What is the use of u_char and u_short... etc ?
Is it possible to use char instead of u_char?
2 Replies  19705 Views  Last Post: 26 February 2009  08:28 AM
#1
What is u_char, u_short usef for? Why not use simply char and short..?
Posted 26 February 2009  12:37 AM
Replies To: What is u_char, u_short usef for? Why not use simply char and short..?
#2
Re: What is u_char, u_short usef for? Why not use simply char and short..?
Posted 26 February 2009  05:50 AM
the u in u_char stands for unsigned.
The char datatype isn't always used to store characters. Since char is the only data type whose size is always 1 byte on any platform, it is used often to store 1 byte data.
1 byte can hold 255 values but the regular char datatype is a signed type and hence stores values from 127 to 127 i.e. After 127, the number is represented in 2's complement notation and hence the numbers are represented as negative.
To use only the values 0 to 255, the unsigned type is used. In this case, everything is considered as a positive number and 2's complement is not taken.
The char datatype isn't always used to store characters. Since char is the only data type whose size is always 1 byte on any platform, it is used often to store 1 byte data.
1 byte can hold 255 values but the regular char datatype is a signed type and hence stores values from 127 to 127 i.e. After 127, the number is represented in 2's complement notation and hence the numbers are represented as negative.
To use only the values 0 to 255, the unsigned type is used. In this case, everything is considered as a positive number and 2's complement is not taken.
#3
Re: What is u_char, u_short usef for? Why not use simply char and short..?
Posted 26 February 2009  08:28 AM
In case some don't know what twos compliment addition is:
A negative plus its absolute value is zero (in other words, x + x =0). So given 1 in binary, how do you find its negative?
00000001
+ X

00000000
If you invert all of the bits and add 1, you can get its negetive
00000001 (one)
11111111 (negative one)

00000000 (zero)
You simply add and carry the one across. The extra 1 at the end is not used because there are no more bits to carry over (the computer protects you from that even if there are two variables next to eachother in memory).
So to find twos compliment negative of 7:
00000111 (seven)
11111001 (negative seven)

00000000 (zero)
It ends up that the leftmost bit hints the sign to you. This is how variables work for signed types. However, unsigned types give don't have negatives, so 7 (signed) above is 128+64+32+16+8+1=220 in unsigned notation.
That's as best as i can do for now. Hope this helps
A negative plus its absolute value is zero (in other words, x + x =0). So given 1 in binary, how do you find its negative?
00000001
+ X

00000000
If you invert all of the bits and add 1, you can get its negetive
00000001 (one)
11111111 (negative one)

00000000 (zero)
You simply add and carry the one across. The extra 1 at the end is not used because there are no more bits to carry over (the computer protects you from that even if there are two variables next to eachother in memory).
So to find twos compliment negative of 7:
00000111 (seven)
11111001 (negative seven)

00000000 (zero)
It ends up that the leftmost bit hints the sign to you. This is how variables work for signed types. However, unsigned types give don't have negatives, so 7 (signed) above is 128+64+32+16+8+1=220 in unsigned notation.
That's as best as i can do for now. Hope this helps
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