linux系统编程之错误处理

在linux系统编程中，当系统调用出现错误时，有一个整型变量会被设置，这个整型变量就是errno，这个变量的定义在/usr/include/errno.h文件中

 #ifndef    _ERRNO_H
 
 /* The includer defined __need_Emath if he wants only the definitions
    of EDOM and ERANGE, and not everything else.  */
 #ifndef    __need_Emath
 # define _ERRNO_H
 # include <features.h>
 #endif
 
 __BEGIN_DECLS
 
 /* Get the error number constants from the system-specific file.
    This file will test __need_Emath and _ERRNO_H.  */
 #include <bits/errno.h>
 #undef    __need_Emath
 
 #ifdef    _ERRNO_H
 
 /* Declare the `errno' variable, unless it's defined as a macro by
    bits/errno.h.  This is the case in GNU, where it is a per-thread
    variable.  This redeclaration using the macro still works, but it
    will be a function declaration without a prototype and may trigger
    a -Wstrict-prototypes warning.  */
 #ifndef    errno
 extern int errno;
 #endif
 
 #ifdef __USE_GNU
 
 /* The full and simple forms of the name with which the program was
    invoked.  These variables are set up automatically at startup based on
    the value of ARGV[0] (this works only if you use GNU ld).  */
 extern char *program_invocation_name, *program_invocation_short_name;
 #endif /* __USE_GNU */
 #endif /* _ERRNO_H */
 
 __END_DECLS
 
 #endif /* _ERRNO_H */
 
 /* The Hurd <bits/errno.h> defines `error_t' as an enumerated type so
    that printing `error_t' values in the debugger shows the names.  We
    might need this definition sometimes even if this file was included
    before.  */
 #if defined __USE_GNU || defined __need_error_t
 # ifndef __error_t_defined
 typedef int error_t;
 #  define __error_t_defined
 # endif
 # undef __need_error_t
 #endif

当系统调用出现无错误时，这个整型变量便会被设置成一个值，每个值有一个特殊的含义，每一值用一个宏来表示，/usr/include/asm-generic/errno.h文件中定义

#ifndef _ASM_GENERIC_ERRNO_H
#define _ASM_GENERIC_ERRNO_H
 
#include <asm-generic/errno-base.h>
 
#define    EDEADLK        35    /* Resource deadlock would occur */
#define    ENAMETOOLONG    36    /* File name too long */
#define    ENOLCK        37    /* No record locks available */
#define    ENOSYS        38    /* Function not implemented */
#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 NFS 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

解释完了errno的含义，那么我们来用两个常用的错误输出函数

一个是perror

void perror(const char *s);//根据系统设置好的errno的值来输出错误信息
                          //错误信息会包含程序员自己填写的内容，以参数s传入程序员想要输出的内容

一个是strerror

char *strerror(int errnum);//把errno传入到函数strerror中，strerror返回相应错误信息的文本