OSError and _Py_fstat_noraise rely on winerror_to_errno() inPC/errmap.h in order to translate a Windows system error code into a POSIX errno value.PC/errmap.h is supposed to be generated byPC/generrmap.c, which is based on the old CRT's _dosmapperr() function. However, this function isn't implemented in the Universal CRT, and ucrt's corresponding function, __acrt_errno_from_os_error, isn't exported for public use.If generrmap.c is effectively dead, then it's no longer possible to add custom mappings there, as was done forissue 12802 (ERROR_DIRECTORY -> ENOTDIR) andissue 13063 (ERROR_NO_DATA -> EPIPE). Also, errmap.h hasn't been regenerated in 8 years, and since then the CRT added a new mapped value: ERROR_NO_UNICODE_TRANSLATION (1113) -> EILSEQ. Unless someone can suggest a way to continue automatically generating errmap.h via generrmap.c, then I think winerror_to_errno should be manually implemented in a more readable and maintainable way, and updated from the ucrt source (ucrt\misc\errno.cpp) with each major Python release. The implementation could use a switch statement like it currently does, but with named error codes, grouped by result. For example:    int    winerror_to_errno(int winerror)    {        switch(winerror) {        case ERROR_FILE_NOT_FOUND:            //    2        case ERROR_PATH_NOT_FOUND:            //    3        case ERROR_INVALID_DRIVE:             //   15        case ERROR_NO_MORE_FILES:             //   18        case ERROR_BAD_NETPATH:               //   53        case ERROR_BAD_NET_NAME:              //   67        case ERROR_BAD_PATHNAME:              //  161        case ERROR_FILENAME_EXCED_RANGE:      //  206            return ENOENT;        case ERROR_BAD_ENVIRONMENT:           //   10            return E2BIG;        case ERROR_BAD_FORMAT:                //   11        case ERROR_INVALID_STARTING_CODESEG:  //  188        case ERROR_INVALID_STACKSEG:          //  189        case ERROR_INVALID_MODULETYPE:        //  190        case ERROR_INVALID_EXE_SIGNATURE:     //  191        case ERROR_EXE_MARKED_INVALID:        //  192        case ERROR_BAD_EXE_FORMAT:            //  193        case ERROR_ITERATED_DATA_EXCEEDS_64k: //  194        case ERROR_INVALID_MINALLOCSIZE:      //  195        case ERROR_DYNLINK_FROM_INVALID_RING: //  196        case ERROR_IOPL_NOT_ENABLED:          //  197        case ERROR_INVALID_SEGDPL:            //  198        case ERROR_AUTODATASEG_EXCEEDS_64k:   //  199        case ERROR_RING2SEG_MUST_BE_MOVABLE:  //  200        case ERROR_RELOC_CHAIN_XEEDS_SEGLIM:  //  201        case ERROR_INFLOOP_IN_RELOC_CHAIN:    //  202            return ENOEXEC;        case ERROR_INVALID_HANDLE:            //    6        case ERROR_INVALID_TARGET_HANDLE:     //  114        case ERROR_DIRECT_ACCESS_HANDLE:      //  130            return EBADF;        case ERROR_WAIT_NO_CHILDREN:          //  128        case ERROR_CHILD_NOT_COMPLETE:        //  129            return ECHILD;        case ERROR_NO_PROC_SLOTS:             //   89        case ERROR_MAX_THRDS_REACHED:         //  164        case ERROR_NESTING_NOT_ALLOWED:       //  215            return EAGAIN;        case ERROR_ARENA_TRASHED:             //    7        case ERROR_NOT_ENOUGH_MEMORY:         //    8        case ERROR_INVALID_BLOCK:             //    9        case ERROR_NOT_ENOUGH_QUOTA:          // 1816            return ENOMEM;        case ERROR_ACCESS_DENIED:             //    5        case ERROR_CURRENT_DIRECTORY:         //   16        case ERROR_WRITE_PROTECT:             //   19        case ERROR_BAD_UNIT:                  //   20        case ERROR_NOT_READY:                 //   21        case ERROR_BAD_COMMAND:               //   22        case ERROR_CRC:                       //   23        case ERROR_BAD_LENGTH:                //   24        case ERROR_SEEK:                      //   25        case ERROR_NOT_DOS_DISK:              //   26        case ERROR_SECTOR_NOT_FOUND:          //   27        case ERROR_OUT_OF_PAPER:              //   28        case ERROR_WRITE_FAULT:               //   29        case ERROR_READ_FAULT:                //   30        case ERROR_GEN_FAILURE:               //   31        case ERROR_SHARING_VIOLATION:         //   32        case ERROR_LOCK_VIOLATION:            //   33        case ERROR_WRONG_DISK:                //   34        case ERROR_SHARING_BUFFER_EXCEEDED:   //   36        case ERROR_NETWORK_ACCESS_DENIED:     //   65        case ERROR_CANNOT_MAKE:               //   82        case ERROR_FAIL_I24:                  //   83        case ERROR_DRIVE_LOCKED:              //  108        case ERROR_SEEK_ON_DEVICE:            //  132        case ERROR_NOT_LOCKED:                //  158        case ERROR_LOCK_FAILED:               //  167        case 35:                              //   35 [undefined]            return EACCES;        case ERROR_FILE_EXISTS:               //   80        case ERROR_ALREADY_EXISTS:            //  183            return EEXIST;        case ERROR_NOT_SAME_DEVICE:           //   17            return EXDEV;        case ERROR_DIRECTORY:                 //  267 [bpo-12802]            return ENOTDIR;        case ERROR_TOO_MANY_OPEN_FILES:       //    4            return EMFILE;        case ERROR_DISK_FULL:                 //  112            return ENOSPC;        case ERROR_BROKEN_PIPE:               //  109        case ERROR_NO_DATA:                   //  232 [bpo-13063]            return EPIPE;        case ERROR_DIR_NOT_EMPTY:             //  145            return ENOTEMPTY;        case ERROR_NO_UNICODE_TRANSLATION:    // 1113            return EILSEQ;        case ERROR_INVALID_FUNCTION:          //    1        case ERROR_INVALID_ACCESS:            //   12        case ERROR_INVALID_DATA:              //   13        case ERROR_INVALID_PARAMETER:         //   87        case ERROR_NEGATIVE_SEEK:             //  131        default:            return EINVAL;        }    }

