前言
之前在Android的开发中，调试代码可以使用logcat,很是方便，但在一般的Linux C/C++开发中，就是用不上logcat了，不过我们linux自身提供了相关的功能，今天就对errno进行讨论一下：它定义在errno.h 头文件中，我看有一下，在linux不同的内核版本中errno.h的具体位置调整了一下，目前我使用的是Ubuntu18.04. LTS,基于linux的内核版本为Linux version 5.3.0-26-generic (buildd@lgw01-amd64-039)，我们可以在ubuntu上可以直接看到和使用Linux的API；我们的errno.h被分成了errno-base.h和errno.h两个文件。
tips
如何查看Ubuntu的内核版本？在Ubuntu中，linux版本信息保存在/proc/version;我们之间在终端键入一下命令即可查看
cat /proc/version
errno.h
先上原文件；我们为了叙述方便，将errno-base.h和errno.h文件放在一起说。【errno-base.h和errno.h的本来就是将linux旧版本中的errno.h中分割出来的】
/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
#ifndef _ASM_GENERIC_ERRNO_BASE_H
#define _ASM_GENERIC_ERRNO_BASE_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 /* Argument 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 */errno-base.h和errno.h文件放在一起说。【errno-base.h和errno.h的本来就是将linux了旧版本中的errno.h中分割出来的】
#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 */
/*
* This error code is special: arch syscall entry code will return
* -ENOSYS if users try to call a syscall that doesn't exist. To keep
* failures of syscalls that really do exist distinguishable from
* failures due to attempts to use a nonexistent syscall, syscall
* implementations should refrain from returning -ENOSYS.
*/
#define ENOSYS 38 /* Invalid system call number */
#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 reequest 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 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 */
#define ERFKILL 132 /* Operation not possible due to RF-kill */
#define EHWPOISON 133 /* Memory page has hardware error */
#endif
#endif
errno是啥
errno是个啥呢？上文的文件并没有直接定义errno是个啥？简单点，没那么多的套路，errno是一个整形变量，我们可以把它看成就是个数值。它是系统运行最后发生错误时产生的错误码，通过查看errno我们可以确定发生的错误的原因。
注意：并不是所有的程序代码报错都可以通过errno来记录判断，只有当调用系统api和一些库函数的时候才会最errno做出改变，当调用发生失败，系统就会将errno置为相应的int值；如果一个函数成功，errno是允许被修改成其他的值，但是绝对不会变为0。
errno使用
头文件
#include <errno.h>
实例
感觉说了太多还是在都只是在概念层面的，我们不可能把那么多的知识点都记在脑子里，所以，就以下面的一个代开文件的错误反馈来演示一下errno是怎么在我们的代码中使用的：
#include <stdio.h>
#include <iostream>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
using namespace std;
int main()
{
int fd = open("errno.txt", O_RDONLY);
if (fd == -1)
{
cout << "open file failed , errno = " << errno << endl;
}
close(fd);
return 0;
}
结果预览
实例说明
我没有创建这个errno.txt的文件，但是现在要去打开这个文件，并且参数使用的是只读模式；当然会报错了，返回的errno错误码是2；errno = 2，接下来对照上面的errno.h 头文件看看这个2对应的是什么描述？
....
#define ENOENT 2 /* No such file or directory */
...
果然，errno 为2对应的描述为/* No such file or directory */，翻译中文为没有这个文件或者目录。即我们在代码中要打开的errmo.txt不存在。
接下来，我们就可以愉快地在自己的代码中运用errno来帮助我们来调试啦
参考
http://www.man7.org/linux/man-pages/man3/errno.3.html
