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_ */