errno简介
Linux中系统调用的错误都存储于errno中，errno由操作系统维护，存储就近发生的错误，即下一次的错误码会覆盖掉上一次的错误。
errno是一个包含在<errno.h>中的预定义的外部int变量，用于表示最近一个函数调用是否产生了错误。若为0，则无错误，其它值均表示一类错误。
Linux中系统调用的错误都存储于 errno中，errno由操作系统维护，存储就近发生的错误，即下一次的错误码会覆盖掉上一次的错误。
编程时需要包含#include <errno.h>，可以直接查看errno的值例如：
#include <errno.h>
当一个系统调用或着库函数的调用失败时，将会重置错误代码errno。用户在判断程序出错后，立即检验errno的值可以获取错误代码和错误信息。
errno是线程安全的吗
是的，它是线程安全的。在Linux上，全局errno变量是特定于线程的。POSIX要求errno必须是线程安全的。
参见http://www.unix.org/whitepapers/reentrant.html
在POSIX.1中，errno被定义为外部全局变量。但是此定义在多线程环境中是不可接受的，因为使用它会导致不确定的结果。问题是两个或多个线程可能会遇到错误，所有错误都会导致设置相同的错误号。在这种情况下，一个线程可能已经被另一个线程更新后，最终检查errno。
为了避免产生不确定性，POSIX.1c将errno重新定义为可以访问每个线程错误号的服务，如下所示（ISO / IEC 9945：1-1996，§2.4）：
某些函数可能在通过符号errno访问的变量中提供错误号。errno符号是通过包括C标准所指定的标头来定义的。对于进程的每个线程，errno的值不应受函数调用或其他线程对errno的分配的影响。
另请参阅http://linux.die.net/man/3/errno
errno是线程本地的；在一个线程中设置它不会影响在其他任何线程中的值
我们可以通过在机器上运行一个简单的程序来进行检查。
#include <stdio.h>
#include <pthread.h>
#include <errno.h>
#define NTHREADS 5
void *thread_function(void *);
int
main()
{
pthread_t thread_id[NTHREADS];
int i, j;
for(i=0; i < NTHREADS; i++)
{
pthread_create( &thread_id[i], NULL, thread_function, NULL );
}
for(j=0; j < NTHREADS; j++)
{
pthread_join( thread_id[j], NULL);
}
return 0;
}
void *thread_function(void *dummyPtr)
{
printf("Thread number %ld addr(errno):%p\n", pthread_self(), &errno);
}output:
Thread number 140672336922368 addr(errno):0x7ff0d4ac0698
Thread number 140672345315072 addr(errno):0x7ff0d52c1698
Thread number 140672328529664 addr(errno):0x7ff0d42bf698
Thread number 140672320136960 addr(errno):0x7ff0d3abe698
Thread number 140672311744256 addr(errno):0x7ff0d32bd698 含义：
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <string.h>
int main(void){
for(int i = 0; i < 140; i++){
printf("%4d:%s\n", i, strerror(i));
}
exit(0);
}
output:
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: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