轉自：http://blog.csdn.net/romandion/archive/2008/01/11/2036975.aspx
一、errno的由來
在C編程中，errno是個不可缺少的變量，特別是在網絡編程中。如果你沒有用過errno，那隻能說明你的程序不夠健壯。當然，如果你是WIN32平臺的GetLastError()，效果也是一樣的。
爲什麼會使用errno呢？個人認爲，這是系統庫設計中的一個無奈之舉，他更多的是個技巧，而不是架構上的需要。我們觀察下函數結構，可以發現，函數的參
數返回值只有一個，這個返回值一般可以攜帶錯誤信息，比如負數表示錯誤，而正數表述正確的返回值，比如recv函數。但是對於一些返回指針的函數，如：
char
*get_str()；這個方法顯然沒有用的。NULL可以表示發生錯誤，但是發生什麼錯誤卻毫無辦法。於是，errno就誕生了。全局變量errno可
以存放錯誤原因，當錯誤發生時，函數的返回值是可以通過非法值來提示錯誤的發生。
二、errno的線程安全
errno是全局變量，但是在多線程環境下，就會變得很恐怖。當你調用一個函數時，發現這個函數發生了錯誤，但當你使用錯誤原因時，他卻變成了另外一個線程的錯誤提示。想想就會覺得是件可怕的事情。
將errno設置爲線程局部變量是個不錯的主意，事實上，GCC中就是這麼幹的。他保證了線程之間的錯誤原因不會互相串改，當你在一個線程中串行執行一系列過程，那麼得到的errno仍然是正確的。
看下，bits/errno.h的定義：
# ifndef __ASSEMBLER__
/* Function to get address of global `errno' variable. */
extern int *__errno_location (void) __THROW __attribute__ ((__const__));
# if !defined _LIBC || defined _LIBC_REENTRANT
/* When using threads, errno is a per-thread value. */
# define errno (*__errno_location ())
# endif
# endif /* !__ASSEMBLER__ */
而errno.h中是這樣定義的：
/* Declare the `errno' variable, unless it's defined as a macro by
bits/errno.h. This is the case in GNU, where it is a per-thread
variable. This redeclaration using the macro still works, but it
will be a function declaration without a prototype and may trigger
a -Wstrict-prototypes warning. */
#ifndef errno
extern int errno;
#endif
顯然，errno實際上，並不是我們通常認爲的是個整型數值，而是通過整型指針來獲取值的。這個整型就是線程安全的。
三、errno的實現
static pthread_key_t key;
static pthread_once_t key_once = PTHREAD_ONCE_INIT;
static void make_key()
{
(void) pthread_key_create(&key, NULL);
}
int *_errno()
{
int *ptr ;
(void) pthread_once(&key_once, make_key);
if ((ptr = pthread_getspecific(key)) == NULL)
{
ptr = malloc(sizeof(int));
(void) pthread_setspecific(key, ptr);
}
return ptr ;
}
四、errno的應用
errno在庫中得到廣泛的應用，但是，錯誤編碼實際上不止那麼多。我們需要在自己的系統中增加更多的錯誤編碼。一種方式就是直接利用errno，另外一種方式就是定義自己的user_errno。
使用errno，strerror可能無法解析，這需要自己解決。但errno使用線程變量的方式值得借鑑。
另附：
轉自：http://www.cnblogs.com/riky/archive/2008/02/02/1062750.html
errno.h- -
以下來自linux 2.4.20-18的內核代碼中的/usr/include/asm/errno.h
#ifndef _I386_ERRNO_H
#define _I386_ERRNO_H
#define EPERM 1 /* Operation not permitted */
#define ENOENT 2 /* No such file or directory */
#define ESRCH 3 /* No such process */
#define EINTR 4 /* Interrupted system call */
#define EIO 5 /* I/O error */
#define ENXIO 6 /* No such device or address */
#define E2BIG 7 /* Arg list too long */
#define ENOEXEC 8 /* Exec format error */
#define EBADF 9 /* Bad file number */
#define ECHILD 10 /* No child processes */
#define EAGAIN 11 /* Try again */
#define ENOMEM 12 /* Out of memory */
#define EACCES 13 /* Permission denied */
#define EFAULT 14 /* Bad address */
#define ENOTBLK 15 /* Block device required */
#define EBUSY 16 /* Device or resource busy */
#define EEXIST 17 /* File exists */
#define EXDEV 18 /* Cross-device link */
#define ENODEV 19 /* No such device */
#define ENOTDIR 20 /* Not a directory */
#define EISDIR 21 /* Is a directory */
#define EINVAL 22 /* Invalid argument */
#define ENFILE 23 /* File table overflow */
#define EMFILE 24 /* Too many open files */
#define ENOTTY 25 /* Not a typewriter */
#define ETXTBSY 26 /* Text file busy */
#define EFBIG 27 /* File too large */
#define ENOSPC 28 /* No space left on device */
#define ESPIPE 29 /* Illegal seek */
#define EROFS 30 /* Read-only file system */
#define EMLINK 31 /* Too many links */
#define EPIPE 32 /* Broken pipe */
#define EDOM 33 /* Math argument out of domain of func */
#define ERANGE 34 /* Math result not representable */
#define EDEADLK 35 /* Resource deadlock would occur */
#define ENAMETOOLONG 36 /* File name too long */
#define ENOLCK 37 /* No record locks available */
#define ENOSYS 38 /* Function not implemented */
#define ENOTEMPTY 39 /* Directory not empty */
#define ELOOP 40 /* Too many symbolic links encountered */
#define EWOULDBLOCK EAGAIN /* Operation would block */
#define ENOMSG 42 /* No message of desired type */
#define EIDRM 43 /* Identifier removed */
#define ECHRNG 44 /* Channel number out of range */
#define EL2NSYNC 45 /* Level 2 not synchronized */