Special casing of Winsock error codes should be handled in winerror_to_errno rather than in oserror_parse_args, so it's consistently applied everywhere. And it should use the proper range for Winsock, 10000-11999 -- not 10000-2147483647. Codes above the Winsock range should map to EINVAL. It's not a non-issue since this range has the following blocks allocated:    IPSEC     13000-13999    SXS       14000-14999    EVT       15000-15079    EC        15080-15099    MUI       15100-15199    MCA       15200-15249    SYSPART   15250-15299    VORTEX    15300-15320    GPIO      15321-15340    RUNLEVEL  15400-15500    COMTASK   15501-15510    APPX      15600-15699    APPMODEL  15700-15720    APPXSTATE 15800-15840    API       15841-15860    STORE     15861-15880Another thing winerror_to_errno should do is unwrap HRESULT codes for Win32 errors (0x80070000-0x8007FFFF). In other words, 0x80070005 is effectively the same as ERROR_ACCESS_DENIED (5). It should be mapped to EACCES and raise PermissionError.For example:    int    winerror_to_errno(int winerror)    {        /* Unwrap FACILITY_WIN32 HRESULT errors. */        if ((winerror & 0xFFFF0000) == 0x80070000) {            winerror &= 0x0000FFFF;        }        /* Winsock error codes (10000-11999) are errno values. */        if (winerror >= 10000 && winerror < 12000) {            return winerror;        }        switch(winerror) {                /* ... */                }    }

While I'm inclined to think it's okay to find the CRT sources (e.g. "C:\Program Files (x86)\Windows Kits\10\Source\10.0.18362.0\ucrt\misc\errno.cpp") and extract the table from there as part of build, the problem I have is that it is woefully incomplete.I'd much rather have a manually managed table. It'd also be nice to expose it through the errno module somehow, probably just as a dict.It also seems like at least some of the WSA* constants (e.g. WSAEBADF==10009) are the equivalent errno plus 10000 (e.g.EBADF==9). Should we be mapping those back to the errno value?

> It also seems like at least some of the WSA* constants (e.g. > WSAEBADF==10009) are the equivalent errno plus 10000 > (e.g.EBADF==9). Should we be mapping those back to the > errno value?Mapping WSAEINTR (10004) -> EINTR (4) and WSAEACCES (10013) -> EACCES (13) would allow special casing them as InterruptedError and PermissionError. This is different from the aliases such as EWOULDBLOCK = WSAEWOULDBLOCK, but it should be fine since the six affected values are singled out in WinSock2.h as C errno values. So the Winsock section becomes a switch:        /* Winsock error codes (10000-11999) are errno values. */        if (winerror >= 10000 && winerror < 12000) {            switch(winerror) {            case WSAEINTR:            case WSAEBADF:            case WSAEACCES:            case WSAEFAULT:            case WSAEINVAL:            case WSAEMFILE:                /* Winsock definitions of errno values. See WinSock2.h */                return winerror - 10000;            default:                return winerror;            }        }

I have created a pull request for this issue. Please take a look.

I'm going to merge and backport this all the way to 3.7. Adding a publicly accessible dict for use in Python code should be 3.9 only.

New changeset19052a11314e7be7ba003fd6cdbb5400a5d77d96 by Steve Dower (Zackery Spytz) in branch 'master':bpo-37705: Improve the implementation of winerror_to_errno() (GH-15623)https://github.com/python/cpython/commit/19052a11314e7be7ba003fd6cdbb5400a5d77d96

New changeset68e401fa0b1a3407bce395ad893535f65107ee6e by Miss Islington (bot) in branch '3.8':bpo-37705: Improve the implementation of winerror_to_errno() (GH-15623)https://github.com/python/cpython/commit/68e401fa0b1a3407bce395ad893535f65107ee6e

New changesetc25759187829d5732eea73f52a269c88aba28371 by Miss Islington (bot) in branch '3.7':bpo-37705: Improve the implementation of winerror_to_errno() (GH-15623)https://github.com/python/cpython/commit/c25759187829d5732eea73f52a269c88aba28371

New changeset38e0b9efdf164be656782db0f969fb536cfbcaf1 by Oleg Iarygin in branch 'main':bpo-37705: Remove orphanedPC/errmap.mak (GH-29724)https://github.com/python/cpython/commit/38e0b9efdf164be656782db0f969fb536cfbcaf1