How do I index a (not all ascii) utf8 string in C? - c

I want to index the characters in a utf8 string which does not necessarily contain
only ascii characters. I want the same kind of behavior I get in javascript:
> str = "lλך" // i.e. Latin ell, Greek lambda, Hebrew lamedh
'lλך'
> str[0]
'l'
> str[1]
'λ'
> str[2]
'ך'
Following the advice of UTF-8 Everywhere, I am representing my mixed character-length string just as any other sting in c - and not using wchars.
The problem is that, in C, one cannot access the 16th character of a string: only the 16th byte. Because λ is encoded with two bytes in utf-8, I have to access the 16th and 17th bytes of the string in order to print out one λ.
For reference, the output of:
#include <stdio.h>
int main () {
char word_with_greek[] = "this is lambda:_λ";
printf("%s\n",word_with_greek);
printf("The 0th character is: %c\n", word_with_greek[0]);
printf("The 15th character is: %c\n",word_with_greek[15]);
printf("The 16th character is: %c%c\n",word_with_greek[16],word_with_greek[17]);
return 0;
}
is:
this is lambda:_λ
The 0th character is: t
The 15th character is: _
The 16th character is: λ
Is there an easy way to break up the string into characters? It does not seem too difficult to write a function which breaks a string into wchars- but I imagine that someone has already written this yet I cannot find it.

It depends on what your unicode characters can be. Most strings are restricted to the Basic Multilanguage Plane. If yours are (not by accident by because of their very nature: at least no risk for emoji...) you can use the char16_t to represent any character. BTW wchar_t is at least as large as char16_t so in that case it is safe to use it.
If your script can contain emoji character, or other characters not in the BMP or simply if you are unsure, the only foolproof way is to convert everything to char32_t because any unicode character (at least in 2019...) as a code using less than 32 bits.
Converting for UTF8 to 32 (or 16) bits unicode is not that hard, and can be coded by hand, Wikipedia contains enough information for it. But you will find tons of library where this is already coded and tested, mainly the excellent libiconv, but the C11 version of the C standard library contains functions for UTF8 conversions. Not as nice but useable.

Related

Can I store UTF8 in C-style char array

Follow up
Can UTF-8 contain zero byte?
Can I safely store UTF8 string in zero terminated char * ?
I understand strlen() will not return correct information, put "storing", printing and "transferring" the char array, seems to be safe.
Yes.
Just like with ASCII and similiar 8-bit encodings before Unicode, you can't store the NUL character in such a string (the value \u+0000 is the Unicode code point NUL, very much like in ASCII).
As long as you know your strings don't need to contain that (and regular text doesn't), it's fine.
In C a 0 byte is the string terminator. As long as the Unicode point 0, U+0000 is not in the Unicode string there is no problem.
To be able to store 0 bytes in Unicode, one may use modified UTF-8 that convert not only code points >= 128, but also 0 to a multi-byte sequence (every byte thereof having its high bit set, >= 128). This is done in java for some APIs, like DataOutputStream.writeUTF. It ensures you can transmit strings with an embedded 0.
It formally is no longer UTF-8 as UTF-8 requires the shortest encoding. Also this is only possible when determining the length i.o. strlen when unpacking to non-UTF-8.
So the most feasible solution is not to accept U+0000 in strings.

Will gcc functions in string.h break UTF-8 string?

I don't know the following cases in GCC, who can help me?
Whether a valid UTF-8 character (except code point 0) still contains zero byte? If so, I think function such as strlen will break that UTF-8 character.
Whether a valid UTF-8 character contains a byte whose value is equal to '\n'? If so, I think function such as "gets" will break that UTF-8 character.
Whether a valid UTF-8 character contains a byte whose value is equal to ' ' or '\t'? If so, I think function such as scanf("%s%s") will break that UTF-8 character and be interpreted as two or more words.
The answer to all your questions are the same: No.
It's one of the advantages of UTF-8: all ASCII bytes do not occur when encoding non-ASCII code points into UTF-8.
For example, you can safely use strlen on a UTF-8 string, only that its result is the number of bytes instead of UTF-8 code points.

