enum类型的本质

自从C语言开始enum类型就被作为用户自定义分类有限集合常量的方法被引入到了语言
当中，而且一度成为C++中定义编译期常量的唯一方法（后来在类中引入了静态整型常量）。
    根据上面对enum类型的描述，到底enum所定义出来的类型是一个什么样的类型呢？作为
一个用户自定义的类型其所占用的内存空间是多少呢？使用enum类型是否真的能够起到有限
集合常量的边界约束呢？大家可能都知道enum类型和int类型具有隐示（自动）转换的规则，
那么是否真的在任何地方都可以使用enum类型的变量来代替int类型的变量呢？下面会逐一
回答这些问题。
    1. 到底enum所定义出来的类型是一个什么样的类型呢？
       在C++中大家都知道仅仅有两种大的类型分类：POD类型和类类型（不清楚的可以参
       见我的其他文章）。enum所定义的类型其实属于POD类型，也就是说它会参与到POD
       类型的隐示转换规则当中去，所以才会出现enum类型与int类型之间的隐示转换现象。
       那么也就是说enum所定义的类型不具备名字空间限定能力（因为不属于类类型），
       其所定义的常量子具备和enum类型所在名字空间相同的可见性，由于自身没有名字
       限定能力，所以会出现名字冲突现象。如：
           struct CEType
           {
               enum EType1 { e1, e2 };
               enum EType2 { e1, e2 };
           };
       上面的例子会出现e1、e2名字冲突编译时错误，原因就在于枚举子（e1、e2）是
       CEType名字空间中的名字，同样在引用该CEType中的枚举子时必须采用CEType::e1
       这样的方式进行，而不是CEType::EType1::e1来进行引用。
    2. 作为一个用户自定义的类型其所占用的内存空间是多少呢？
       该问题就是sizeof( EType1 )等于多少的问题，是不是每一个用户自定义的枚举类
       型都具有相同的尺寸呢？在大多数的32位编译器下（如：VC++、gcc等）一个枚举类
       型的尺寸其实就是一个sizeof( int )的大小，难道枚举类型的尺寸真的就应该是int
       类型的尺寸吗？其实不是这样的，在C++标准文档（ISO14882）中并没有这样来定义，
       标准中是这样说明的：“枚举类型的尺寸是以能够容纳最大枚举子的值的整数的尺寸”，
       同时标准中也说名了：“枚举类型中的枚举子的值必须要能够用一个int类型表述”，
       也就是说，枚举类型的尺寸不能够超过int类型的尺寸，但是是不是必须和int类型
       具有相同的尺寸呢？上面的标准已经说得很清楚了，只要能够容纳最大的枚举子的
       值的整数就可以了，那么就是说可以是char、short和int。例如：
           enum EType1 { e1 = CHAR_MAX };
           enum EType2 { e2 = SHRT_MAX };
           enum EType3 { e3 = INT_MAX  };
       上面的三个枚举类型分别可以用char、short、int的内存空间进行表示，也就是：
           sizeof( EType1 ) == sizeof( char  );
           sizeof( EType2 ) == sizeof( short );
           sizeof( EType3 ) == sizeof( int   );
       那为什么在32位的编译器下都会将上面三个枚举类型的尺寸编译成int类型的尺寸呢？
       主要是从32位数据内存对其方面的要求进行考虑的，在某些计算机硬件环境下具有对
       齐的强制性要求（如：sun SPARC），有些则是因为采用一个完整的32位字长CPU处理
       效率非常高的原因（如：IA32）。所以不可以简单的假设枚举类型的尺寸就是int类
       型的尺寸，说不定会遇到一个编译器为了节约内存而采用上面的处理策略。
    3. 使用enum类型是否真的能够起到有限集合常量的边界约束呢？
       首先看一下下面这个例子：
           enum EType { e1 = 0, e2 };
           void func1( EType e )
           {
               if ( e == e1 )
               {
                   // do something
               }
               // do something because e != e1 must e == e2
           }
           void func2( EType e )
           {
               if ( e == e1 )
               {
                   // do something
               }
               else if ( e == e2 )
               {
                   // do something
               }
           }
           
           func1( static_cast<EType>( 2  ) );
           func2( static_cast<EType>( -1 ) );
       上面的代码应该很清楚的说明了这样一种异常的情况了，在使用一个操出范围的整
       型值调用func1函数时会导致函数采取不该采取的行为，而第二个函数可能会好一些
       他仅仅是忽略了超出范围的值。这就说明枚举所定义的类型并不是一个真正强类型
       的有限常量集合，这样一种条件下和将上述的两个函数参数声明成为整数类型没有
       任何差异。所以以后要注意标准定义中枚举类型的陷阱。（其实只有类类型才是真
       正的强类型）
       
