Question

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.

Answer 1

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'

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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') {}.