关于errno的后事妥善处理

前言

当UNIX函数出现错误时，常常返回一个负值，而且整型变量errno通常被设置为含有附加信息的一个值。例如，open函数如果成功执行则返回一个非负文件描述符，如出错则返回-1.在open出错时，有大约15种不同的errno值（文件不存在、权限问题等）。某些函数并不返回负值而是使用另一种约定。例如，返回一个指向对象指针的大多数函数，在出错时，将返回一个NULL指针。

文件==<errno.h>==中定义了符号errno以及可以赋予的各个常量，这些常量都以字符E开头。例如，若errno等于常量EACESS，这表示产生了权限问题（例如，没有打开所要求文件的足够权限）。

POSIX和ISO C将errno定义为这一一个符号，它扩展成为一个可以修改的整型左值。这可以是包含出错编号的一个整数，或是一个返回出错编号指针的函数。以前使用的定义是：

extern int errno;

但是在支持线程的环境中，多个线程共享进程地址空间，每个线程都有属于自己的局部errno以避免一个线程干扰另一个线程。

对于errno应当知道两条规则。第一条规则：如果没有出错，则其值不会被一个例程清除。因此，仅当函数返回指明出错时，才检验其值。第二条：任何一个函数都不会将errno值设置为0，在<errno.h>中定义的所有常量都不为0。

C标准定义了两个函数，它们帮助打印出错信息。

#include <string.h>
char *strerror(int errnum);

此函数将errnum（它通常就是errno值）映射到一个出错信息字符串，并且返回此字符串的指针。

perror函数基于errno的当前值，在标准错误上产生一条出错信息，

#include <stdio.h>
 
void perror(const char *msg);

它首先输出由msg指向的字符串，然后是一个冒号，一个空格，接着是对应于errno值的出错信息，最后是一个换行符。

出错恢复

可与将<errno.h>中定义的各种出错分出致命性的和非致命性的两类。对于致命性的错误，无法执行恢复动作，最多只能在用户屏幕上打印一条出错信息，或者将一条出错信息写到日志文件中，然后终止。关于这点可以看我的：​​段错误？打的就是段错误！​​

而对于非致命性错误，有课可以较为妥善的处理。大多数非致命性错误在本质上是暂时的，例如资源短缺，当系统中活动较少时，这种出错很可能就不会发生。

与资源相关的非致命性出错包括EAGAIN. ENFILE、ENOBUFS、ENOLCK、ENOSPC、ENOSR、EWOULDBLOCK, 有时ENOMEM也是非致命性出错。当EBUSY指明共享资源正在使用时，也可将它作为非致命性出错处理。当EINTR中断- -慢速系统调用时，可将它作为非致命性出错处理。

对于资源相关的非致命性出错，一般恢复动作是延迟一些时间， 然后再试。这种技术可应用于其他情况。例如，假设-个 出错表明一个 网络连接不再起作用， 那么应用程序可以在短时间延迟后重建该连接。某些应用使用指数补偿算法，在每次重复中等待更长时间。

最后，取决于应用程序的开发者，他可以决定那些出错是可恢复的。如若使用一种从错误中恢复的合理策略，那么由于避免了应用程序的异常终止，就能改善应用程序的健壮性。

栗子

对于中断的read、write系 统调用，POSIX.I的语 义在该标准的2001版有所改变。对于如何处理已read、write部分数据量的相应系统调用，早期版本允许实现进行选择。如若read系统调用已接收并传送数据至应用程序缓冲区，但尚未接收到应用程序请求的全部数据，此时被中断、操作系统可以认为该系统调用失败、并将errno设置为EINTR;另一种处理方式是允许该系统调用成功返回，返回已接收到的部分数据量。与此类似，如若write巳传输了应用程序缓冲区中的部分数据，然后被中断，操作系统可以认为该系统调用失败，并将ermo设置为EINTR;另一种处理方式是允许该系统调用成功返回，返回已写的部分数据量。历史上，从系统V派生的实现，将这种系统调用视为失败，而BSD派生的实现则处理为部分成功返回。POSIX.1 标准的2001版采用BSD风格的语义。

假定一个读操作，它被中断，我们要恢复它：

again:
if(n= read(fd, buf, BUFSIZE)) < 0)
  if (errno == EINTR)
    goto again;/* just an interrupted system call * /
/* handle other errors */

errno表

其实看不看这个表意义不大，但是有的人他就是想看，那我就贴一份吧。

查看系统中所有的errno所代表的含义，可以采用如下的代码：

/* Function: obtain the errno string
*   char *strerror(int errno)
*/

#include <stdio.h>
#include <string.h>     //for strerror()
//#include <errno.h>
int main()
{
    int tmp = 0;
    for(tmp = 0; tmp <=256; tmp++)
    {
        printf("errno: %2d\t%s\n",tmp,strerror(tmp));
    }
    return 0;
}

输出信息如下：

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，在头文件 /usr/include/asm-generic/errno-base.h 对基础常用errno进行了宏定义：

#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，对剩余的errno做了宏定义：

#ifndef _ASM_GENERIC_ERRNO_H
#define _ASM_GENERIC_ERRNO_H

#include <asm-generic/errno-base.h>

#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