    4. 是否真的在任何地方都可以使用enum类型的变量来代替int类型的变量呢？
       通过上面的讨论，其实枚举类型的变量和整型变量具有了太多的一致性和可互换性，
       那么是不是在每一个可以使用int类型的地方都可以很好的用枚举类型来替代呢？
       其实也不是这样的，毕竟枚举类型是一个在编译时可区分的类型，同时第2点的分析
       枚举类型不一定和int类型具有相同的尺寸，这两个差异就决定了在某些场合是不可
       以使用枚举类型来代替int类型的。如：
           第一种情况：
               enum EType { e1 = 0, e2, e3 };
               EType val;
               std::cin >> val;
           第二种情况：
               enum EType { e1 = 0, e2, e3 };
               EType val;
               std::scanf( "%d", &val );
       上面的两种情况看是基本上属于同一种类型的问题，其实不然。第一种情况会导致
       编译时错误，会因为std::cin没有定义对应的枚举类型的重载>>运算符而出错，这
       就说明枚举类型是一种独立和鉴别的类型；而第二种情况不会有任何编译时问题，
       但是可能会导致scanf函数栈被破坏而使得程序运行非法，为什么会这样呢？上面
       已经分析过了枚举类型变量的尺寸不一定和int类型相同，这样一来我们采用%d就
       是说将枚举类型变量val当作4字节的int变量来看待并进行参数压栈，而在某些编
       译器下sizeof( val )等于1字节，这样scanf函数就会将val变量地址中的后续的三
       字节地址也压入栈中，并对其进行赋值，也许val变量后续的三个字节的地址没有
       特殊含义可以被改写（比如是字节对齐的空地址空间），可能会认为他不会出现错
       误，其实不然，在scanf函数调用结束后会进行栈清理，这样一来会导致scanf函数
       清理了过多的地址空间，从而破坏了外围函数的栈指针的指向，从而必然会导致程
       序运行时错误。

    由上面的说明枚举类型有那么多的缺点，那我们怎样才能够有一个类型安全的枚举类型
呢？其实可以采用类类型来模拟枚举类型的有限常量集合的概念，同时得到类型安全的好处，
具体参见后续的文章。   

文章转自：http://www.cppblog.com/chemz/archive/2007/06/05/25578.html

131 Linux Error Codes for C Programming Language using errno

Programmers should handle all kinds of errors to protect the program from failure.

In C programming language, there is no direct support for error handling. You have to detect the failure and handle the error. In C programming language, return values represents success or failure. Inside a C program, when a function fails, you should handle the errors accordingly, or at least record the errors in a log file.

When you are running some program on Linux environment, you might notice that it gives some error number. For example, “Error no is : 17″, which doesn’t really say much. You really need to know what error number 17 means.

This article shows all available error numbers along with it descriptions. This article might be a handy reference for you, when you encounter an error number and you would like to know what it means.

• In C programming language, there is an external variable called “errno”.
• From this errno variable you can use some error handling functions to find out the error description and handle it appropriately.
• You have to include errno.h header file to use external variable errno.
• perror function prints error description in standard error.
• The strerror function returns a string describing the error code passed in the argument errnum.

The following C code snippet tries to open a file through open system call. There are two flags in the open call. O_CREAT flag is to create a file, if the file does not exist. O_EXCL flag is used with O_CREAT, if the file is already exist open call will fail with the proper error number.

 $ cat fileopen.c #include <stdio.h> #include <fcntl.h> #include <errno.h> #include <string.h> main() { // Declaration of a file descriptor int fd;  // Opening a file fd = open("/root/sasikala/testing",O_CREAT|O_EXCL); // If Open is failed if ( fd < 0 ) {         printf("Opening file : Failed\n");         printf ("Error no is : %d\n", errno);         printf("Error description is : %s\n",strerror(errno)); } // If Open is success else         printf("Opening file : Success\n");  }  $ cc -o fileopen fileopen.c $ ./fileopen  Opening file : Success  $ ./fileopen Opening file : Failed Error no is : 17 Error description is : File exists

At first execution, open got executed successfully, and it created the file since the file was not available. In next execution, it throws an error number 17, which is “File already exist”.

The following table shows list of error numbers and its descriptions in Linux operation system

