文章目录提示：以下案例仅供参考
- 进程退出的形式
- 进程退出的几种方法
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 1 # include2 3 int main(void) 4 { 5 printf("i am testingn"); 6 return 0; 7 }
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//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 1 # include2 # include 3 4 int main(void) 5 { 6 for(int i = 0; i < 135; i++) 7 { 8 printf("%d: %sn", i, strerror(i)); 9 } 10 return 0; 11 }
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//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。
1 # include2 # include 3 # include 4 5 int main(void) 6 { 7 printf("i am testingn"); 8 exit(EXIT_SUCCESS); 9 return 0; 10 }
1 # include2 # include 3 # include 4 5 int main(void) 6 { 7 printf("i am testingn"); 8 exit(EXIT_FAILURE); //修改exit函数的参数 9 return 0; 10 }
【调用_exit函数】
//所在头文件 #include//_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。
1 # include2 # include 3 # include 4 # include 5 6 int main(void) 7 { 8 printf("i am testing"); //没有“n” 9 sleep(2); 10 exit(EXIT_SUCCESS); 11 return 0; 12 }
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。
1 # include2 # include 3 # include 4 # include 5 6 int main(void) 7 { 8 printf("i am testing"); //没有“n” 9 sleep(2); 10 _exit(EXIT_SUCCESS); 11 return 0; 12 }
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。