#define EL3HLT 46 /* Level 3 halted */
#define EL3RST 47 /* Level 3 reset */
#define ELNRNG 48 /* Link number out of range */
#define EUNATCH 49 /* Protocol driver not attached */
#define ENOCSI 50 /* No CSI structure available */
#define EL2HLT 51 /* Level 2 halted */
#define EBADE 52 /* Invalid exchange */
#define EBADR 53 /* Invalid request descriptor */
#define EXFULL 54 /* Exchange full */
#define ENOANO 55 /* No anode */
#define EBADRQC 56 /* Invalid request code */
#define EBADSLT 57 /* Invalid slot */
#define EDEADLOCK EDEADLK
#define EBFONT 59 /* Bad font file format */
#define ENOSTR 60 /* Device not a stream */
#define ENODATA 61 /* No data available */
#define ETIME 62 /* Timer expired */
#define ENOSR 63 /* Out of streams resources */
#define ENONET 64 /* Machine is not on the network */
#define ENOPKG 65 /* Package not installed */
#define EREMOTE 66 /* Object is remote */
#define ENOLINK 67 /* Link has been severed */
#define EADV 68 /* Advertise error */
#define ESRMNT 69 /* Srmount error */
#define ECOMM 70 /* Communication error on send */
#define EPROTO 71 /* Protocol error */
#define EMULTIHOP 72 /* Multihop attempted */
#define EDOTDOT 73 /* RFS specific error */
#define EBADMSG 74 /* Not a data message */
#define EOVERFLOW 75 /* Value too large for defined data type */
#define ENOTUNIQ 76 /* Name not unique on network */
#define EBADFD 77 /* File descriptor in bad state */
#define EREMCHG 78 /* Remote address changed */
#define ELIBACC 79 /* Can not access a needed shared library */
#define ELIBBAD 80 /* Accessing a corrupted shared library */
#define ELIBSCN 81 /* .lib section in a.out corrupted */
#define ELIBMAX 82 /* Attempting to link in too many shared libraries */
#define ELIBEXEC 83 /* Cannot exec a shared library directly */
#define EILSEQ 84 /* Illegal byte sequence */
#define ERESTART 85 /* Interrupted system call should be restarted */
#define ESTRPIPE 86 /* Streams pipe error */
#define EUSERS 87 /* Too many users */
#define ENOTSOCK 88 /* Socket operation on non-socket */
#define EDESTADDRREQ 89 /* Destination address required */
#define EMSGSIZE 90 /* Message too long */
#define EPROTOTYPE 91 /* Protocol wrong type for socket */
#define ENOPROTOOPT 92 /* Protocol not available */
#define EPROTONOSUPPORT 93 /* Protocol not supported */
#define ESOCKTNOSUPPORT 94 /* Socket type not supported */
#define EOPNOTSUPP 95 /* Operation not supported on transport endpoint */
#define EPFNOSUPPORT 96 /* Protocol family not supported */
#define EAFNOSUPPORT 97 /* Address family not supported by protocol */
#define EADDRINUSE 98 /* Address already in use */
#define EADDRNOTAVAIL 99 /* Cannot assign requested address */
#define ENETDOWN 100 /* Network is down */
#define ENETUNREACH 101 /* Network is unreachable */
#define ENETRESET 102 /* Network dropped connection because of reset */
#define ECONNABORTED 103 /* Software caused connection abort */
#define ECONNRESET 104 /* Connection reset by peer */
#define ENOBUFS 105 /* No buffer space available */
#define EISCONN 106 /* Transport endpoint is already connected */
#define ENOTCONN 107 /* Transport endpoint is not connected */
#define ESHUTDOWN 108 /* Cannot send after transport endpoint shutdown */
#define ETOOMANYREFS 109 /* Too many references: cannot splice */
#define ETIMEDOUT 110 /* Connection timed out */
#define ECONNREFUSED 111 /* Connection refused */
#define EHOSTDOWN 112 /* Host is down */
#define EHOSTUNREACH 113 /* No route to host */
#define EALREADY 114 /* Operation already in progress */
#define EINPROGRESS 115 /* Operation now in progress */
#define ESTALE 116 /* Stale NFS file handle */
#define EUCLEAN 117 /* Structure needs cleaning */
#define ENOTNAM 118 /* Not a XENIX named type file */
#define ENAVAIL 119 /* No XENIX semaphores available */
#define EISNAM 120 /* Is a named type file */
#define EREMOTEIO 121 /* Remote I/O error */
#define EDQUOT 122 /* Quota exceeded */
#define ENOMEDIUM 123 /* No medium found */
#define EMEDIUMTYPE 124 /* Wrong medium type */
#endif
