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 addressn");,　　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 : %sn”, h->h_name);,　　printf(“IP Address : %sn”, 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。,　　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linux 2.4.20-18的内核代码中的/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=%dn",errno);,　　char * mesg = strerror(errno);,　　printf("Mesg:%sn",mesg);,　　},　　exit(0);,　　},　　dsp设备忙的话将输出如下：,　　errno=16,　　Mesg:Device or resource busy,