每个进程维护一个全局整型变量errno, 记录系统(调用)的最后一次错误代码. errno及其常量值定义位于<errno.h>. 以前, errno不是线程安全的, 其定义是:

extern int errno;

现在, 为支持多线程环境, errno定义是:

extern int *__errno_location(void);
#define errno (*__errno_location())

errno与多线程: 每个线程都有属于直接的局部errno(errno副本), 可以避免一个线程干扰另外一个线程.

errno使用规则:

如果没有出错, errno值不会被例程清除. 只有函数返回值表明出错时, 才检验errno值; 任何函数都不会将errno设置为0, errno.h定义的所有常量也不为0;

errno错误输出: C标准定义了2个函数, 用于打印出错信息: perror和strerror.

2. 错误输出perror

perror基于当前errno值, 输出除外信息到stderr. 也就是说, 只有函数出错返回时, 会设置errno时, 才能用perror输出错误信息.

#include <stdio.h>

void perror(const char *msg);

如系统调用fork失败, 可以输出失败的详细信息:

int ret = fork();
if (ret < 0) {
    perorr("fork failed");
    exit(1);
}

3. 错误输出strerror

strerror将errnum(通常是errno值)转化为出错消息字符串. 有些函数/系统调用, 如pthread_create, 是不会设置errnor的, 此时就可以利用strerror将函数返回错误码, 转化为出错字符串, 然后输出到stdout/stderr. 至于函数调用失败时, 如何确认是否设置errno, 可以查看man手册.

#include <string.h>

char *strerror(int errnum);

pthread_t th1;
int ret = pthread_create(&th1, NULL, thr_func, NULL);
if (ret < 0) {
    printf("%s\n", strerror(ret));
    exit(1);
}

4. errno出错常量

errno值宏定义值位于 error.h, 用命令man 3 errno查看. 不过, 这种方法只能看到宏和对应注释含义, 无法看到定义的数值.

或者安装ctags后, 可以输入vim -t命令查看任意一个标识符, 如查看EINTR

$ vim -t EINTR

所有errno取值:

#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 */
#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 request 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 */