前言
當UNIX函數出現錯誤時，常常返回一個負值，而且整型變量errno通常被設置爲含有附加信息的一個值。例如，open函數如果成功執行則返回一個非負文件描述符，如出錯則返回-1.在open出錯時，有大約15種不同的errno值（文件不存在、權限問題等）。某些函數並不返回負值而是使用另一種約定。例如，返回一個指向對象指針的大多數函數，在出錯時，將返回一個NULL指針。
文件==<errno.h>==中定義了符號errno以及可以賦予的各個常量，這些常量都以字符E開頭。例如，若errno等於常量EACESS，這表示產生了權限問題（例如，沒有打開所要求文件的足夠權限）。
POSIX和ISO C將errno定義爲這一一個符號，它擴展成爲一個可以修改的整型左值。這可以是包含出錯編號的一個整數，或是一個返回出錯編號指針的函數。以前使用的定義是：
extern int errno;
但是在支持線程的環境中，多個線程共享進程地址空間，每個線程都有屬於自己的局部errno以避免一個線程干擾另一個線程。
對於errno應當知道兩條規則。第一條規則：如果沒有出錯，則其值不會被一個例程清除。因此，僅當函數返回指明出錯時，才檢驗其值。第二條：任何一個函數都不會將errno值設置爲0，在<errno.h>中定義的所有常量都不爲0。
C標準定義了兩個函數，它們幫助打印出錯信息。
#include <string.h>
char *strerror(int errnum);
此函數將errnum（它通常就是errno值）映射到一個出錯信息字符串，並且返回此字符串的指針。
perror函數基於errno的當前值，在標準錯誤上產生一條出錯信息，
#include <stdio.h>
void perror(const char *msg);
它首先輸出由msg指向的字符串，然後是一個冒號，一個空格，接着是對應於errno值的出錯信息，最後是一個換行符。
出錯恢復
可與將<errno.h>中定義的各種出錯分出致命性的和非致命性的兩類。對於致命性的錯誤，無法執行恢復動作，最多隻能在用戶屏幕上打印一條出錯信息，或者將一條出錯信息寫到日誌文件中，然後終止。關於這點可以看我的：段錯誤？打的就是段錯誤！
而對於非致命性錯誤，有課可以較爲妥善的處理。大多數非致命性錯誤在本質上是暫時的，例如資源短缺，當系統中活動較少時，這種出錯很可能就不會發生。
與資源相關的非致命性出錯包括EAGAIN. ENFILE、ENOBUFS、ENOLCK、ENOSPC、ENOSR、EWOULDBLOCK, 有時ENOMEM也是非致命性出錯。當EBUSY指明共享資源正在使用時，也可將它作爲非致命性出錯處理。當EINTR中斷- -慢速系統調用時，可將它作爲非致命性出錯處理。
對於資源相關的非致命性出錯，一般恢復動作是延遲一些時間， 然後再試。這種技術可應用於其他情況。例如，假設-個 出錯表明一個 網絡連接不再起作用， 那麼應用程序可以在短時間延遲後重建該連接。某些應用使用指數補償算法，在每次重複中等待更長時間。
最後，取決於應用程序的開發者，他可以決定那些出錯是可恢復的。如若使用一種從錯誤中恢復的合理策略，那麼由於避免了應用程序的異常終止，就能改善應用程序的健壯性。
栗子
對於中斷的read、write系 統調用，POSIX.I的語 義在該標準的2001版有所改變。對於如何處理已read、write部分數據量的相應系統調用，早期版本允許實現進行選擇。如若read系統調用已接收並傳送數據至應用程序緩衝區，但尚未接收到應用程序請求的全部數據，此時被中斷、操作系統可以認爲該系統調用失敗、並將errno設置爲EINTR;另一種處理方式是允許該系統調用成功返回，返回已接收到的部分數據量。與此類似，如若write巳傳輸了應用程序緩衝區中的部分數據，然後被中斷，操作系統可以認爲該系統調用失敗，並將ermo設置爲EINTR;另一種處理方式是允許該系統調用成功返回，返回已寫的部分數據量。歷史上，從系統V派生的實現，將這種系統調用視爲失敗，而BSD派生的實現則處理爲部分成功返回。POSIX.1 標準的2001版採用BSD風格的語義。
假定一個讀操作，它被中斷，我們要恢復它：
again:
if(n= read(fd, buf, BUFSIZE)) < 0)
if (errno == EINTR)
goto again;/* just an interrupted system call * /
/* handle other errors */
errno表
其實看不看這個表意義不大，但是有的人他就是想看，那我就貼一份吧。
查看系統中所有的errno所代表的含義，可以採用如下的代碼：
/* Function: obtain the errno string
* char *strerror(int errno)
*/
#include <stdio.h>
#include <string.h> //for strerror()
//#include <errno.h>
int main()
{
int tmp = 0;
for(tmp = 0; tmp <=256; tmp++)
{
printf("errno: %2d\t%s\n",tmp,strerror(tmp));
}
return 0;
}
輸出信息如下：
errno: 0 Success
errno: 1 Operation not permitted
errno: 2 No such file or directory
errno: 3 No such process
errno: 4 Interrupted system call
errno: 5 Input/output error
errno: 6 No such device or address
errno: 7 Argument list too long
errno: 8 Exec format error
errno: 9 Bad file descriptor
errno: 10 No child processes
errno: 11 Resource temporarily unavailable
errno: 12 Cannot allocate memory
errno: 13 Permission denied
errno: 14 Bad address
errno: 15 Block device required
errno: 16 Device or resource busy
errno: 17 File exists
errno: 18 Invalid cross-device link
errno: 19 No such device
errno: 20 Not a directory
errno: 21 Is a directory
errno: 22 Invalid argument
errno: 23 Too many open files in system
errno: 24 Too many open files
errno: 25 Inappropriate ioctl for device
errno: 26 Text file busy
errno: 27 File too large
errno: 28 No space left on device
errno: 29 Illegal seek
errno: 30 Read-only file system
errno: 31 Too many links
errno: 32 Broken pipe
errno: 33 Numerical argument out of domain
errno: 34 Numerical result out of range
errno: 35 Resource deadlock avoided
errno: 36 File name too long
errno: 37 No locks available
errno: 38 Function not implemented
errno: 39 Directory not empty
errno: 40 Too many levels of symbolic links
errno: 41 Unknown error 41
errno: 42 No message of desired type
errno: 43 Identifier removed
errno: 44 Channel number out of range
errno: 45 Level 2 not synchronized
errno: 46 Level 3 halted
errno: 47 Level 3 reset
errno: 48 Link number out of range
errno: 49 Protocol driver not attached
errno: 50 No CSI structure available
errno: 51 Level 2 halted
errno: 52 Invalid exchange
errno: 53 Invalid request descriptor
