https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108976
Bug ID: 108976 Summary: codecvt for Unicode allows surrogate code points Product: gcc Version: 13.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: libstdc++ Assignee: unassigned at gcc dot gnu.org Reporter: dmjpp at hotmail dot com Target Milestone: --- Text in valid Unicode should never contain surrogate code POINTS. Those are only allowed in UTF-16, but only as code UNITS and must be properly paired. UTF-8 text in its strictest form must not contain surrogates but in a slightly relaxed form surrogates can be easily encoded as 3-byte sequences. Same can be said for UTF-32 and UCS-2. Only UTF-16 is immune to the error of surrogate code POINT (they are treated as UNITS). Codecvts in libstdc++ currently allow surrogate code points in some cases. Here is a minimal reproduction (asserts are the correct behavior): #include <locale> #include <cassert> void u32() { using namespace std; auto& f = use_facet<codecvt<char32_t, char, mbstate_t>>(locale::classic()); char u8str[] = "\uC800\uCBFF\uCC00\uCFFF"; u8str[0] = u8str[3] = u8str[6] = u8str[9] = 0xED; // turn the C into D. // now the string is D800, DBFF, DC00 and DFFF encoded in relaxed UTF-8 // that allows surrogate code points. char32_t u32str[] = {0xD800, 0xDBFF, 0xDC00, 0xDFFF, 0}; char32_t u32out[1]; const char* from_next; char32_t* to_next; mbstate_t st = {}; auto res = f.in(st, u8str, u8str+3, from_next, u32out, u32out+1, to_next); assert(res == f.error); assert(from_next == u8str); assert(to_next == u32out); st = {}; auto l = f.length(st, u8str, u8str+3, 1); assert(l == 0); char u8out[3]; const char32_t* from_next2; char* to_next2; st = {}; res = f.out(st, u32str, u32str+1, from_next2, u8out, u8out+3, to_next2); assert(res == f.error); assert(from_next2 == u32str); assert(to_next2 == u8out); } void u16() { using namespace std; auto& f = use_facet<codecvt<char16_t, char, mbstate_t>>(locale::classic()); char u8str[] = "\uC800\uCBFF\uCC00\uCFFF"; u8str[0] = u8str[3] = u8str[6] = u8str[9] = 0xED; // turn the C into D. // now the string is D800, DBFF, DC00 and DFFF encoded in relaxed UTF-8 // that allows surrogates. char16_t u16out[1]; const char* from_next; char16_t* to_next; mbstate_t st = {}; auto res = f.in(st, u8str, u8str+3, from_next, u16out, u16out+1, to_next); assert(res == f.error); assert(from_next == u8str); assert(to_next == u16out); st = {}; auto l = f.length(st, u8str, u8str+3, 1); assert(l == 0); } int main() { u32(); u16(); } >From reading the file codecvt.cc the following conversions have the bug: - From UTF-8 to any other encoding. - From UTF-32/UCS-4 to any other encoding. Those that read from UCS-2 seem to me like they properly report the error. Reading from UTF-16 can not have this bug by definition. From what I checked, the functions for reading UTF-16 properly treat unpaired surrogate code units as error.