Is it true that whether the architecture is big or little endian ,only the memory layout of numbers differ,that of the string is the same.
If you have a simple 8-bit character representation (e.g. extended ASCII), then no, endianness does not affect the layout, because each character is one byte.
If you have a multi-byte representation, such as UTF-16, then yes, endianness is still important (see e.g. http://en.wikipedia.org/wiki/UTF-16#Byte_order_encoding_schemes).
For strings of 1-byte characters that is correct. For unicode strings (2 bytes/character) there will be a difference.
That's generally not true. Depending on the circumstances, more than one byte might be used for characters, in which case there is a difference between little endian encoding of characters and big endian encoding of characters.
For the most part, but you should understand why. Big vs little endian refers to the ordering of bytes in multi-byte data types like integers. ASCII characters are just a single byte.
Note however that unicode characters are multiple bytes, so the byte order matters. The entire point of unicode is that the single byte in ASCII can only encode 256 different values, which is not enough for all the languages in the world.
Refer here for more informantion about what endianness means:
http://www.cs.umd.edu/class/sum2003/cmsc311/Notes/Data/endian.html
Related
I have a strange validation program that validates wheather a utf-8 string is a valid host name(Zend Framework Hostname valdiator in PHP). It allows IDNs(internationalized domain names). It will compare each subdomain with sets of characters defined by their HEX bytes representation. Two such sets are D800-DB7F and DC00-DFFF. Php regexp comparing function called preg_match fails during these comparsions and it says that DC00-DFFF characters are not allowed in this function. From wikipedia I learned these bytes are called surrogate characters in UTF-8. What are thay and which characters they actually correspond to? I read in several places I still don't understand what they are.
What are surrogate characters in UTF-8?
This is almost like a trick question.
Approximate answer #1: 4 bytes (if paired and encoded in UTF-8).
Approximate answer #2: Invalid (if not paired).
Approximate answer #3: It's not UTF-8; It's Modified UTF-8.
Synopsis: The term doesn't apply to UTF-8.
Unicode codepoints have a range that needs 21 bits of data.
UTF-16 code units are 16 bits. UTF-16 encodes some ranges of Unicode codepoints as one code unit and others as pairs of two code units, the first from a "high" range, the second from a "low" range. Unicode reserves the codepoints that match the ranges of the high and low pairs as invalid. They are sometimes called surrogates but they are not characters. They don't mean anything by themselves.
UTF-8 code units are 8 bits. UTF-8 encodes several distinct ranges of codepoints in one to four code units, respectively.
#1 It happens that the codepoints that UTF-16 encodes with two 16-bit code units, UTF-8 encodes with 4 8-bit code units, and vice versa.
#2 You can apply the UTF-8 encoding algorithm to the invalid codepoints, which is invalid. They can't be decoded to a valid codepoint. A compliant reader would throw an exception or throw out the bytes and insert a replacement character (�).
#3 Java provides a way of implementing functions in external code with a system called JNI. The Java String API provides access to String and char as UTF-16 code units. In certain places in JNI, presumably as a convenience, string values are modified UTF-8. Modified UTF-8 is the UTF-8 encoding algorithm applied to UTF-16 code units instead of Unicode codepoints.
Regardless, the fundamental rule of character encodings is to read with the encoding that was used to write. If any sequence of bytes is to be considered text, you must know the encoding; Otherwise, you have data loss.
If the largest 32 bit number I can express is 0xFFFFFFFF, then in ascii representation, is this 64 bits in size (in hex)?
Yes, assuming you use one octet per ascii character, as is conventional. However you might also need a terminating nul, and maybe the "0x" prefix. Also you can use a 7 bit representation for ascii, it's still an ascii encoding (though hard to work with on 8-bit based platforms).
I'm still confused about the bits and bytes although I've been searching through the internet. Is that one character of ASCII = 1 bytes = 8 bits? So 8 bits have 256 unique pattern that covered all the ASCII code, what form is it stored in our computer?
And if I typed "Hello" does that mean this consists of 5 bytes?
Yes to everything you wrote. "Bit" is a binary digit: a 0 or a 1. Historically there existed bytes of smaller sizes; now "byte" only ever means "8 bits of information", or a number between 0 and 255.
No. ASCII is a character set with 128 codepoints stored as the values 0-127. Modern computers predominantly address 8-bit memory and disk locations so a 7-bit ASCII value takes up 8 bits.
There is no text but encoded text. An encoding maps a member of a character set to one or more bytes. Unless you absolutely know you are using ASCII, you probably aren't. There are quite a few character sets with encodings that cover all 256 byte values and use any combination of byte values to encode a string.
There are several character sets that are similar but have a few less than 256 characters. And others that use more than one byte to encode a codepoint and don't use every combination of byte values.
