本篇记录c语言中error的用法，学习中后面会继续更新

C语言中用于保存open函数执行错误代号的变量为errno(error number)，每种打开错误都会返回一个错误代号，并且保存在error这个变量中。

如打开文件已存在，error的值为2。其指代的错误为EEXIST          File exists (POSIX.1)

如果想要将errno数值，以其对应的错误解析输出的话，程序需要调用strerror(errno)这个函数。此函数会直接返回errno的错误信息。

将errno以及strerror两者公用之后的效果与perror相同。

用errno可以用于分析所有open失败的情况。

以下参考于http://blog.youkuaiyun.com/jediael_lu/article/details/8589194

   0:Success
   1:Operation not permitted
   2:No such file or directory
   3:No such process
   4:Interrupted system call
   5:Input/output error
   6:No such device or address
   7:Argument list too long
   8:Exec format error
   9:Bad file descriptor
  10:No child processes
  11:Resource temporarily unavailable
  12:Cannot allocate memory
  13:Permission denied
  14:Bad address
  15:Block device required
  16:Device or resource busy
  17:File exists
  18:Invalid cross-device link
  19:No such device
  20:Not a directory
  21:Is a directory
  22:Invalid argument
  23:Too many open files in system
  24:Too many open files
  25:Inappropriate ioctl for device
  26:Text file busy
  27:File too large
  28:No space left on device
  29:Illegal seek
  30:Read-only file system
  31:Too many links
  32:Broken pipe
  33:Numerical argument out of domain
  34:Numerical result out of range
  35:Resource deadlock avoided
  36:File name too long
  37:No locks available
  38:Function not implemented
  39:Directory not empty
  40:Too many levels of symbolic links
  41:Unknown error 41
  42:No message of desired type
  43:Identifier removed
  44:Channel number out of range
  45:Level 2 not synchronized
  46:Level 3 halted
  47:Level 3 reset
  48:Link number out of range
  49:Protocol driver not attached
  50:No CSI structure available
  51:Level 2 halted
  52:Invalid exchange
  53:Invalid request descriptor
  54:Exchange full
  55:No anode
  56:Invalid request code
  57:Invalid slot
  58:Unknown error 58
  59:Bad font file format
  60:Device not a stream
  61:No data available
  62:Timer expired
  63:Out of streams resources
  64:Machine is not on the network
  65:Package not installed
  66:Object is remote
  67:Link has been severed
  68:Advertise error
  69:Srmount error
  70:Communication error on send
  71:Protocol error
  72:Multihop attempted
  73:RFS specific error
  74:Bad message
  75:Value too large for defined data type
  76:Name not unique on network
  77:File descriptor in bad state
  78:Remote address changed
  79:Can not access a needed shared library
  80:Accessing a corrupted shared library
  81:.lib section in a.out corrupted
  82:Attempting to link in too many shared libraries
  83:Cannot exec a shared library directly
  84:Invalid or incomplete multibyte or wide character
  85:Interrupted system call should be restarted
  86:Streams pipe error
  87:Too many users
  88:Socket operation on non-socket
  89:Destination address required
  90:Message too long
  91:Protocol wrong type for socket
  92:Protocol not available
  93:Protocol not supported
  94:Socket type not supported
  95:Operation not supported
  96:Protocol family not supported
  97:Address family not supported by protocol
  98:Address already in use
  99:Cannot assign requested address
 100:Network is down
 101:Network is unreachable
 102:Network dropped connection on reset
 103:Software caused connection abort
 104:Connection reset by peer
 105:No buffer space available
 106:Transport endpoint is already connected
 107:Transport endpoint is not connected
 108:Cannot send after transport endpoint shutdown
 109:Too many references: cannot splice
 110:Connection timed out
 111:Connection refused
 112:Host is down
 113:No route to host
 114:Operation already in progress
 115:Operation now in progress
 116:Stale NFS file handle
 117:Structure needs cleaning
 118:Not a XENIX named type file
 119:No XENIX semaphores available
 120:Is a named type file
 121:Remote I/O error
 122:Disk quota exceeded
 123:No medium found
 124:Wrong medium type
 125:Operation canceled
 126:Required key not available

printf("");

 127:Key has expired
 128:Key has been revoked
 129:Key was rejected by service
 130:Owner died
 131:State not recoverable
 132:Operation not possible due to RF-kill
 133:Unknown error 133
 134:Unknown error 134
 135:Unknown error 135
 136:Unknown error 136
 137:Unknown error 137
 138:Unknown error 138
 139:Unknown error 139

==================================2017/2/24添加==================================

之前学会用errno这个方法之后，就经常把perror替换成printf("%s",strerror(errno));这样的形式，但是有时候要打很长代码的时候又发现很麻烦，又换了过来直接用perror输出，发现errno很累赘，貌似完全可以被perror函数替代。

前几天遇到个问题，我在arm开发板上搭建了一个boa服务器，需要用到cgi编程

在cgi编程的时候发现了一个问题，我的网站需要登陆才能访问，使用过cgi搭建boa网站的人应该知道所有的html标签都是c语言通过printf输出到浏览器，浏览器捕获到这些标记语言显示对应的内容，例如：

printf("<p>Hello, world!</p>",);

可以在网页中输出 "hello world!" 的字样。

但是我要看到我的代码运行到什么位置的时候，就必须要用页面访问的方式，才可以，因为我在页面中加入了html中post获取，如果直接在arm板上运行的话，获取不到post数据就会出现错误没办法正常运行（后来发现这也只是因为post操作之后，没有判断获取的数据如果为NULL的操作才会出现的错误）。

如果用perror输出的话，访问的时候也不会在页面中显示，因为perror是对标准输出文件的输出，但是浏览器并不是ARM开发板的标准输出，错误信息不会显示在浏览器中。

后来我想起了errno。原来errno是用在这种场合

printf('<script>alert("error information:%s")</script>',strerror(errno));

只要这样的话，所有的错误都可以借用script 的alert方法将错误信息以弹框的方式显示出来。如果遇到了用不了调试器，又没办法简单的perror的话，用errno这种方式将数据转换成%s字符串输出，会方便很多！

tips
errno    头文件  errno.h
strerror 头文件  string.h