TCL_UTF_MAX is the maximum number of bytes that Tcl_UtfToUniChar can consume in a single call.
Tcl_UniCharToUtf stores the character ch as a UTF-8 string in starting at buf. The return value is the number of bytes stored in buf. If ch is a high surrogate (range U+D800 - U+DBFF), then the return value will be 1 and a single byte in the range 0xF0 - 0xF4 will be stored. If you still want to produce UTF-8 output for it (even though knowing it's an illegal code-point on its own), just call Tcl_UniCharToUtf again specifying ch = -1.
Tcl_UtfToUniChar reads one UTF-8 character starting at src and stores it as a Tcl_UniChar in *chPtr. The return value is the number of bytes read from src. The caller must ensure that the source buffer is long enough such that this routine does not run off the end and dereference non-existent or random memory; if the source buffer is known to be null-terminated, this will not happen. If the input is a byte in the range 0x80 - 0x9F, Tcl_UtfToUniChar assumes the cp1252 encoding, stores the corresponding Tcl_UniChar in *chPtr and returns 1. If the input is otherwise not in proper UTF-8 format, Tcl_UtfToUniChar will store the first byte of src in *chPtr as a Tcl_UniChar between 0x00A0 and 0x00FF and return 1.
Tcl_UniCharToUtfDString converts the given Unicode string to UTF-8, storing the result in a previously initialized Tcl_DString. The return value is a pointer to the UTF-8 representation of the Unicode string. Storage for the return value is appended to the end of the Tcl_DString.
Tcl_UtfToUniCharDString converts the given UTF-8 string to Unicode, storing the result in the previously initialized Tcl_DString. In the argument length, you may either specify the length of the given UTF-8 string in bytes or “-1”, in which case Tcl_UtfToUniCharDString uses strlen to calculate the length. The return value is a pointer to the Unicode representation of the UTF-8 string. Storage for the return value is appended to the end of the Tcl_DString. The Unicode string is terminated with a Unicode null character.
Tcl_UniCharLen corresponds to strlen for Unicode characters. It accepts a null-terminated Unicode string and returns the number of Unicode characters (not bytes) in that string.
Tcl_UniCharNcmp and Tcl_UniCharNcasecmp correspond to strncmp and strncasecmp, respectively, for Unicode characters. They accept two null-terminated Unicode strings and the number of characters to compare. Both strings are assumed to be at least numChars characters long. Tcl_UniCharNcmp compares the two strings character-by-character according to the Unicode character ordering. It returns an integer greater than, equal to, or less than 0 if the first string is greater than, equal to, or less than the second string respectively. Tcl_UniCharNcasecmp is the Unicode case insensitive version.
Tcl_UniCharCaseMatch is the Unicode equivalent to Tcl_StringCaseMatch. It accepts a null-terminated Unicode string, a Unicode pattern, and a boolean value specifying whether the match should be case sensitive and returns whether the string matches the pattern.
Tcl_UtfNcmp corresponds to strncmp for UTF-8 strings. It accepts two null-terminated UTF-8 strings and the number of characters to compare. (Both strings are assumed to be at least numChars characters long.) Tcl_UtfNcmp compares the two strings character-by-character according to the Unicode character ordering. It returns an integer greater than, equal to, or less than 0 if the first string is greater than, equal to, or less than the second string respectively.
Tcl_UtfNcasecmp corresponds to strncasecmp for UTF-8 strings. It is similar to Tcl_UtfNcmp except comparisons ignore differences in case when comparing upper, lower or title case characters.
Tcl_UtfCharComplete returns 1 if the source UTF-8 string src of length bytes is long enough to be decoded by Tcl_UtfToUniChar/Tcl_UtfNext, or 0 otherwise. This function does not guarantee that the UTF-8 string is properly formed. This routine is used by procedures that are operating on a byte at a time and need to know if a full Unicode character has been seen.
Tcl_NumUtfChars corresponds to strlen for UTF-8 strings. It returns the number of Tcl_UniChars that are represented by the UTF-8 string src. The length of the source string is length bytes. If the length is negative, all bytes up to the first null byte are used.
Tcl_UtfFindFirst corresponds to strchr for UTF-8 strings. It returns a pointer to the first occurrence of the Unicode character ch in the null-terminated UTF-8 string src. The null terminator is considered part of the UTF-8 string.
Tcl_UtfFindLast corresponds to strrchr for UTF-8 strings. It returns a pointer to the last occurrence of the Unicode character ch in the null-terminated UTF-8 string src. The null terminator is considered part of the UTF-8 string.
Given src, a pointer to some location in a UTF-8 string, Tcl_UtfNext returns a pointer to the next UTF-8 character in the string. The caller must not ask for the next character after the last character in the string if the string is not terminated by a null character. Tcl_UtfCharComplete can be used in that case to make sure enough bytes are available before calling Tcl_UtfNext.
Tcl_UtfPrev is used to step backward through but not beyond the UTF-8 string that begins at start. If the UTF-8 string is made up entirely of complete and well-formed characters, and src points to the lead byte of one of those characters (or to the location one byte past the end of the string), then repeated calls of Tcl_UtfPrev will return pointers to the lead bytes of each character in the string, one character at a time, terminating when it returns start.
When the conditions of completeness and well-formedness may not be satisfied, a more precise description of the function of Tcl_UtfPrev is necessary. It always returns a pointer greater than or equal to start; that is, always a pointer to a location in the string. It always returns a pointer to a byte that begins a character when scanning for characters beginning from start. When src is greater than start, it always returns a pointer less than src and greater than or equal to (src - 4). The character that begins at the returned pointer is the first one that either includes the byte src[-1], or might include it if the right trail bytes are present at src and greater. Tcl_UtfPrev never reads the byte src[0] nor the byte start[-1] nor the byte src[-5].
Tcl_UniCharAtIndex corresponds to a C string array dereference or the Pascal Ord() function. It returns the Unicode character represented at the specified character (not byte) index in the UTF-8 string src. The source string must contain at least index characters. If a negative index is given or index points to the second half of a surrogate pair, it returns -1.
Tcl_UtfAtIndex returns a pointer to the specified character (not byte) index in the UTF-8 string src. The source string must contain at least index characters. This is equivalent to calling Tcl_UtfToUniChar index times, except if that would return a pointer to the second byte of a valid 4-byte UTF-8 sequence, in which case, Tcl_UtfToUniChar will be called once more to find the end of the sequence. If a negative index is given, the returned pointer points to the first character in the source string.
Tcl_UtfBackslash is a utility procedure used by several of the Tcl commands. It parses a backslash sequence and stores the properly formed UTF-8 character represented by the backslash sequence in the output buffer dst. At most 4 bytes are stored in the buffer. Tcl_UtfBackslash modifies *readPtr to contain the number of bytes in the backslash sequence, including the backslash character. The return value is the number of bytes stored in the output buffer.
See the Tcl manual entry for information on the valid backslash sequences. All of the sequences described in the Tcl manual entry are supported by Tcl_UtfBackslash.