Is there a difference between an string and an ASCII string in C?

In my computer architecture course we are being asked to write a program that takes in a pointer to an 'ASCII' string as one of our function arguments. Being unaware that there may be different variations of strings, I would have just declared an array of characters as usual, but how would someone declare an array of characters that are specific to an ASCII string. Please provide an example of a pointer to an ASCII string declaration. Please comment below if anything mentioned was unclear.
Every string in C is ASCII. Standard ASCII characters are 7-bits wide. Your professor just want to emphasis you should use byte array to store the string. Unlike let's say UTF-8 which can use up to 4 bytes for encoding a certain characters.
char *str = "example";

What's the use of universal characters on POSIX system?

In C one can pass unicode characters to printf() like this:
printf("some unicode char: %c\n", "\u00B1");
But the problem is that on POSIX compliant systems `char' is always 8 bits and most of UTF-8 character such as the above are wider and don't fit into char and as the result nothing is printed on the terminal. I can do this to achieve this effect however:
printf("some unicode char: %s\n", "\u00B1");
%s placeholder is expanded automatically and a unicode character is printed on the terminal. Also, in a standard it says:
If the hexadecimal value for a universal character name is less than
0x20 or in the range 0x7F-0x9F (inclusive), or if the universal
character name designates a character in the basic source character
set, then the program is illformed.
When I do this:
printf("letter a: %c\n", "\u0061");
gcc says:
error: \u0061 is not a valid universal character
So this technique is also unusable for printing ASCII characters. In this article on Wikipedia http://en.wikipedia.org/wiki/Character_(computing)#cite_ref-3 it says:
A char in the C programming language is a data type with the size of
exactly one byte, which in turn is defined to be large enough to
contain any member of the basic execution character set and UTF-8 code
units.
But is this doable on POSIX systems?
Use of universal characters in byte-based strings is dependent on the compile-time and run-time character encodings matching, so it's generally not a good idea except in certain situations. However they work very well in wide string and wide character literals: printf("%ls", L"\u00B1"); or printf("%lc", L'\00B1'); will print U+00B1 in the correct encoding for your locale.

