NAME

编辑
errno - number of last error
if (somecall() == -1) {
printf("somecall() failed\n");
if (errno == ...) { ... }
}
这样的例子并不能得到somecall这个函数的运行所产生的错误代码，因为很可能是printf这个函数产生的。
if (somecall() == -1) {
int errsv = errno;
printf("somecall() failed\n");
if (errsv == ...) { ... }
这样才能真正得到运行somecall函数所带来的错误代码。
}
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。

errno的一些错误定义

编辑
以下主要来自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 */