Index: src/loadext.c
==================================================================
--- src/loadext.c
+++ src/loadext.c
@@ -412,11 +412,13 @@
   sqlite3_value_subtype,
   sqlite3_result_subtype,
   /* Version 3.10.0 and later */
   sqlite3_status64,
   sqlite3_strlike,
-  sqlite3_db_cacheflush
+  sqlite3_db_cacheflush,
+  /* Version 3.12.0 and later */
+  sqlite3_system_errno
 };
 
 /*
 ** Attempt to load an SQLite extension library contained in the file
 ** zFile.  The entry point is zProc.  zProc may be 0 in which case a

Index: src/main.c
==================================================================
--- src/main.c
+++ src/main.c
@@ -2238,10 +2238,13 @@
   if( !db || db->mallocFailed ){
     return SQLITE_NOMEM_BKPT;
   }
   return db->errCode;
 }
+int sqlite3_system_errno(sqlite3 *db){
+  return db ? db->iSysErrno : 0;
+}  
 
 /*
 ** Return a string that describes the kind of error specified in the
 ** argument.  For now, this simply calls the internal sqlite3ErrStr()
 ** function.

Index: src/os.c
==================================================================
--- src/os.c
+++ src/os.c
@@ -260,10 +260,13 @@
   return pVfs->xRandomness(pVfs, nByte, zBufOut);
 }
 int sqlite3OsSleep(sqlite3_vfs *pVfs, int nMicro){
   return pVfs->xSleep(pVfs, nMicro);
 }
+int sqlite3OsGetLastError(sqlite3_vfs *pVfs){
+  return pVfs->xGetLastError ? pVfs->xGetLastError(pVfs, 0, 0) : 0;
+}
 int sqlite3OsCurrentTimeInt64(sqlite3_vfs *pVfs, sqlite3_int64 *pTimeOut){
   int rc;
   /* IMPLEMENTATION-OF: R-49045-42493 SQLite will use the xCurrentTimeInt64()
   ** method to get the current date and time if that method is available
   ** (if iVersion is 2 or greater and the function pointer is not NULL) and

Index: src/os.h
==================================================================
--- src/os.h
+++ src/os.h
@@ -195,10 +195,11 @@
 void (*sqlite3OsDlSym(sqlite3_vfs *, void *, const char *))(void);
 void sqlite3OsDlClose(sqlite3_vfs *, void *);
 #endif /* SQLITE_OMIT_LOAD_EXTENSION */
 int sqlite3OsRandomness(sqlite3_vfs *, int, char *);
 int sqlite3OsSleep(sqlite3_vfs *, int);
+int sqlite3OsGetLastError(sqlite3_vfs*);
 int sqlite3OsCurrentTimeInt64(sqlite3_vfs *, sqlite3_int64*);
 
 /*
 ** Convenience functions for opening and closing files using 
 ** sqlite3_malloc() to obtain space for the file-handle structure.

Index: src/os_unix.c
==================================================================
--- src/os_unix.c
+++ src/os_unix.c
@@ -6262,27 +6262,22 @@
 }
 #else
 # define unixCurrentTime 0
 #endif
 
-#ifndef SQLITE_OMIT_DEPRECATED
 /*
-** We added the xGetLastError() method with the intention of providing
-** better low-level error messages when operating-system problems come up
-** during SQLite operation.  But so far, none of that has been implemented
-** in the core.  So this routine is never called.  For now, it is merely
-** a place-holder.
+** The xGetLastError() method is designed to return a better
+** low-level error message when operating-system problems come up
+** during SQLite operation.  Only the integer return code is currently
+** used.
 */
 static int unixGetLastError(sqlite3_vfs *NotUsed, int NotUsed2, char *NotUsed3){
   UNUSED_PARAMETER(NotUsed);
   UNUSED_PARAMETER(NotUsed2);
   UNUSED_PARAMETER(NotUsed3);
-  return 0;
+  return errno;
 }
-#else
-# define unixGetLastError 0
-#endif
 
 
 /*
 ************************ End of sqlite3_vfs methods ***************************
 ******************************************************************************/

Index: src/os_win.c
==================================================================
--- src/os_win.c
+++ src/os_win.c
@@ -5582,12 +5582,14 @@
 ** However if an error message is supplied, it will be incorporated
 ** by sqlite into the error message available to the user using
 ** sqlite3_errmsg(), possibly making IO errors easier to debug.
 */
 static int winGetLastError(sqlite3_vfs *pVfs, int nBuf, char *zBuf){
+  DWORD e = osGetLastError();
   UNUSED_PARAMETER(pVfs);
-  return winGetLastErrorMsg(osGetLastError(), nBuf, zBuf);
+  if( nBuf>0 ) winGetLastErrorMsg(e, nBuf, zBuf);
+  return e;
 }
 
 /*
 ** Initialize and deinitialize the operating system interface.
 */

Index: src/sqlite.h.in
==================================================================
--- src/sqlite.h.in
+++ src/sqlite.h.in
@@ -7913,10 +7913,22 @@
 ** ^This function does not set the database handle error code or message
 ** returned by the [sqlite3_errcode()] and [sqlite3_errmsg()] functions.
 */
 int sqlite3_db_cacheflush(sqlite3*);
 