Trouble comparing UTF-8 characters using wchar.h

I am in the process of making a small program that reads a file, that contains UTF-8 elements, char by char. After reading a char it compares it with a few other characters and if there is a match it replaces the character in the file with an underscore '_'.
(Well, it actually makes a duplicate of that file with specific letters replaced by underscores.)
I'm not sure where exactly I'm messing up here but it's most likely everywhere.
Here is my code:
FILE *fpi;
FILE *fpo;
char ifilename[FILENAME_MAX];
char ofilename[FILENAME_MAX];
wint_t sample;
fpi = fopen(ifilename, "rb");
fpo = fopen(ofilename, "wb");
while (!feof(fpi)) {
fread(&sample, sizeof(wchar_t*), 1, fpi);
if ((wcscmp(L"ά", &sample) == 0) || (wcscmp(L"ε", &sample) == 0) ) {
fwrite(L"_", sizeof(wchar_t*), 1, fpo);
} else {
fwrite(&sample, sizeof(wchar_t*), 1, fpo);
}
}
I have omitted the code that has to do with the filename generation because it has nothing to offer to the case. It is just string manipulation.
If I feed this program a file containing the words γειά σου κόσμε. I would want it to return this:
γει_ σου κόσμ_.
Searching the internet didn't help much as most results were very general or talking about completely different things regarding UTF-8. It's like nobody needs to manipulate single characters for some reason.
Anything pointing me the right way is most welcome.
I am not, necessarily, looking for a straightforward fixed version of the code I submitted, I would be grateful for any insightful comments helping me understand how exactly the wchar mechanism works. The whole wbyte, wchar, L, no-L, thing is a mess to me.
Thank you in advance for your help.
C has two different kinds of characters: multibyte characters and wide characters.
Multibyte characters can take a varying number of bytes. For instance, in UTF-8 (which is a variable-length encoding of Unicode), a takes 1 byte, while α takes 2 bytes.
Wide characters always take the same number of bytes. Additionally, a wchar_t must be able to hold any single character from the execution character set. So, when using UTF-32, both a and α take 4 bytes each. Unfortunately, some platforms made wchar_t 16 bits wide: such platforms cannot correctly support characters beyond the BMP using wchar_t. If __STDC_ISO_10646__ is defined, wchar_t holds Unicode code-points, so must be (at least) 4 bytes long (technically, it must be at least 21-bits long).
So, when using UTF-8, you should use multibyte characters, which are stored in normal char variables (but beware of strlen(), which counts bytes, not multibyte characters).
Unfortunately, there is more to Unicode than this.
ά can be represented as a single Unicode codepoint, or as two separate codepoints:
U+03AC GREEK SMALL LETTER ALPHA WITH TONOS ← 1 codepoint ← 1 multibyte character ← 2 bytes (0xCE 0xAC) = 2 char's.
U+03B1 GREEK SMALL LETTER ALPHA U+0301 COMBINING ACUTE ACCENT ← 2 codepoints ← 2 multibyte characters ← 4 bytes (0xCE 0xB1 0xCC 0x81) = 4 char's.
U+1F71 GREEK SMALL LETTER ALPHA WITH OXIA ← 1 codepoint ← 1 multibyte character ← 3 bytes (0xE1 0xBD 0xB1) = 3 char's.
All of the above are canonical equivalents, which means that they should be treated as equal for all purposes. So, you should normalize your strings on input/output, using one of the Unicode normalization algorithms (there are 4: NFC, NFD, NFKC, NFKD).
First of all, please do take the time to read this great article, which explains UTF8 vs Unicode and lots of other important things about strings and encodings: http://www.joelonsoftware.com/articles/Unicode.html
What you are trying to do in your code is read in unicode character by character, and do comparisons with those. That's won't work if the input stream is UTF8, and it's not really possible to do with quite this structure.
In short: Fully unicode strings can be encoded in several ways. One of them is using a series of equally-sized "wide" chars, one for each character. That is what the wchar_t type (sometimes WCHAR) is for. Another way is UTF8, which uses a variable number of raw bytes to encode each character, depending on the value of the character.
UTF8 is just a stream of bytes, which can encode a unicode string, and is commonly used in files. It is not the same as a string of WCHARs, which are the more common in-memory representation. You can't poke through a UTF8 stream reliably, and do character replacements within it directly. You'll need to read the whole thing in and decode it, and then loop through the WCHARs that result to do your comparisons and replacement, and then map that result back to UTF8 to write to the output file.
On Win32, use MultiByteToWideChar to do the decoding, and you can use the corresponding WideCharToMultiByte to go back.
When you use a "string literal" with regular quotes, you're creating a nul-terminated ASCII string (char*), which does not support Unicode. The L"string literal" with the L prefix will create a nul-terminated string of WCHARs (wchar_t *), which you can use in string or character comparisons. The L prefix also works with single-quote character literals, like so: L'ε'
As a commenter noted, when you use fread/fwrite, you should be using sizeof(wchar_t) and not its pointer type, since the amount you are trying to read/write is an actual wchar, not the size of a pointer to one. This advice is just code feedback independent of the above-- you don't want to be reading the input character by character anyways.
Note too that when you do string comparisons (wcscmp), you should use actual wide strings (which are terminated with a nul wide char)-- not use single characters in memory as input. If (when) you want to do character-to-character comparisons, you don't even need to use the string functions. Since a WCHAR is just a value, you can compare directly: if (sample == L'ά') {}.

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