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 虽然errno是非线程安全的，但是可以通过几种机制保证其安全。。

 最近在使用的过程中获得了errno，程序无法执行，也不知道如何解决问题。因此，理解每一个返回的errno整数值的含义是很重要的。。

这个我们可以通过strerror函数来实现。。

strerror(返回整数errno对应的错误原因的描述字符串)

所需头文件：  #include
函数声明：    char * strerror(int errnum);

输入：          整数errno

输出：         返回描述错误原因的字符串指针
功能：         用来依参数errnum 的错误代码来查询其错误原因的描述字符串,然后将该字符串指针返回。
示例：
#include
#include

#define NUM    150
int main(void)
{
    int i;    
    for ( i=0; i    {
        printf( "%d:%s\n", i, strerror(i) );
    }       
    return 0;
}

运行结果：

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
127:Key has expired
128:Key has been revoked
129:Key was rejected by service
130:Owner died
131:State not recoverable
132:Unknown error 132
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
140:Unknown error 140
141:Unknown error 141
142:Unknown error 142
143:Unknown error 143
144:Unknown error 144
145:Unknown error 145
146:Unknown error 146
147:Unknown error 147
148:Unknown error 148
149:Unknown error 149

通过这个我们终于理解errno的含义了。。那天做的一个使用非阻塞connect式爬虫，无论如何也不好使，也不知道怎么解决，程序并没有出现段错误，最终定位在write套接字时就退出，而通过FD_ISSET测试此套接字可写，于是不知道怎么解决了。通过捕捉errno值，发现是104.而如果当时我可以知道errno=104是指“Connection reset by peer”，那么我就知道了网络爬虫式客户端需要增加对于SIGPIPE信号的处理了。。因为SIGPIPE默认处理方式是退出，改成自己添加信号处理函数或者使用忽略就可以了。。。

所以列出这些信息，一方面能对大家有所帮助，另一方面方便以后查询。

   1#ifndef _ASM_GENERIC_ERRNO_BASE_H
   2#define _ASM_GENERIC_ERRNO_BASE_H
   3
   4#define EPERM            1      /* Operation not permitted */
   5#define ENOENT           2      /* No such file or directory */
   6#define ESRCH            3      /* No such process */
   7#define EINTR            4      /* Interrupted system call */
   8#define EIO              5      /* I/O error */
   9#define ENXIO            6      /* No such device or address */
  10#define E2BIG            7      /* Argument list too long */
  11#define ENOEXEC          8      /* Exec format error */
  12#define EBADF            9      /* Bad file number */
  13#define ECHILD          10      /* No child processes */
  14#define EAGAIN          11      /* Try again */
  15#define ENOMEM          12      /* Out of memory */
  16#define EACCES          13      /* Permission denied */
  17#define EFAULT          14      /* Bad address */
  18#define ENOTBLK         15      /* Block device required */
  19#define EBUSY           16      /* Device or resource busy */
  20#define EEXIST          17      /* File exists */
  21#define EXDEV           18      /* Cross-device link */
  22#define ENODEV          19      /* No such device */
  23#define ENOTDIR         20      /* Not a directory */
  24#define EISDIR          21      /* Is a directory */
  25#define EINVAL          22      /* Invalid argument */
  26#define ENFILE          23      /* File table overflow */
  27#define EMFILE          24      /* Too many open files */
  28#define ENOTTY          25      /* Not a typewriter */
  29#define ETXTBSY         26      /* Text file busy */
  30#define EFBIG           27      /* File too large */
  31#define ENOSPC          28      /* No space left on device */
  32#define ESPIPE          29      /* Illegal seek */
  33#define EROFS           30      /* Read-only file system */
  34#define EMLINK          31      /* Too many links */
  35#define EPIPE           32      /* Broken pipe */
  36#define EDOM            33      /* Math argument out of domain of func */
  37#define ERANGE          34      /* Math result not representable */
  38
  39#endif