linux中errno使用(转)

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);

#include <math.h>
#include <errno.h>
#include <stdio.h>

int main(void)
{
    errno = 0;
    int s = sqrt(-1);
    if (errno) {
　　　　  //使用printf打印错误号
        printf("errno = %d\n", errno); // errno = 33
　　　　　//使用perror打印错误信息
        perror("sqrt failed"); 　　　　 // sqrt failed: Numerical argument out of domain
         //使用strerror打印错误信息
        printf("error: %s\n", strerror(errno)); // error: Numerical argument out of domain
    }

    return 0;
}

　　

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

#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);
}

if((h=gethostbyname(argv[1])) == NULL)
{

herror(“gethostbyname”);
exit(1);
}

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。

ERRNO的定义

#ifndef _SYS_ERRNO_H_
#define _SYS_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  /* Input/output error */
#define ENXIO    6  /* Device not configured */
#define E2BIG    7  /* Argument list too long */
#define ENOEXEC  8  /* Exec format error */
#define EBADF    9  /* Bad file descriptor */
#define ECHILD   10  /* No child processes */
#define EDEADLK  11  /* Resource deadlock avoided */
                     /* 11 was EAGAIN */
#define ENOMEM   12  /* Cannot allocate memory */
#define EACCES   13  /* Permission denied */
#define EFAULT   14  /* Bad address */
#define ENOTBLK  15  /* Block device required */
#define EBUSY    16  /* Device busy */
#define EEXIST   17  /* File exists */
#define EXDEV    18  /* Cross-device link */
#define ENODEV   19  /* Operation not supported by device */
#define ENOTDIR  20  /* Not a directory */
#define EISDIR   21  /* Is a directory */
#define EINVAL   22  /* Invalid argument */
#define ENFILE   23  /* Too many open files in system */
#define EMFILE   24  /* Too many open files */
#define ENOTTY   25  /* Inappropriate ioctl for device */
#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 */
/* math software */
#define EDOM     33  /* Numerical argument out of domain */
#define ERANGE   34  /* Result too large or too small */
/* non-blocking and interrupt i/o */
#define EAGAIN   35  /* Resource temporarily unavailable */
#define EWOULDBLOCK EAGAIN  /* Operation would block */
#define EINPROGRESS 36  /* Operation now in progress */
#define EALREADY    37  /* Operation already in progress */
/* ipc/network software -- argument errors */
#define ENOTSOCK    38  /* Socket operation on non-socket */
#define EDESTADDRREQ 39  /* Destination address required */
#define EMSGSIZE     40  /* Message too long */
#define EPROTOTYPE   41  /* Protocol wrong type for socket */
#define ENOPROTOOPT  42  /* Protocol not available */
#define EPROTONOSUPPORT 43  /* Protocol not supported */
#define ESOCKTNOSUPPORT 44  /* Socket type not supported */
#define EOPNOTSUPP   45  /* Operation not supported */
#define EPFNOSUPPORT 46  /* Protocol family not supported */
#define EAFNOSUPPORT 47  /* Address family not supported by protocol family */
#define EADDRINUSE   48  /* Address already in use */
#define EADDRNOTAVAIL 49  /* Can't assign requested address */
/* ipc/network software -- operational errors */
#define ENETDOWN     50  /* Network is down */
#define ENETUNREACH  51  /* Network is unreachable */
#define ENETRESET    52  /* Network dropped connection on reset */
#define ECONNABORTED 53  /* Software caused connection abort */
#define ECONNRESET   54  /* Connection reset by peer */
#define ENOBUFS  55  /* No buffer space available */
#define EISCONN  56  /* Socket is already connected */
#define ENOTCONN 57  /* Socket is not connected */
#define ESHUTDOWN    58  /* Can't send after socket shutdown */
#define ETOOMANYREFS 59  /* Too many references: can't splice */
#define ETIMEDOUT    60  /* Operation timed out */
#define ECONNREFUSED 61  /* Connection refused */
#define ELOOP    62  /* Too many levels of symbolic links */
#define ENAMETOOLONG 63  /* File name too long */
/* should be rearranged */
#define EHOSTDOWN    64  /* Host is down */
#define EHOSTUNREACH 65  /* No route to host */
#define ENOTEMPTY    66  /* Directory not empty */
/* quotas & mush */
#define EPROCLIM     67  /* Too many processes */
#define EUSERS       68  /* Too many users */
#define EDQUOT   69  /* Disc quota exceeded */
/* Network File System */
#define ESTALE   70  /* Stale NFS file handle */
#define EREMOTE  71  /* Too many levels of remote in path */
#define EBADRPC  72  /* RPC struct is bad */
#define ERPCMISMATCH 73  /* RPC version wrong */
#define EPROGUNAVAIL 74  /* RPC prog. not avail */
#define EPROGMISMATCH 75  /* Program version wrong */
#define EPROCUNAVAIL 76  /* Bad procedure for program */
#define ENOLCK   77  /* No locks available */
#define ENOSYS   78  /* Function not implemented */
#define EFTYPE   79  /* Inappropriate file type or format */
#define EAUTH    80  /* Authentication error */
#define ENEEDAUTH 81  /* Need authenticator */
/* SystemV IPC */
#define EIDRM    82  /* Identifier removed */
#define ENOMSG   83  /* No message of desired type */
#define EOVERFLOW 84  /* Value too large to be stored in data type */
/* Wide/multibyte-character handling, ISO/IEC 9899/AMD1:1995 */
#define EILSEQ   85  /* Illegal byte sequence */
/* From IEEE Std 1003.1-2001 */
/* Base, Realtime, Threads or Thread Priority Scheduling option errors */
#define ENOTSUP  86  /* Not supported */
/* Realtime option errors */
#define ECANCELED 87  /* Operation canceled */
/* Realtime, XSI STREAMS option errors */
#define EBADMSG  88  /* Bad or Corrupt message */
/* XSI STREAMS option errors  */
#define ENODATA  89  /* No message available */
#define ENOSR  90  /* No STREAM resources */
#define ENOSTR  91  /* Not a STREAM */
#define ETIME  92  /* STREAM ioctl timeout */
#define ELAST  92  /* Must equal largest errno */
#ifdef _KERNEL
/* pseudo-errors returned inside kernel to modify return to process */
#define EJUSTRETURN -2  /* don't modify regs, just return */
#define ERESTART -3  /* restart syscall */
#define EPASSTHROUGH -4  /* ioctl not handled by this layer */
#endif
#endif /* !_SYS_ERRNO_H_ */