前言
當(dāng)UNIX函數(shù)出現(xiàn)錯(cuò)誤時(shí)，常常返回一個(gè)負(fù)值，而且整型變量errno通常被設(shè)置為含有附加信息的一個(gè)值。例如，open函數(shù)如果成功執(zhí)行則返回一個(gè)非負(fù)文件描述符，如出錯(cuò)則返回-1.在open出錯(cuò)時(shí)，有大約15種不同的errno值（文件不存在、權(quán)限問題等）。某些函數(shù)并不返回負(fù)值而是使用另一種約定。例如，返回一個(gè)指向?qū)ο笾羔樀拇蠖鄶?shù)函數(shù)，在出錯(cuò)時(shí)，將返回一個(gè)NULL指針。
文件====中定義了符號errno以及可以賦予的各個(gè)常量，這些常量都以字符E開頭。例如，若errno等于常量EACESS，這表示產(chǎn)生了權(quán)限問題（例如，沒有打開所要求文件的足夠權(quán)限）。
POSIX和ISO C將errno定義為這一一個(gè)符號，它擴(kuò)展成為一個(gè)可以修改的整型左值。這可以是包含出錯(cuò)編號的一個(gè)整數(shù)，或是一個(gè)返回出錯(cuò)編號指針的函數(shù)。以前使用的定義是：
extern int errno;
但是在支持線程的環(huán)境中，多個(gè)線程共享進(jìn)程地址空間，
每個(gè)線程都有屬于自己的局部errno以避免一個(gè)線程干擾另一個(gè)線程。
對于errno應(yīng)當(dāng)知道兩條規(guī)則。第一條規(guī)則：如果沒有出錯(cuò)，則其值不會(huì)被一個(gè)例程清除。因此，僅當(dāng)函數(shù)返回指明出錯(cuò)時(shí)，才檢驗(yàn)其值。第二條：任何一個(gè)函數(shù)都不會(huì)將errno值設(shè)置為0，在中定義的所有常量都不為0。
C標(biāo)準(zhǔn)定義了兩個(gè)函數(shù)，它們幫助打印出錯(cuò)信息。
#include char *strerror(int errnum);
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此函數(shù)將errnum（它通常就是errno值）映射到一個(gè)出錯(cuò)信息字符串，并且返回此字符串的指針。
perror函數(shù)基于errno的當(dāng)前值，在標(biāo)準(zhǔn)錯(cuò)誤上產(chǎn)生一條出錯(cuò)信息，
#include void perror(const char *msg);
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它首先輸出由msg指向的字符串，然后是一個(gè)冒號，一個(gè)空格，接著是對應(yīng)于errno值的出錯(cuò)信息，最后是一個(gè)換行符。
出錯(cuò)恢復(fù)
可與將中定義的各種出錯(cuò)分出
致命性的
和
非致命性的
兩類。對于致命性的錯(cuò)誤，無法執(zhí)行恢復(fù)動(dòng)作，最多只能在用戶屏幕上打印一條出錯(cuò)信息，或者將一條出錯(cuò)信息寫到日志文件中，然后終止。關(guān)于這點(diǎn)可以看我的：段錯(cuò)誤？打的就是段錯(cuò)誤！
而對于非致命性錯(cuò)誤，有課可以較為妥善的處理。大多數(shù)非致命性錯(cuò)誤在本質(zhì)上是暫時(shí)的，例如資源短缺，當(dāng)系統(tǒng)中活動(dòng)較少時(shí)，這種出錯(cuò)很可能就不會(huì)發(fā)生。
與資源相關(guān)的非致命性出錯(cuò)包括EAGAIN. ENFILE、ENOBUFS、ENOLCK、ENOSPC、ENOSR、EWOULDBLOCK, 有時(shí)ENOMEM也是非致命性出錯(cuò)。當(dāng)EBUSY指明共享資源正在使用時(shí)，也可將它作為非致命性出錯(cuò)處理。當(dāng)EINTR中斷- -慢速系統(tǒng)調(diào)用時(shí)，可將它作為非致命性出錯(cuò)處理。
對于資源相關(guān)的非致命性出錯(cuò)，一般恢復(fù)動(dòng)作是延遲一些時(shí)間， 然后再試。這種技術(shù)可應(yīng)用于其他情況。例如，假設(shè)-個(gè) 出錯(cuò)表明一個(gè) 網(wǎng)絡(luò)連接不再起作用， 那么應(yīng)用程序可以在短時(shí)間延遲后重建該連接。某些應(yīng)用使用指數(shù)補(bǔ)償算法，在每次重復(fù)中等待更長時(shí)間。
最后，取決于應(yīng)用程序的開發(fā)者，他可以決定那些出錯(cuò)是可恢復(fù)的。如若使用一種從錯(cuò)誤中恢復(fù)的合理策略，那么由于避免了應(yīng)用程序的異常終止，就能改善應(yīng)用程序的健壯性。
栗子
對于中斷的read、write系 統(tǒng)調(diào)用，POSIX.I的語 義在該標(biāo)準(zhǔn)的2001版有所改變。對于如何處理已read、write部分?jǐn)?shù)據(jù)量的相應(yīng)系統(tǒng)調(diào)用，早期版本允許實(shí)現(xiàn)進(jìn)行選擇。如若read系統(tǒng)調(diào)用已接收并傳送數(shù)據(jù)至應(yīng)用程序緩沖區(qū)，但尚未接收到應(yīng)用程序請求的全部數(shù)據(jù)，此時(shí)被中斷、操作系統(tǒng)可以認(rèn)為該系統(tǒng)調(diào)用失敗、并將errno設(shè)置為EINTR;另一種處理方式是允許該系統(tǒng)調(diào)用成功返回，返回已接收到的部分?jǐn)?shù)據(jù)量。與此類似，如若write巳傳輸了應(yīng)用程序緩沖區(qū)中的部分?jǐn)?shù)據(jù)，然后被中斷，操作系統(tǒng)可以認(rèn)為該系統(tǒng)調(diào)用失敗，并將ermo設(shè)置為EINTR;另一種處理方式是允許該系統(tǒng)調(diào)用成功返回，返回已寫的部分?jǐn)?shù)據(jù)量。歷史上，從系統(tǒng)V派生的實(shí)現(xiàn)，將這種系統(tǒng)調(diào)用視為失敗，而BSD派生的實(shí)現(xiàn)則處理為部分成功返回。POSIX.1 標(biāo)準(zhǔn)的2001版采用BSD風(fēng)格的語義。
假定一個(gè)讀操作，它被中斷，我們要恢復(fù)它：
again: if(n= read(fd, buf, BUFSIZE)) < 0) if (errno == EINTR) goto again;/* just an interrupted system call * / /* handle other errors */
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errno表
其實(shí)看不看這個(gè)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(tǒng)中所有的errno所代表的含義，可以采用如下的代碼： /* Function: obtain the errno string * char *strerror(int errno) */ #include #include //for strerror() //#include 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(chǔ)常用errno進(jìn)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，對剩余的errno做了宏定義： #ifndef _ASM_GENERIC_ERRNO_H #define _ASM_GENERIC_ERRNO_H #include #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
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