Linux中errno使用

当linux中的C api函数发生异常时,一般会将errno变量(需include errno.h)赋一个整数值,不同的值表示不同的含义,可以通过查看该值推测出错的原因，在实际编程中用这一招解决了不少原本看来莫名其妙的问题。但是errno是一个数字，代表的具体含义还要到errno.h中去阅读宏定义，而每次查阅是一件很繁琐的事情。有下面几种方法可以方便的得到错误信息
(1)void perror(const char *s)
函数说明
perror ( )用来将上一个函数发生错误的原因输出到标准错误(stderr),参数s 所指的字符串会先打印出,后面再加上错误原因 字符串。此错误原因依照全局变量 errno 的值来决定要输出的字符串。
(2) char *strerror(int errno)
将错误代码转换为字符串错误信息,可以将该字符串和其它的信息组合输出到用户界面例如
fprintf(stderr,"error in CreateProcess %s, Process ID %d ",strerror(errno),processID)
注：假设processID是一个已经获取了的整形ID
(3)printf("%m", errno);
另外不是所有的地方发生错误的时候都可以通过error获取错误代码，例如下面的代码段
/*注：下面的头文件使用""而没有直接使用尖括号是因为博客大巴中尖括号当作html符号，所以其内部的头文件名字会被直接忽略*/
#include"stdio.h"
#include "stdlib.h"
#include "errno.h"
#include "netdb.h"
#include "sys/types.h"
#include "netinet/in.h"
int main (int argc, char *argv[])
{
struct hostent *h;
if (argc != 2)
{
fprintf (stderr ,"usage: getip address\n");
exit(1);
}
/* 取得主机信息 */
if((h=gethostbyname(argv[1])) == NULL)
{
/* 如果gethostbyname 失败，则给出错误信息 */
herror(“gethostbyname”);
exit(1);
}
/* 列印程序取得的信息 */
printf(“Host name : %s\n”, h->h_name);
printf(“IP Address : %s\n”, inet_ntoa (*((struct in_addr *)h->h_addr))）;
return 0;
}

/*************************************/
通过上面的代码可以看到：使用gethostbyname()函数，你不能使用perror()来输出错误信息（因为错误代码存储在 h_errno 中而不是errno 中。所以，你需要调用herror()函数。
你简单的传给gethostbyname() 一个机器名（“bbs.tsinghua.edu.cn”），然后就从返回的结构struct hostent 中得到了IP 等其他信息．程序中输出IP 地址的程序需要解释一下：h->h_addr 是一个char*，但是inet_ntoa()函数需要传递的是一个struct in_addr 结构。所以上面将h->h_addr 强制转换为struct in_addr*，然后通过它得到了所有数据。

from http://wzgyantai.blogbus.com/logs/24470871.html

----------------------------------------------

errno.h中定义的错误代码值如下：

查 看错误代码errno是调试程序的一个重要方法。当linuc C api函数发生异常时,一般会将errno变量(需include errno.h)赋一个整数值,不同的值表示不同的含义,可以通过查看该值推测出错的原因。在实际编程中用这一招解决了不少原本看来莫名其妙的问题。比较 麻烦的是每次都要去linux源代码里面查找错误代码的含义，现在把它贴出来，以后需要查时就来这里看了。
以下来自的linux2.6.18-53.e15内核代码中的/usr/include/asm/errno.h

#ifndef _I386_ERRNO_H

#define _I386_ERRNO_H

#include

#endif

/usr/include/asm-generic/errno.h中内容如下：

#ifndef _ASM_GENERIC_ERRNO_H

#define _ASM_GENERIC_ERRNO_H

#include

#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 NFS 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 */

#endif

同时也可以使用strerror（）来自己翻译
如：
#include
#include
#include  
int main(void)
{
int   fd;
extern int errno;

if((fd = open("/dev/dsp",O_WRONLY)) < 0)
{
printf("errno=%d\n",errno); 
char * mesg = strerror(errno);
printf("Mesg:%s\n",mesg); 
}

exit(0);
}
dsp设备忙的话将输出如下：
errno=16
Mesg:Device or resource busy