errno.c
#include <stdio.h>
#include <string.h>
#include <errno.h>
int main(int argc, char *argv[])
{
fprintf(stderr, "EACCES : %s\n",strerror(EACCES));
errno = ENOENT;
perror(argv[0]);
return 0;
}函数fprintf：
int fprintf(FILE *stream, const char *format, ...); //根据参数format字符串来转换并格式化数据，然后将结果输出到参数stream指定的文件中，直到出现字符串结束为主。关于参数format字符串的格式参考printf()。成功返回世界输出的字符串，失败返回-1.
函数strerror：
char *strerror(int errnum);//依参数errnum的错误代码来查询其错误原因的描述字符串，然后将该字符串指针返回。
函数perror：
void perror(const char *s); //将函数发生的错误原因输出的标准设备(stderr)。参数s所指的字符串会先打印出，后面再加上错误原因字符串。此错误原因依照全局变量errno的值来决定要输出的字符串
errno ：
errno是一个全局变量，当调用某些函数出错时，该函数会重设errno的值。
errno的一下错误定义如下
E2BIG Argument list too long (POSIX.1)
EACCES Permission denied (POSIX.1)
EADDRINUSE Address already in use (POSIX.1)
EADDRNOTAVAIL Address not available (POSIX.1)
EAFNOSUPPORT Address family not supported (POSIX.1)
EAGAIN Resource temporarily unavailable (may be the same value as EWOULDBLOCK)(POSIX.1)
EALREADY Connection already in progress (POSIX.1)
EBADE Invalid exchange
EBADF Bad file descriptor (POSIX.1)
EBADFD File descriptor in bad state
EBADMSG Bad message (POSIX.1)
EBADR Invalid request descriptor
EBADRQC Invalid request code
EBADSLT Invalid slot
EBUSY Device or resource busy (POSIX.1)
ECANCELED Operation canceled (POSIX.1)
ECHILD No child processes (POSIX.1)
ECHRNG Channel number out of range
ECOMM Communication error on send
ECONNABORTED Connection aborted (POSIX.1)
ECONNREFUSED Connection refused (POSIX.1)
ECONNRESET Connection reset (POSIX.1)
EDEADLK Resource deadlock avoided (POSIX.1)
EDEADLOCK Synonym for EDEADLK
EDESTADDRREQ Destination address required (POSIX.1)
EDOM Mathematics argument out of domain of function (POSIX.1, C99)
EDQUOT Disk quota exceeded (POSIX.1)
EEXIST File exists (POSIX.1)
EFAULT Bad address (POSIX.1)
EFBIG File too large (POSIX.1)
EHOSTDOWN Host is down
EHOSTUNREACH Host is unreachable (POSIX.1)
EIDRM Identifier removed (POSIX.1)
EILSEQ Illegal byte sequence (POSIX.1, C99)
EINPROGRESS Operation in progress (POSIX.1)
EINTR Interrupted function call (POSIX.1); see signal(7).
EINVAL Invalid argument (POSIX.1)
EIO Input/output error (POSIX.1)
EISCONN Socket is connected (POSIX.1)
EISDIR Is a directory (POSIX.1)
EISNAM Is a named type file
EKEYEXPIRED Key has expired
EKEYREJECTED Key was rejected by service
EKEYREVOKED Key has been revoked
EL2HLT Level 2 halted
EL2NSYNC Level 2 not synchronized
EL3HLT Level 3 halted
EL3RST Level 3 halted
ELIBACC Cannot access a needed shared library
ELIBBAD Accessing a corrupted shared library
ELIBMAX Attempting to link in too many shared libraries
ELIBSCN lib section in a.out corrupted
ELIBEXEC Cannot exec a shared library directly
ELOOP Too many levels of symbolic links (POSIX.1)
EMEDIUMTYPE Wrong medium type
EMFILE Too many open files (POSIX.1)
EMLINK Too many links (POSIX.1)
EMSGSIZE Message too long (POSIX.1)
EMULTIHOP Multihop attempted (POSIX.1)
ENAMETOOLONG Filename too long (POSIX.1)
ENETDOWN Network is down (POSIX.1)
ENETRESET Connection aborted by network (POSIX.1)
ENETUNREACH Network unreachable (POSIX.1)
ENFILE Too many open files in system (POSIX.1)
ENOBUFS No buffer space available (POSIX.1 (XSI STREAMS option))
ENODATA No message is available on the STREAM head read queue (POSIX.1)
ENODEV No such device (POSIX.1)
ENOENT No such file or directory (POSIX.1)
ENOEXEC Exec format error (POSIX.1)
ENOKEY Required key not available
ENOLCK No locks available (POSIX.1)
ENOLINK Link has been severed (POSIX.1)
ENOMEDIUM No medium found
ENOMEM Not enough space (POSIX.1)
ENOMSG No message of the desired type (POSIX.1)
ENONET Machine is not on the network
ENOPKG Package not installed
ENOPROTOOPT Protocol not available (POSIX.1)
ENOSPC No space left on device (POSIX.1)
ENOSR No STREAM resources (POSIX.1 (XSI STREAMS option))
ENOSTR Not a STREAM (POSIX.1 (XSI STREAMS option))
ENOSYS Function not implemented (POSIX.1)
ENOTBLK Block device required
ENOTCONN The socket is not connected (POSIX.1)
ENOTDIR Not a directory (POSIX.1)
ENOTEMPTY Directory not empty (POSIX.1)
ENOTSOCK Not a socket (POSIX.1)
ENOTSUP Operation not supported (POSIX.1)
ENOTTY Inappropriate I/O control operation (POSIX.1)
ENOTUNIQ Name not unique on network
ENXIO No such device or address (POSIX.1)
EOPNOTSUPP Operation not supported on socket (POSIX.1)
(ENOTSUP and EOPNOTSUPP have the same value on Linux, but according to POSIX.1
these error values should be distinct.)
EOVERFLOW Value too large to be stored in data type (POSIX.1)
EPERM Operation not permitted (POSIX.1)
EPFNOSUPPORT Protocol family not supported
EPIPE Broken pipe (POSIX.1)
EPROTO Protocol error (POSIX.1)
EPROTONOSUPPORT Protocol not supported (POSIX.1)
EPROTOTYPE Protocol wrong type for socket (POSIX.1)
ERANGE Result too large (POSIX.1, C99)
EREMCHG Remote address changed
EREMOTE Object is remote
EREMOTEIO Remote I/O error
ERESTART Interrupted system call should be restarted
EROFS Read-only file system (POSIX.1)
ESHUTDOWN Cannot send after transport endpoint shutdown
ESPIPE Invalid seek (POSIX.1)
ESOCKTNOSUPPORT Socket type not supported
ESRCH No such process (POSIX.1)
ESTALE Stale file handle (POSIX.1)
This error can occur for NFS and for other file systems
ESTRPIPE Streams pipe error
ETIME Timer expired (POSIX.1 (XSI STREAMS option))
(POSIX.1 says "STREAM ioctl(2) timeout")
ETIMEDOUT Connection timed out (POSIX.1)
ETXTBSY Text file busy (POSIX.1)
EUCLEAN Structure needs cleaning
EUNATCH Protocol driver not attached
EUSERS Too many users
EWOULDBLOCK Operation would block (may be same value as EAGAIN) (POSIX.1)
EXDEV Improper link (POSIX.1)
EXFULL Exchange full