Linux 系统错误码 errno 剖析

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一、errno 介绍

1.1 errno 简介

Linux 中系统调用的错误都存储于错误码 errno 中。errno 由操作系统维护,存储就近发生的错误,即下一次的错误码会覆盖掉上一次的错误。

errno 是一个包含在 <errno.h> 中的预定义的外部 int 变量,用于表示最近一个函数调用是否产生了错误。

  • 若为0,则无错误;
  • 其它值均表示某一种错误。

代码中需要包含 #include <errno.h>,当一个系统调用或着库函数的调用失败时,将会重置错误码 errno。用户在判断程序出错后,立即检验 errno 的值可以获取错误码和错误信息。

1.2 errno是线程安全的

在 Linux 上,全局 errno 变量是特定于线程的。POSIX 要求 errno 必须是线程安全的。

参阅:Thread-safety and POSIX.1 (unix.org)

在 POSIX.1 中,errno 被定义为外部全局变量。但是此定义在多线程环境中是不可接受的,因为使用它会导致不确定的结果。问题是两个或多个线程可能会遇到错误,所有错误都会导致设置相同的错误号。在这种情况下,一个线程可能已经被另一个线程更新后,最终检查 errno。

为了避免产生不确定性,POSIX.1c 将 errno 重新定义为可以访问每个线程错误号的服务:

  • 某些函数可能在通过符号 errno 访问的变量中提供错误号。
  • errno 符号是通过包括 C 标准所指定的标头来定义的。
  • 对于进程的每个线程,errno 的值不应受函数调用或其他线程对 errno 的分配的影响。

参阅:errno(3): number of last error - Linux man page (die.net)

errno 是线程本地的;在一个线程中设置它不会影响在其他任何线程中的值

我们可以通过在机器上运行一个简单的程序来进行检查。

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

#define NTHREADS 5

void *thread_function(void *dummyPtr)

    printf("Thread number %ld addr(errno):%p\\n", pthread_self(), &errno);


int main()

    pthread_t thread_id[NTHREADS];
    int i, j;

    for (i = 0; i < NTHREADS; i++)
    
        pthread_create(&thread_id[i], NULL, thread_function, NULL);
    

    for (j = 0; j < NTHREADS; j++)
    
        pthread_join(thread_id[j], NULL);
    

    return 0;

输出结果:

1.3 errno 宏定义

在头文件「/usr/include/asm-generic/errno-base.h」中对基础的常用 errno 进行了宏定义:

define errno explain
EPERM 1 Operation not permitted
ENOENT 2 No such file or directory
ESRCH 3 No such process
EINTR 4 Interrupted system call
EIO 5 I/O error
ENXIO 6 No such device or address
E2BIG 7 Argument list too long
ENOEXEC 8 Exec format error
EBADF 9 Bad file number
ECHILD 10 No child processes
EAGAIN 11 Try again
ENOMEM 12 Out of memory
EACCES 13 Permission denied
EFAULT 14 Bad address
ENOTBLK 15 Block device required
EBUSY 16 Device or resource busy
EEXIST 17 File exists
EXDEV 18 Cross-device link
ENODEV 19 No such device
ENOTDIR 20 Not a directory
EISDIR 21 Is a directory
EINVAL 22 Invalid argument
ENFILE 23 File table overflow
EMFILE 24 Too many open files
ENOTTY 25 Not a typewriter
ETXTBSY 26 Text file busy
EFBIG 27 File too large
ENOSPC 28 No space left on device
ESPIPE 29 Illegal seek
EROFS 30 Read-only file system
EMLINK 31 Too many links
EPIPE 32 Broken pipe
EDOM 33 Math argument out of domain of func
ERANGE 34 Math result not representable

在 「/usr/include/asm-generic/errno.h」 中,对剩余的 errno 做了宏定义:

