Linux 系统错误码 errno 剖析

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