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 */