+/*
+** CAPI3REF: Low-level system error code
+**
+** ^Attempt to return the underlying operating system error code or error
+** number that caused the most reason I/O error or failure to open a file.
+** The return value is OS-dependent.  For example, on unix systems, after
+** [sqlite3_open_v2()] returns [SQLITE_CANTOPEN], this interface could be
+** called to get back the underlying "errno" that caused the problem, such
+** as ENOSPC, EAUTH, EISDIR, and so forth.  
+*/
+int sqlite3_system_errno(sqlite3*);
+
 /*
 ** CAPI3REF: Database Snapshot
 ** KEYWORDS: {snapshot}
 ** EXPERIMENTAL
 **

Index: src/sqlite3ext.h
==================================================================
--- src/sqlite3ext.h
+++ src/sqlite3ext.h
@@ -277,10 +277,12 @@
   void (*result_subtype)(sqlite3_context*,unsigned int);
   /* Version 3.10.0 and later */
   int (*status64)(int,sqlite3_int64*,sqlite3_int64*,int);
   int (*strlike)(const char*,const char*,unsigned int);
   int (*db_cacheflush)(sqlite3*);
+  /* Version 3.12.0 and later */
+  int (*system_errno)(sqlite3*);
 };
 
 /*
 ** The following macros redefine the API routines so that they are
 ** redirected through the global sqlite3_api structure.
@@ -520,10 +522,12 @@
 #define sqlite3_result_subtype         sqlite3_api->result_subtype
 /* Version 3.10.0 and later */
 #define sqlite3_status64               sqlite3_api->status64
 #define sqlite3_strlike                sqlite3_api->strlike
 #define sqlite3_db_cacheflush          sqlite3_api->db_cacheflush
+/* Version 3.12.0 and later */
+#define sqlite3_system_errno           sqlite3_api->system_errno
 #endif /* !defined(SQLITE_CORE) && !defined(SQLITE_OMIT_LOAD_EXTENSION) */
 
 #if !defined(SQLITE_CORE) && !defined(SQLITE_OMIT_LOAD_EXTENSION)
   /* This case when the file really is being compiled as a loadable 
   ** extension */

Index: src/sqliteInt.h
==================================================================
--- src/sqliteInt.h
+++ src/sqliteInt.h
@@ -1218,10 +1218,11 @@
   i64 lastRowid;                /* ROWID of most recent insert (see above) */
   i64 szMmap;                   /* Default mmap_size setting */
   unsigned int openFlags;       /* Flags passed to sqlite3_vfs.xOpen() */
   int errCode;                  /* Most recent error code (SQLITE_*) */
   int errMask;                  /* & result codes with this before returning */
+  int iSysErrno;                /* Errno value from last system error */
   u16 dbOptFlags;               /* Flags to enable/disable optimizations */
   u8 enc;                       /* Text encoding */
   u8 autoCommit;                /* The auto-commit flag. */
   u8 temp_store;                /* 1: file 2: memory 0: default */
   u8 mallocFailed;              /* True if we have seen a malloc failure */
@@ -3764,10 +3765,11 @@
 char sqlite3ExprAffinity(Expr *pExpr);
 int sqlite3Atoi64(const char*, i64*, int, u8);
 int sqlite3DecOrHexToI64(const char*, i64*);
 void sqlite3ErrorWithMsg(sqlite3*, int, const char*,...);
 void sqlite3Error(sqlite3*,int);
+void sqlite3SystemError(sqlite3*,int);
 void *sqlite3HexToBlob(sqlite3*, const char *z, int n);
 u8 sqlite3HexToInt(int h);
 int sqlite3TwoPartName(Parse *, Token *, Token *, Token **);
 
 #if defined(SQLITE_NEED_ERR_NAME)

Index: src/test1.c
==================================================================
--- src/test1.c
+++ src/test1.c
@@ -4846,10 +4846,33 @@
   }
 
   Tcl_ResetResult(interp);
   return TCL_OK;
 }
+
+/*
+** Usage:  sqlite3_system_errno DB
+**
+** Return the low-level system errno value.
+*/
+static int test_system_errno(
+  void * clientData,
+  Tcl_Interp *interp,
+  int objc,
+  Tcl_Obj *CONST objv[]
+){
+  sqlite3 *db;
+  int iErrno;
+  if( objc!=2 ){
+    Tcl_WrongNumArgs(interp, 1, objv, "DB");
+    return TCL_ERROR;
+  }
+  if( getDbPointer(interp, Tcl_GetString(objv[1]), &db) ) return TCL_ERROR;
+  iErrno = sqlite3_system_errno(db);
+  Tcl_SetObjResult(interp, Tcl_NewIntObj(iErrno));
+  return TCL_OK;
+}
 
