关于errno的后事妥善处理看，未来的博客-

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: %2dt%sn",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