errno: 54 Exchange full
errno: 55 No anode
errno: 56 Invalid request code
errno: 57 Invalid slot
errno: 58 Unknown error 58
errno: 59 Bad font file format
errno: 60 Device not a stream
errno: 61 No data available
errno: 62 Timer expired
errno: 63 Out of streams resources
errno: 64 Machine is not on the network
errno: 65 Package not installed
errno: 66 Object is remote
errno: 67 Link has been severed
errno: 68 Advertise error
errno: 69 Srmount error
errno: 70 Communication error on send
errno: 71 Protocol error
errno: 72 Multihop attempted
errno: 73 RFS specific error
errno: 74 Bad message
errno: 75 Value too large for defined data type
errno: 76 Name not unique on network
errno: 77 File descriptor in bad state
errno: 78 Remote address changed
errno: 79 Can not access a needed shared library
errno: 80 Accessing a corrupted shared library
errno: 81 .lib section in a.out corrupted
errno: 82 Attempting to link in too many shared libraries
errno: 83 Cannot exec a shared library directly
errno: 84 Invalid or incomplete multibyte or wide character
errno: 85 Interrupted system call should be restarted
errno: 86 Streams pipe error
errno: 87 Too many users
errno: 88 Socket operation on non-socket
errno: 89 Destination address required
errno: 90 Message too long
errno: 91 Protocol wrong type for socket
errno: 92 Protocol not available
errno: 93 Protocol not supported
errno: 94 Socket type not supported
errno: 95 Operation not supported
errno: 96 Protocol family not supported
errno: 97 Address family not supported by protocol
errno: 98 Address already in use
errno: 99 Cannot assign requested address
errno: 100 Network is down
errno: 101 Network is unreachable
errno: 102 Network dropped connection on reset
errno: 103 Software caused connection abort
errno: 104 Connection reset by peer
errno: 105 No buffer space available
errno: 106 Transport endpoint is already connected
errno: 107 Transport endpoint is not connected
errno: 108 Cannot send after transport endpoint shutdown
errno: 109 Too many references: cannot splice
errno: 110 Connection timed out
errno: 111 Connection refused
errno: 112 Host is down
errno: 113 No route to host
errno: 114 Operation already in progress
errno: 115 Operation now in progress
errno: 116 Stale file handle
errno: 117 Structure needs cleaning
errno: 118 Not a XENIX named type file
errno: 119 No XENIX semaphores available
errno: 120 Is a named type file
errno: 121 Remote I/O error
errno: 122 Disk quota exceeded
errno: 123 No medium found
errno: 124 Wrong medium type
errno: 125 Operation canceled
errno: 126 Required key not available
errno: 127 Key has expired
errno: 128 Key has been revoked
errno: 129 Key was rejected by service
errno: 130 Owner died
errno: 131 State not recoverable
errno: 132 Operation not possible due to RF-kill
errno: 133 Memory page has hardware error
errno: 134~255 unknown error!
Linux中，在頭文件 /usr/include/asm-generic/errno-base.h 對基礎常用errno進行了宏定義：
#ifndef _ASM_GENERIC_ERRNO_BASE_H
#define _ASM_GENERIC_ERRNO_BASE_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 /* Argument 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 */
#endif
在 /usr/include/asm-asm-generic/errno.h 中，對剩餘的errno做了宏定義：
#ifndef _ASM_GENERIC_ERRNO_H
#define _ASM_GENERIC_ERRNO_H
#include <asm-generic/errno-base.h>
#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 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 */
#define ECANCELED 125 /* Operation Canceled */
#define ENOKEY 126 /* Required key not available */
#define EKEYEXPIRED 127 /* Key has expired */
#define EKEYREVOKED 128 /* Key has been revoked */
#define EKEYREJECTED 129 /* Key was rejected by service */
/* for robust mutexes */
#define EOWNERDEAD 130 /* Owner died */
#define ENOTRECOVERABLE 131 /* State not recoverable */
#define ERFKILL 132 /* Operation not possible due to RF-kill */
#define EHWPOISON 133 /* Memory page has hardware error */
#endif