ERROR CODE TABLE
Error number	Error Code	Error Description
1	EPERM	Operation not permitted
2	ENOENT	No such file or directory
3	ESRCH	No such process
4	EINTR	Interrupted system call
5	EIO	I/O error
6	ENXIO	No such device or address
7	E2BIG	Argument list too long
8	ENOEXEC	Exec format error
9	EBADF	Bad file number
10	ECHILD	No child processes
11	EAGAIN	Try again
12	ENOMEM	Out of memory
13	EACCES	Permission denied
14	EFAULT	Bad address
15	ENOTBLK	Block device required
16	EBUSY	Device or resource busy
17	EEXIST	File exists
18	EXDEV	Cross-device link
19	ENODEV	No such device
20	ENOTDIR	Not a directory
21	EISDIR	Is a directory
22	EINVAL	Invalid argument
23	ENFILE	File table overflow
24	EMFILE	Too many open files
25	ENOTTY	Not a typewriter
26	ETXTBSY	Text file busy
27	EFBIG	File too large
28	ENOSPC	No space left on device
29	ESPIPE	Illegal seek
30	EROFS	Read-only file system
31	EMLINK	Too many links
32	EPIPE	Broken pipe
33	EDOM	Math argument out of domain of func
34	ERANGE	Math result not representable
35	EDEADLK	Resource deadlock would occur
36	ENAMETOOLONG	File name too long
37	ENOLCK	No record locks available
38	ENOSYS	Function not implemented
39	ENOTEMPTY	Directory not empty
40	ELOOP	Too many symbolic links encountered
42	ENOMSG	No message of desired type
43	EIDRM	Identifier removed
44	ECHRNG	Channel number out of range
45	EL2NSYNC	Level 2 not synchronized
46	EL3HLT	Level 3 halted
47	EL3RST	Level 3 reset
48	ELNRNG	Link number out of range
49	EUNATCH	Protocol driver not attached
50	ENOCSI	No CSI structure available
51	EL2HLT	Level 2 halted
52	EBADE	Invalid exchange
53	EBADR	Invalid request descriptor
54	EXFULL	Exchange full
55	ENOANO	No anode
56	EBADRQC	Invalid request code
57	EBADSLT	Invalid slot
59	EBFONT	Bad font file format
60	ENOSTR	Device not a stream
61	ENODATA	No data available
62	ETIME	Timer expired
63	ENOSR	Out of streams resources
64	ENONET	Machine is not on the network
65	ENOPKG	Package not installed
66	EREMOTE	Object is remote
67	ENOLINK	Link has been severed
68	EADV	Advertise error
69	ESRMNT	Srmount error
70	ECOMM	Communication error on send
71	EPROTO	Protocol error
72	EMULTIHOP	Multihop attempted
73	EDOTDOT	RFS specific error
74	EBADMSG	Not a data message
75	EOVERFLOW	Value too large for defined data type
76	ENOTUNIQ	Name not unique on network
77	EBADFD	File descriptor in bad state
78	EREMCHG	Remote address changed
79	ELIBACC	Can not access a needed shared library
80	ELIBBAD	Accessing a corrupted shared library
81	ELIBSCN	.lib section in a.out corrupted
82	ELIBMAX	Attempting to link in too many shared libraries
83	ELIBEXEC	Cannot exec a shared library directly
84	EILSEQ	Illegal byte sequence
85	ERESTART	Interrupted system call should be restarted
86	ESTRPIPE	Streams pipe error
87	EUSERS	Too many users
88	ENOTSOCK	Socket operation on non-socket
89	EDESTADDRREQ	Destination address required
90	EMSGSIZE	Message too long
91	EPROTOTYPE	Protocol wrong type for socket
92	ENOPROTOOPT	Protocol not available
93	EPROTONOSUPPORT	Protocol not supported
94	ESOCKTNOSUPPORT	Socket type not supported
95	EOPNOTSUPP	Operation not supported on transport endpoint
96	EPFNOSUPPORT	Protocol family not supported
97	EAFNOSUPPORT	Address family not supported by protocol
98	EADDRINUSE	Address already in use
99	EADDRNOTAVAIL	Cannot assign requested address
100	ENETDOWN	Network is down
101	ENETUNREACH	Network is unreachable
102	ENETRESET	Network dropped connection because of reset
103	ECONNABORTED	Software caused connection abort
104	ECONNRESET	Connection reset by peer
105	ENOBUFS	No buffer space available
106	EISCONN	Transport endpoint is already connected
107	ENOTCONN	Transport endpoint is not connected
108	ESHUTDOWN	Cannot send after transport endpoint shutdown
109	ETOOMANYREFS	Too many references: cannot splice
110	ETIMEDOUT	Connection timed out
111	ECONNREFUSED	Connection refused
112	EHOSTDOWN	Host is down
113	EHOSTUNREACH	No route to host
114	EALREADY	Operation already in progress
115	EINPROGRESS	Operation now in progress
116	ESTALE	Stale NFS file handle
117	EUCLEAN	Structure needs cleaning
118	ENOTNAM	Not a XENIX named type file
119	ENAVAIL	No XENIX semaphores available
120	EISNAM	Is a named type file
121	EREMOTEIO	Remote I/O error
122	EDQUOT	Quota exceeded
123	ENOMEDIUM	No medium found
124	EMEDIUMTYPE	Wrong medium type
125	ECANCELED	Operation Canceled
126	ENOKEY	Required key not available
127	EKEYEXPIRED	Key has expired
128	EKEYREVOKED	Key has been revoked
129	EKEYREJECTED	Key was rejected by service
130	EOWNERDEAD	Owner died
131	ENOTRECOVERABLE	State not recoverable

When you see an error number thrown by a C program on a Linux environment, you might find the above table handy to identify what those error number means. Make sure to bookmark this article for future reference.