CTRL key

ctrl key其实是用于扩展键盘，单独一个ctrl键没有什么作用，也没有ascii码，当与其他键相结合时，相当于创造出一个新键。例如：用getchar()侦测输入的字符，当按下ctrl+a时，只输出了一个ascii码1。只按下ctrl键没有输出。因此ctrl+a其实只相当于一个键罢了。

下表为所有的ctrl组合键的ascii码及其名字。

Control			Code			To Enter With			Echo
Abbreviation	Full Name		Dec	Hex		Alt-Key	Ctrl-Key
NUL	Null	0	00		Ctrl-@	^@
SOH	Start of heading	1	01	Alt-1	Ctrl-A	^A
STX	Start of text	2	02	Alt-2	Ctrl-B	^B
ETX	End of text	3	03	Alt-3	Ctrl-C	^C
EOT	End of transmission	4	04	Alt-4	Ctrl-D	^D
ENQ	Enquiry	5	05	Alt-5	Ctrl-E	^E
ACK	Acknowledge	6	06	Alt-6	Ctrl-F	^F
BEL	Bell	7	07	Alt-7	Ctrl-G	^G
BS	Backspace	8	08	Alt-8	Ctrl-H	^H
HT	Horizontal tab	9	09	Alt-9	Ctrl-I	^I
LF	Line feed	10	0A	Alt-10	Ctrl-J	^J
VT	Vertical tab	11	0B	Alt-11	Ctrl-K	^K
FF	Form feed	12	0C	Alt-12	Ctrl-L	^L
CR	Carriage return	13	0D	Alt-13	Ctrl-M	^M
SO	Shift out	14	0E	Alt-14	Ctrl-N	^N
SI	Shift in	15	0F	Alt-15	Ctrl-O	^O
DLE	Data line escape	16	10	Alt-16	Ctrl-P	^P
DC1	Device control 1	17	11	Alt-17	Ctrl-Q	^Q
DC2	Device control 2	18	12	Alt-18	Ctrl-R	^R
DC3	Device control 3	19	13	Alt-19	Ctrl-S	^S
DC4	Device control 4	20	14	Alt-20	Ctrl-T	^T
NAK	Negative acknowledge	21	15	Alt-21	Ctrl-U	^U
SYN	Synchronous idle	22	16	Alt-22	Ctrl-V	^V
ETB	End transmission block	23	17	Alt-23	Ctrl-W	^W
CAN	Cancel	24	18	Alt-24	Ctrl-X	^X
EM	End of medium	25	19	Alt-25	Ctrl-Y	^Y
SUB	Substitute	26	1A	Alt-26	Ctrl-Z	^Z
ESC	Escape	27	1B	Alt-27	Ctrl-[	^[
FS	File separator	28	1C	Alt-28	Ctrl-\	^\
GS	Group separator	29	1D	Alt-29	Ctrl-]	^]
RS	Record separator	30	1E	Alt-30	Ctrl-^	^^
US	Unit separator	31	1F	Alt-31	Ctrl-_	^_