注意：只有当一个库函数失败时，errno才会被设置。当函数成功运行时，errno的值不会被修改。这意味着我们不能通过测试errno的值来判断是否有错误存在。反之，只有当被调用的函数提示有错误发生时检查errno的值才有意义。

查看错误代码errno是调试程序的一个重要方法。当linux C api函数发生异常时,一般会将errno变量(需include errno.h)赋一个整数值,不同的值表示不同的含义,可以通过查看该值推测出错的原因。在实际编程中用这一招解决了不少原本看来莫名其妙的问题。

以下来主要自2.6.32的内核代码中的/usr/include/asm-generic/errno.h及errno-base.h，输出错误原因定义归纳整理如下：

#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 */

#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 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 */

#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 */