Just so you know, Unicode is the predominant character set except in very specialized situations. It has several encodings. UTF-8 is often used for storage and streams. UTF-16 is often used in memory, particularly in Java, .NET, JavaScript, XML, …. When text is communicated between systems, there has to be an agreement, specification, standard, or indication about which character set and encoding it uses so a sequence of bytes can be interpreted as characters.
To add to the confusion, programming languages have data types called char, Character, etc. You have to look at the specific language's reference manual to see what they mean. For example in C, char is simply an integer that is defined as the size of the encoding of character used by that C implementation. (C also calls this a "byte" and it is not necessarily 8 bits. In all other contexts, people mean 8 bits when they say "byte". If they want to be exceedingly unambiguous they might say "octet".)
"Hello" is five characters. In a specific character set, it is five codepoints. In a specific encoding for that character set, it could be 5, 10 or 20, or ??? bytes.
Also, in the source code of a specific language, a literal string like that might be "null-terminated". This means that you could say it is 6 "characters". Other languages might store a string as a counted sequence of code units. Again, you have to look at the language reference to know the underlying data structure of strings. Of, if the language and the libraries used with it are sufficiently high-level, you might never need to know such internals.
I am reading a (.txt) file, the contents of first line are just the four alphabet letters: "abcd".
when I display the ASCII code of these letters, I expect I to find 97,98,99 and 100 respectively for a,b,c and b. But I found tow special characters which their ASCII code are 255 and 254 for ÿ and þ.
Therefore the length of read line is 6 not 4 because of "ÿþabcd". Are these special character must-to-insert at the start of any sequential text file or is there any way to avoid both of them?
ASCII is only used in niche or archaic systems. Your data proved to you that your file is not ASCII. You must find out which character set and encoding the file was stored in.
Character Sets
All text is an encoding of elements of a character set. Elements of a character set are called codepoints. A character set consists of a list of codepoints and their descriptions. The description states how the codepoint is used semantically in text, such as LATIN CAPITAL LETTER A (A) or N-ARY PRODUCT (∏). (The style in which the codepoint is rendered is the purview of typefaces/fonts.)
Encodings
Codepoints are numbered with non-negative integers. The number is encoded into bytes. Most character sets have only one encoding, which is the number as an unsigned integer in the smallest size that can represent all of the codepoints. For example, Windows-1252 has 251 codepoints, with numbers between 0 and 255. A byte is big enough to represent any of them. The Unicode character set has about 1 million codepoints, numbered from 0 to 1,114,112. A 32-bit integer is big enough to represent all of them. That's the UTF-32 encoding.
Byte order
Computer memory is usually byte-addressable and file are byte sequences so for a large integer the question becomes, in which order the bytes are stored: The most significant byte first (big endian) or least significant byte first (little endian). Software adapts to or assumes one way or the other. So, UTF-32 actually identifies one of two encodings: UTF32BE or UTF-32LE. UTF-32 is shorthand for the endianness that the software assumes. Typically, the OS assumes the endianness of the hardware it is running on and programs follow suit.
Unicode Encodings
UTF-32 takes a lot of space. The most commonly used codepoints are numbered below 65,536. So there can be savings if codepoints are represented variable number of smaller integers. The size of the integer is called a code unit. The value of a code unit contains some of the bits of a code unit and indicates if there are more code units to follow that have more of the bits. So, there are UTF-16LE and UTF-16BE and UTF-8 (and more) encodings for Unicode. UTF-16 uses one or two 16-bit code units for a codepoint and UTF-8 uses one to four 8-bit code units for a codepoint.
Files are data outside of programs. So for a program to read text, it has to know the character set and encoding. Often this metadata is not stored with the file (within or beside). That's how you made the mistake of believing your file is ASCII. If you don't know the encoding of a file, you've lost data. You might be able to recover it through guessing. It is notable that the CP437 character set has 256 codepoints, numbered 0 to 255 and encoded in one byte. So every file can be read as CP437; The question is, is that right? Even if it looks right, it's probably not right unless it's from a Western culture circa 1990.
Unicode Byte-order Mark
A strong clue about which character set and encoding to guess is called the byte-order mark (BOM). Recall encodings with code units larger than one byte have an endianness. Endianness is a hardware concern. So, although a file can be passed between systems with agreement on which character set and encoding scheme is used, the endianness attribute of encoding is critical to each system. It has become standard to indicate byte order within the file, as the first bytes. Unicode specifies a codepoint to use for this purpose, as long as it is at the beginning of the file. (That means that programs reading Unicode from a file must separate this metadata from the data.) Many file writing libraries write the BOM codepoint regardless of the code unit size. So, you'll see it at the beginning of UTF-8 files. Since the Unicode BOM looks different in each of the Unicode encodings, it completely identifies which Unicode encoding is being used.
