果不正確）
• 異常退出（非自愿，代碼異常直接終止）
• 被其他進程終止（非自愿）

自愿退出會返回一個退出碼，由父進程接收。

在Linux上可以使用命令echo $?顯示最近一次退出的進程返回的退出碼

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

運行以上代碼……

使用命令echo $?顯示退出碼

正常退出的退出碼為0，錯誤退出的退出碼為非0

退出碼反應了進程退出的原因。所以有必要知道這些退出碼究竟代表著什么。

strerror（）函數的作用就是以字符的形式返回退出碼。

//所在頭文件
<string.h>
//函數原型
char* strerror(int errNum);

//可以使用下面代碼查看退出碼對應的信息
 # 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系統上一共有134個退出碼，退出碼“134”就已經不認識它了。

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上一切皆為文件，指令也是一個文件，當使用不存在的指令時，就會出現報錯信息。

其退出碼為2，退出碼2代表的含義為： No such file or directory(沒有這樣的文件或目錄)

進程退出的幾種方法

正常終止的3種方法：

• main函數返回
• 調用exit
• 調用_exit

最常見的進程終止莫過于main函數返回了！需要注意main函數中返回才是進程退出，而非main函數返回并不能正常終止進程，只是函數返回。

【調用exit】

//所在頭文件
 #include <stdlib.h>
 //exit函數原型
 void exit(int status);

exit函數是語言層面的函數，是對系統調用函數_exit的封裝。

它可以在main函數中調用，也可以在非main函數中調用，都可以達到終止進程的目的。

C標準還規定了兩個宏EXIT_SUCCESS和EXIT_FAILURE可以作為exit的參數，用來指示這個進程是正常退出還是錯誤退出。

 # 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函數的參數                                                                                                                                      
   return 0;
 }

【調用_exit函數】

//所在頭文件
#include <unistd.h>
//_exit函數原型
void _exit(int status);

_exit是系統調用函數。_exit函數的本質就是調用了系統調用函數_exit。

exit函數封裝了系統調用_exit，exit函數還做了其他的事情。

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

代碼運行的現象：休眠2秒后，在屏幕上打印“i am testing”.

因此，可以看出，exit函數會沖刷緩沖區……

現在換做_exit函數來終止進程。

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

代碼運行的現象：休眠兩秒后，并沒有在屏幕上顯示預期的“i am testing”，就終止進程了。

因此可看出，_exit函數并不會沖刷緩沖區……

實際上，_exit系統調用函數只是簡單地終止了進程，并不會做其他的工作。而exit函數在調用_exit函數之前還會做一些其他的工作：

• 執行用戶定義的清理函數
• 沖刷緩沖區，關閉所有的流等。
• 調用_exit函數

main函數返回，本質上是把返回值作為參數傳給exit函數，最后還是調用exit函數。