What is the difference between octet string and char ? how octet string can be used ? can anybody write a small C program on Octet string.How octet strings are stored in memory ?
+1
A:
- An octet is another word for a 8-bit byte.
- A char is usually 8 bits, but may be another size on some architectures.
Sjoerd
2010-07-08 19:20:55
signed char (signed being optional on most compilers, a changeable behavior) in ANSI C is not less than 8bits, not more than 8bits, and 8bits exactely. It's decimal representation, after 2-complements conversion, goes from -127 to 128. unsigned char goes from 0 to 255.
jpinto3912
2010-07-08 19:36:24
@jpinto3912: it goes _at least_ from -127 to 127 or from 0 to 255, but it can cover a wider range. In fact, signed chars usually cover a wider range, from -128 to 127.
ninjalj
2010-07-08 19:43:15
@jp: http://stackoverflow.com/questions/2098149/what-platforms-have-something-other-than-8-bit-char
KennyTM
2010-07-08 19:52:19
@ninjalj, thanks for pointing that, my bad... it was a speed-typo, I did meant -128 to 127, as in what you get from 2-complements on 8-bits. But you raised a very interesting and valid point: "at least -127", which is what you would get on a 1-complements storing computer.
jpinto3912
2010-07-08 19:55:28
@Kenny, Ansi C has a default of char=8bits... apparently this can be changed (haven't checked, and wonder what mental conditions would trigger someone to change that). So char can (sic) be more than 8 bits. I eat my hat.
jpinto3912
2010-07-08 20:05:04
@jp: All I know is ISO C requires `CHAR_BIT ≥ 8`. I don't know how it is in ANSI C.
KennyTM
2010-07-08 20:08:36
@jpinto3912: The mental conditions nowadays typically involve writing/porting a C compiler for a 16-bit or 32-bit DSP. Also, ISO C supports 2-complement, 1-complement and sign-magnitude representations for integers.
ninjalj
2010-07-08 20:37:22
@ninjalj, I think it's a bit mental to have the compiler treat char as more than 8bits, when we have int to play with. It's well understood that DSP/CPU memory word or instruction operands length might imply using 8bit var/const is less fast than using 16 or 32bit... but the compiler can make those padding/alignment changes as speed optimizations (i.e. use a whole 16/32bit mem-position and trunc at 8).
jpinto3912
2010-07-13 10:41:50
A:
An octet string is simply a sequence of bits grouped into chunks of 8. Those 8-sized groups often represent characters. Octet string is a basic data type used for SNMP.
Matt
2010-07-08 19:23:42
+3
A:
Standards (and such) use "octet" to explicitly state that they're talking about 8-bit groups. While most current computers work with bytes that are also 8 bits in size, that's not necessarily the case. In fact, "byte" is rather poorly defined, with considerable disagreement over what it means for sure -- so it's generally avoided when precision is needed.
Nonetheless, on a typical computer, an octet is going to be the same thing as a byte, and an octet stream will be stored in a series of bytes.
Jerry Coffin
2010-07-08 19:24:42
A:
An octet is 8 bits meant to be handled together (hence the "oct" in "octet"). It's what we think of when we say "byte" these days.
A char is basically a byte -- it's defined as the smallest addressable unit of memory, which on almost all modern computers is the same as an octet. But there have been computers with 9-bit, 16-bit, even 36-bit "words" that qualify as chars by that definition. You only need to care about those computers (and thus, about the difference between a char and an octet) if you have one -- let the people who have the weird hardware worry about how to make their programs run on it.
cHao
2010-07-08 19:27:21
This doesn't only happen in ancient PDP's. Some DSP's from TI have 16 or 32-bit chars,
ninjalj
2010-07-08 19:44:51
A:
A string used to be a set of octets, which is turn is a set of 8 bits.
A string in C, is always a null-terminated, memory contiguous, set of bytes. Back in the day, each byte, an octet, represented a character. That's why they named the type used to make strings, char.
The ASCII table, that goes from 0 to 127, with the graphics/accents version going from 0 to 255, was no longer enough for displaying characters in a string, so someone though of adding bits to a character representation. Dumb-asses from CS though of 9bit and so forth, to what HW guys replied "are you nuts??? keep it a multiple of memory addressing unit", which was the byte, back then. Enter wide-character strings, i.e. 16bits per character. On a WC string, each character is represented by 2 bytes... there goes your char=1 byte rule down the drain.
To keep an exact description of a string, if it's a set of characters-represented-by-8bits (in Earth, following the ASCII table, but I've been to Mars), it's an "octet string".
If it's not "octet string" it may or may not be WC... Joel was a nice post on this.
jpinto3912
2010-07-08 19:48:53