（二三）errno變量 · C語言天天學

引言：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，那只能說明你的程序不夠健壯。?為什么會使用errno呢？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。但是對于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，但是發生什么錯誤卻毫無辦法。于是，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。問題：很多庫函數，當執行失敗時會通過一個名稱為errno的外部變量，通知程序該函數調用失敗。該如何利用errno這個外部變量檢查錯誤發生的原因呢？解決辦法：錯誤的方法一： ~~~ /*調用庫函數*/ if(errno) /*處理錯誤*/ ~~~ 錯誤原因：在庫函數調用沒有失敗的情況下，并沒有強制要求庫函數一定要設置errno為0，這樣errno的值可能是前一個執行失敗的庫函數設置的值。程序器啟動的似乎errno的初始值是零。許多庫函數在遭遇一些確定的錯誤以后將保證設置它為某個確定的非零值。當這些函數運行成功的時候它將不會修改 errno的值；因此，當一個成功的調用以后errno的值不必要是零，而且你不能用errno來判斷一次調用是否失敗了。正確的做法是檢查每個函數報道的結果，如果調用失敗了，再來檢查errno。錯誤的方法二： ~~~ errno = 0; /*調用庫函數*/ if(errno) /*處理錯誤*/ ~~~ 錯誤原因：庫函數在調用成功時，既沒有強制要求對errno清零，但同時也沒有禁止設置errno。正確方法： ~~~ /*調用庫函數*/ if(返回的錯誤值) 檢查errno ~~~ 參考： 1、《C陷阱與缺陷》 2、[http://blog.csdn.net/wang_517766334/article/details/7561495](http://blog.csdn.net/wang_517766334/article/details/7561495) 3、[http://blog.csdn.net/romandion/article/details/2036975](http://blog.csdn.net/romandion/article/details/2036975) 附錄：errno變量可設置為的值及每個值代表的意義。 ~~~ #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 */ ~~~