 /*
 ** Usage:  sqlite3_db_filename DB DBNAME
 **
 ** Return the name of a file associated with a database.
@@ -7084,10 +7107,11 @@
      { "uses_stmt_journal",             uses_stmt_journal ,0 },
 
      { "sqlite3_release_memory",        test_release_memory,     0},
      { "sqlite3_db_release_memory",     test_db_release_memory,  0},
      { "sqlite3_db_cacheflush",         test_db_cacheflush,      0},
+     { "sqlite3_system_errno",          test_system_errno,       0},
      { "sqlite3_db_filename",           test_db_filename,        0},
      { "sqlite3_db_readonly",           test_db_readonly,        0},
      { "sqlite3_soft_heap_limit",       test_soft_heap_limit,    0},
      { "sqlite3_thread_cleanup",        test_thread_cleanup,     0},
      { "sqlite3_pager_refcounts",       test_pager_refcounts,    0},

Index: src/util.c
==================================================================
--- src/util.c
+++ src/util.c
@@ -114,18 +114,42 @@
 ** a poiner to that other string.
 */
 const char *sqlite3StrNext(const char *z){
   return z + strlen(z) + 1;
 }
+
+/*
+** Helper function for sqlite3Error() - called rarely.  Broken out into
+** a separate routine to avoid unnecessary register saves on entry to
+** sqlite3Error().
+*/
+static SQLITE_NOINLINE void  sqlite3ErrorFinish(sqlite3 *db, int err_code){
+  if( db->pErr ) sqlite3ValueSetNull(db->pErr);
+  sqlite3SystemError(db, err_code);
+}
 
 /*
 ** Set the current error code to err_code and clear any prior error message.
+** Also set iSysErrno (by calling sqlite3System) if the err_code indicates
+** that would be appropriate.
 */
 void sqlite3Error(sqlite3 *db, int err_code){
   assert( db!=0 );
   db->errCode = err_code;
-  if( db->pErr ) sqlite3ValueSetNull(db->pErr);
+  if( err_code || db->pErr ) sqlite3ErrorFinish(db, err_code);
+}
+
+/*
+** Load the sqlite3.iSysErrno field if that is an appropriate thing
+** to do based on the SQLite error code in rc.
+*/
+void sqlite3SystemError(sqlite3 *db, int rc){
+  if( rc==SQLITE_IOERR_NOMEM ) return;
+  rc &= 0xff;
+  if( rc==SQLITE_CANTOPEN || rc==SQLITE_IOERR ){
+    db->iSysErrno = sqlite3OsGetLastError(db->pVfs);
+  }
 }
 
 /*
 ** Set the most recent error code and error string for the sqlite
 ** handle "db". The error code is set to "err_code".
@@ -148,10 +172,11 @@
 ** to NULL.
 */
 void sqlite3ErrorWithMsg(sqlite3 *db, int err_code, const char *zFormat, ...){
   assert( db!=0 );
   db->errCode = err_code;
+  sqlite3SystemError(db, err_code);
   if( zFormat==0 ){
     sqlite3Error(db, err_code);
   }else if( db->pErr || (db->pErr = sqlite3ValueNew(db))!=0 ){
     char *z;
     va_list ap;

Index: src/vdbe.c
==================================================================
--- src/vdbe.c
+++ src/vdbe.c
@@ -6824,10 +6824,11 @@
   assert( rc );
   if( p->zErrMsg==0 && rc!=SQLITE_IOERR_NOMEM ){
     sqlite3VdbeError(p, "%s", sqlite3ErrStr(rc));
   }
   p->rc = rc;
+  sqlite3SystemError(db, rc);
   testcase( sqlite3GlobalConfig.xLog!=0 );
   sqlite3_log(rc, "statement aborts at %d: [%s] %s", 
                    (int)(pOp - aOp), p->zSql, p->zErrMsg);
   sqlite3VdbeHalt(p);
   if( rc==SQLITE_IOERR_NOMEM ) sqlite3OomFault(db);

Index: test/capi3.test
==================================================================
--- test/capi3.test
+++ test/capi3.test
@@ -170,18 +170,19 @@
 } {SQLITE_OK}
 do_test capi3-3.3 {
   catch {
     set db2 [sqlite3_open /bogus/path/test.db {}]
   }
-  sqlite3_extended_errcode $db2
-} {SQLITE_CANTOPEN}
+  set ::capi3_errno [sqlite3_system_errno $db2]
+  list [sqlite3_extended_errcode $db2] [expr {$::capi3_errno!=0}]
+} {SQLITE_CANTOPEN 1}
 do_test capi3-3.4 {
   sqlite3_errmsg $db2
 } {unable to open database file}
 do_test capi3-3.5 {
-  sqlite3_close $db2
-} {SQLITE_OK}
+  list [sqlite3_system_errno $db2] [sqlite3_close $db2]
+} [list $::capi3_errno SQLITE_OK]
 if {[clang_sanitize_address]==0} {
   do_test capi3-3.6.1-misuse {
     sqlite3_close $db2
   } {SQLITE_MISUSE}
   do_test capi3-3.6.2-misuse {