Linux中errno使用

當linux中的C api函數發生異常時,一般會將errno變量(需include errno.h)賦一個整數值,不同的值表示不同的含義,可以通過查看該值推測出錯的原因，在實際編程中用這一招解決了不少原本看來莫名其妙的問題。但是errno是一個數字，代表的具體含義還要到errno.h中去閱讀宏定義，而每次查閱是一件很繁瑣的事情。有下面幾種方法可以方便的得到錯誤信息

  (1)void perror(const char *s)
函數說明
perror ( )用來將上一個函數發生錯誤的原因輸出到標准錯誤(stderr),參數s 所指的字符串會先打印出,後面再加上錯誤原因 字符串。此錯誤原因依照全局變量 errno 的值來決定要輸出的字符串。
  (2) char *strerror(int errno)
將錯誤代碼轉換為字符串錯誤信息,可以將該字符串和其它的信息組合輸出到用戶界面例如
fprintf(stderr,"error in CreateProcess %s, Process ID %d ",strerror(errno),processID)
注：假設processID是一個已經獲取了的整形ID
  (3)printf("%m", errno);
另外不是所有的地方發生錯誤的時候都可以通過error獲取錯誤代碼，例如下面的代碼段
/*注：下面的頭文件使用""而沒有直接使用尖括號是因為博客大巴中尖括號當作html符號，所以其內部的頭文件名字會被直接忽略*/
#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);
}
/* 取得主機信息 */
if((h=gethostbyname(argv[1])) == NULL)
{
/* 如果gethostbyname 失敗，則給出錯誤信息 */
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;
}

/*************************************/
  通過上面的代碼可以看到：使用gethostbyname()函數，你不能使用perror()來輸出錯誤信息（因為錯誤代碼存儲在 h_errno 中而不是errno 中。所以，你需要調用herror()函數。
你簡單的傳給gethostbyname() 一個機器名（“bbs.tsinghua.edu.cn”），然後就從返回的結構struct hostent 中得到了IP 等其他信息．程序中輸出IP 地址的程序需要解釋一下：h->h_addr 是一個char*，但是inet_ntoa()函數需要傳遞的是一個struct in_addr 結構。所以上面將h->h_addr 強制轉換為struct in_addr*，然後通過它得到了所有數據。

 

errno.h中定義的錯誤代碼值如下：

查 看錯誤代碼errno是調試程序的一個重要方法。當linuc C api函數發生異常時,一般會將errno變量(需include errno.h)賦一個整數值,不同的值表示不同的含義,可以通過查看該值推測出錯的原因。在實際編程中用這一招解決了不少原本看來莫名其妙的問題。比較 麻煩的是每次都要去linux源代碼裡面查找錯誤代碼的含義，現在把它貼出來，以後需要查時就來這裡看了。
以下來自linux 2.4.20-18的內核代碼中的/usr/include/asm/errno.h
#ifndef _I386_ERRNO_H
#define _I386_ERRNO_H
#define EPERM   1 /* Operation not permitted */
#define ENOENT   2 /* No such file or directory */
#define ESRCH   3 /* No such process */
#define EINTR   4 /* Interrupted system call */
#define EIO       5 /* I/O error */
#define ENXIO   6 /* No such device or address */
#define E2BIG   7 /* Arg list too long */
#define ENOEXEC   8 /* Exec format error */
#define EBADF   9 /* Bad file number */
#define ECHILD 10 /* No child processes */
#define EAGAIN 11 /* Try again */
#define ENOMEM 12 /* Out of memory */
#define EACCES 13 /* Permission denied */
#define EFAULT 14 /* Bad address */
#define ENOTBLK 15 /* Block device required */
#define EBUSY 16 /* Device or resource busy */
#define EEXIST 17 /* File exists */
#define EXDEV 18 /* Cross-device link */
#define ENODEV 19 /* No such device */
#define ENOTDIR 20 /* Not a directory */
#define EISDIR 21 /* Is a directory */
#define EINVAL 22 /* Invalid argument */
#define ENFILE 23 /* File table overflow */
#define EMFILE 24 /* Too many open files */
#define ENOTTY 25 /* Not a typewriter */
#define ETXTBSY 26 /* Text file busy */
#define EFBIG 27 /* File too large */
#define ENOSPC 28 /* No space left on device */
#define ESPIPE 29 /* Illegal seek */
#define EROFS 30 /* Read-only file system */
#define EMLINK 31 /* Too many links */
#define EPIPE 32 /* Broken pipe */
#define EDOM 33 /* Math argument out of domain of func */
#define ERANGE 34 /* Math result not representable */
#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 */
#endif

同時也可以使用strerror（）來自己翻譯
如：
#include <stdio.h>
#include <string.h>
#include <errno.h>
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設備忙的話將輸出如下：
errno=16
Mesg:Device or resource busy