原文链接：http://blog.chinaunix.net/xmlrpc.php?r=blog/article&uid=116213&id=3376727
最近在网络编程使用的过程中，发现errno会经常使用。因此决定在此做个留用，以备以后使用。。。
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<num; i++="" )<br=""> {
printf( "%d:%s
", 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