define errno explain
EDEADLK 35 Resource deadlock would occur
ENAMETOOLONG 36 File name too long
ENOLCK 37 No record locks available
ENOSYS 38 Function not implemented
ENOTEMPTY 39 Directory not empty
ELOOP 40 Too many symbolic links encountered
EWOULDBLOCK EAGAIN Operation would block
ENOMSG 42 No message of desired type
EIDRM 43 Identifier removed
ECHRNG 44 Channel number out of range
EL2NSYNC 45 Level 2 not synchronized
EL3HLT 46 Level 3 halted
EL3RST 47 Level 3 reset
ELNRNG 48 Link number out of range
EUNATCH 49 Protocol driver not attached
ENOCSI 50 No CSI structure available
EL2HLT 51 Level 2 halted
EBADE 52 Invalid exchange
EBADR 53 Invalid request descriptor
EXFULL 54 Exchange full
ENOANO 55 No anode
EBADRQC 56 Invalid request code
EBADSLT 57 Invalid slot
EDEADLOCK EDEADLK
EBFONT 59 Bad font file format
ENOSTR 60 Device not a stream
ENODATA 61 No data available
ETIME 62 Timer expired
ENOSR 63 Out of streams resources
ENONET 64 Machine is not on the network
ENOPKG 65 Package not installed
EREMOTE 66 Object is remote
ENOLINK 67 Link has been severed
EADV 68 Advertise error
ESRMNT 69 Srmount error
ECOMM 70 Communication error on send
EPROTO 71 Protocol error
EMULTIHOP 72 Multihop attempted
EDOTDOT 73 RFS specific error
EBADMSG 74 Not a data message
EOVERFLOW 75 Value too large for defined data type
ENOTUNIQ 76 Name not unique on network
EBADFD 77 File descriptor in bad state
EREMCHG 78 Remote address changed
ELIBACC 79 Can not access a needed shared library
ELIBBAD 80 Accessing a corrupted shared library
ELIBSCN 81 .lib section in a.out corrupted
ELIBMAX 82 Attempting to link in too many shared libraries
ELIBEXEC 83 Cannot exec a shared library directly
EILSEQ 84 Illegal byte sequence
ERESTART 85 Interrupted system call should be restarted
ESTRPIPE 86 Streams pipe error
EUSERS 87 Too many users
ENOTSOCK 88 Socket operation on non-socket
EDESTADDRREQ 89 Destination address required
EMSGSIZE 90 Message too long
EPROTOTYPE 91 Protocol wrong type for socket
ENOPROTOOPT 92 Protocol not available
EPROTONOSUPPORT 93 Protocol not supported
ESOCKTNOSUPPORT 94 Socket type not supported
EOPNOTSUPP 95 Operation not supported on transport endpoint
EPFNOSUPPORT 96 Protocol family not supported
EAFNOSUPPORT 97 Address family not supported by protocol
EADDRINUSE 98 Address already in use
EADDRNOTAVAIL 99 Cannot assign requested address
ENETDOWN 100 Network is down
ENETUNREACH 101 Network is unreachable
ENETRESET 102 Network dropped connection because of reset
ECONNABORTED 103 Software caused connection abort
ECONNRESET 104 Connection reset by peer
ENOBUFS 105 No buffer space available
EISCONN 106 Transport endpoint is already connected
ENOTCONN 107 Transport endpoint is not connected
ESHUTDOWN 108 Cannot send after transport endpoint shutdown
ETOOMANYREFS 109 Too many references: cannot splice
ETIMEDOUT 110 Connection timed out
ECONNREFUSED 111 Connection refused
EHOSTDOWN 112 Host is down
EHOSTUNREACH 113 No route to host
EALREADY 114 Operation already in progress
EINPROGRESS 115 Operation now in progress
ESTALE 116 Stale file handle
EUCLEAN 117 Structure needs cleaning
ENOTNAM 118 Not a XENIX named type file
ENAVAIL 119 No XENIX semaphores available
EISNAM 120 Is a named type file
EREMOTEIO 121 Remote I/O error
EDQUOT 122 Quota exceeded
ENOMEDIUM 123 No medium found
EMEDIUMTYPE 124 Wrong medium type
ECANCELED 125 Operation Canceled
ENOKEY 126 Required key not available
EKEYEXPIRED 127 Key has expired
EKEYREVOKED 128 Key has been revoked
EKEYREJECTED 129 Key was rejected by service
EOWNERDEAD 130 Owner died
ENOTRECOVERABLE 131 State not recoverable
ERFKILL 132 Operation not possible due to RF-kill
EHWPOISON 133 Memory page has hardware error

二、打印 errno

若想要打印 errno,需要包含头文件 #include <errno.h>

2.1 使用 perror 打印错误信息

函数原型:void perror(const char *s)

头 文 件:#include <stdio.h>

作 用:打印系统错误信息

2.2 使用 strerror 显示错误信息

函数原型:char *strerror(int errnum);

头 文 件:#include <string.h>

作 用:将错误码以字符串的信息显示出来

三、输出 errno 的小 Demo

#include <stdio.h>
#include <string.h>

static void __Process(char *str)

    if (strstr(str, "_ASM_GENERIC_") != NULL || strstr(str, "define") == NULL)
    
        return;
    
    if (strstr(str, "EWOULDBLOCK") != NULL || strstr(str, "EDEADLOCK") != NULL)
    
        return;
    

    char szDefine[64] = 0;
    int errno = -1;
    char szExplain[64] = 0;
    sscanf(str, "%*[^A-Z]%s%*[^0-9]%d %*[^*]* %[^*]s", szDefine, &errno, szExplain);
    szExplain[strlen(szExplain) - 1] = 0; // 去除最后的空格

    char buf[1024] = 0;
    snprintf(buf, sizeof(buf), "|%s|%d|%s|", szDefine, errno, szExplain);
    puts(buf);


int main()

    freopen("/usr/include/asm-generic/errno-base.h", "r", stdin); // 读文件
    // freopen("E:\\\\Documents\\\\stdin&&stdout\\\\stdout\\\\文件名", "w", stdout); // 写文件

    while (1)
    
        char str[1024] = 0;
        fgets(str, sizeof(str), stdin);

        if (0 == strncmp(str, "#endif", 6))
            break;

        __Process(str);
    

    puts("-----------------------------------------------------------");

    freopen("/usr/include/asm-generic/errno.h", "r", stdin); // 读文件
    while (1)
    
        char str[1024] = 0;
        fgets(str, sizeof(str), stdin);

        if (0 == strncmp(str, "#endif", 6))
            break;

        __Process(str);
    

参考资料

linxu中系统错误码errno

Unix errno值、

  •   只要一个Unix函数(如,某个套接字函数)中有错误发生,全局变量errno就被设置为一个指明该错误类型的正直,函数本身则通过返回-1.
  •   errno的值只在函数发生错误时被设置,如果函数不返回错误,errno的值就没有定义。
  •   errno的所有证书错误值都是常值,具有以“E”开头的全大写字母名字,并通常在<sys/errno.h>头文件中定义,0值不表示任何错误;
  •   sys_errlist
#include <iostream>
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <errno.h>
#include <string.h> int main(int argc, char **argv) { int sockfd = 0; if ((sockfd = socket(0, SOCK_STREAM, 0)) < 0) { if (errno >= 0 && errno <= sys_nerr) printf("socket created, failed[%d], reason:%s ", errno, sys_errlist[errno]);
     printf("%s ",strerror(errno)); //头文件string.h内 }
else { printf("socket successful "); } return 0; }

输出结果:

socket created, failed[97], reason:Address family not supported by protocol
Address family not supported by protocol

 

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