Guessing
Your file begins with the UTF-16LE BOM. Read it as UTF-16LE (and discard the BOM codepoint if your library doesn't already.)
Given the specificness of the Unicode BOM, its presence is a strong indicator that the file is encoded in Unicode and the actual bytes tell which Unicode encoding. However, as noted above, it's possible that this guess is wrong.
As #Lưu Vĩnh Phúc points out, it is unclear how you are reading "ÿþabcd" from what you say is a 6-byte file. Open the file in a hex editor. UTF-16LE should be FF FE 61 00 62 00 63 00 64 00.
Does the term multibyte refer to a charset whose characters can - but don't have to be - wider than 1 byte, (e.g. UTF-8) or does it refer to character sets which are in any case wider than 1 byte (e.g. UTF-16) ? In other words: What is meant if anybody talks about multibyte character sets?
The term is ambiguous, but in my internationalization work, we typically avoided the term "multibyte character sets" to refer to Unicode-based encodings. Generally, we used the term only for legacy encoding schemes that had one or more bytes to define each character (excluding encodings that require only one byte per character).
Shift-jis, jis, euc-jp, euc-kr, along with Chinese encodings are typically included.
Most of the legacy encodings, with some exceptions, require a sort of state machine model (or, more simply, a page swapping model) to process, and moving backwards in a text stream is complicated and error-prone. UTF-8 and UTF-16 do not suffer from this problem, as UTF-8 can be tested with a bitmask and UTF-16 can be tested against a range of surrogate pairs, so moving backward and forward in a non-pathological document can be done safely without major complexity.
A few legacy encodings, for languages like Thai and Vietnamese, have some of the complexity of multibyte character sets but are really just built on combining characters, and aren't generally lumped in with the broad term "multibyte."
What is meant if anybody talks about multibyte character sets?
That, as usual, depends on who is doing the talking!
Logically, it should include UTF-8, Shift-JIS, GB etc.: the variable-length encodings. UTF-16 would often not be considered in this group (even though it kind of is, what with the surrogates; and certainly it's multiple bytes when encoded into bytes via UTF-16LE/UTF-16BE).
But in Microsoftland the term would more typically be used to mean a variable-length default system codepage (for legacy non-Unicode applications, of which there are sadly still plenty). In this usage, UTF-8 and UTF-16LE/UTF-16BE cannot be included because the system codepage on Windows cannot be set to either of these encodings.
Indeed, in some cases “mbcs” is no more than a synonym for the system codepage, otherwise known (even more misleadingly) as “ANSI”. In this case a “multibyte” character set could actually be something as trivial as cp1252 Western European, which only uses one byte per character!
My advice: use “variable-length” when you mean that, and avoid the ambiguous term “multibyte”; when someone else uses it you'll need to ask for clarification, but typically someone with a Windows background will be talking about a legacy East Asian codepage like cp932 (Shift-JIS) and not a UTF.
All character sets where you dont have a 1 byte = 1 character mapping. All Unicode variants, but also asian character sets are multibyte.
For more information, I suggest reading this Wikipedia article.
A multibyte character will mean a character whose encoding requires more than 1 byte. This does not imply however that all characters using that particular encoding will have the same width (in terms of bytes). E.g: UTF-8 and UTF-16 encoded character may use multiple bytes sometimes whereas all UTF-32 encoded characters always use 32-bits.
References:
IBM: Multibyte Characters
Unicode and MultiByte Character Set (archived), Unicode and Multibyte Character Set (MBCS) Support | Microsoft Docs
Unicode Consortium Website
A multibyte character set may consist of both one-byte and two-byte
characters. Thus a multibyte-character string may contain a mixture of
single-byte and double-byte characters.
Ref: Single-Byte and Multibyte Character Sets
UTF-8 is multi-byte, which means that each English character (ASCII) is stored in 1 byte while non-english character like Chinese, Thai, is stored in 3 bytes. When you mix Chinese/Thai with English, like "ทt", the first Thai character "ท" uses 3 bytes while the second English character "t" uses only 1 byte. People who designed multi-byte encoding realized that English character shouldn't be stored in 3 bytes while it can fit in 1 byte due to the waste of storage space.
UTF-16 stores each character either English or non-English in a fixed 2 byte length so it is not multi-byte but called a wide character. It is very suitable for Chinese/Thai languages where each character fits entirely in 2 bytes but printing to utf-8 console output need a conversion from wide character to multi-byte format by using function wcstombs().
UTF-32 stores each character in a fixed 4 byte length but nobody use it to store character due to a waste of storage space.
Typically the former, i.e. UTF-8-like. For more info, see Variable-width encoding.
The former - although the term "variable-length encoding" would be more appropriate.
I generally use it to refer to any character that can have more than one byte per character.