diff --git a/common.mk b/common.mk
index ec13d25ca22a56..71ae5eb6e6485a 100644
--- a/common.mk
+++ b/common.mk
@@ -10997,6 +10997,7 @@ process.$(OBJEXT): $(top_srcdir)/internal/fixnum.h
 process.$(OBJEXT): $(top_srcdir)/internal/gc.h
 process.$(OBJEXT): $(top_srcdir)/internal/hash.h
 process.$(OBJEXT): $(top_srcdir)/internal/imemo.h
+process.$(OBJEXT): $(top_srcdir)/internal/io.h
 process.$(OBJEXT): $(top_srcdir)/internal/numeric.h
 process.$(OBJEXT): $(top_srcdir)/internal/object.h
 process.$(OBJEXT): $(top_srcdir)/internal/process.h
diff --git a/debug.c b/debug.c
index d3f41f1a3a1ebe..b5cba590bab5d3 100644
--- a/debug.c
+++ b/debug.c
@@ -70,7 +70,7 @@ const union {
         RUBY_FMODE_NOREVLOOKUP		= 0x00000100,
         RUBY_FMODE_TRUNC		= FMODE_TRUNC,
         RUBY_FMODE_TEXTMODE		= FMODE_TEXTMODE,
-        RUBY_FMODE_PREP 		= 0x00010000,
+        RUBY_FMODE_EXTERNAL		= 0x00010000,
         RUBY_FMODE_SETENC_BY_BOM	= FMODE_SETENC_BY_BOM,
         RUBY_FMODE_UNIX 		= 0x00200000,
         RUBY_FMODE_INET 		= 0x00400000,
diff --git a/ext/pty/pty.c b/ext/pty/pty.c
index acec33f9bf5342..0aca10bfa070b5 100644
--- a/ext/pty/pty.c
+++ b/ext/pty/pty.c
@@ -448,8 +448,10 @@ pty_close_pty(VALUE assoc)
 
     for (i = 0; i < 2; i++) {
         io = rb_ary_entry(assoc, i);
-        if (RB_TYPE_P(io, T_FILE) && 0 <= RFILE(io)->fptr->fd)
+        if (RB_TYPE_P(io, T_FILE)) {
+            /* it's OK to call rb_io_close again even if it's already closed */
             rb_io_close(io);
+        }
     }
     return Qnil;
 }
@@ -499,28 +501,21 @@ pty_open(VALUE klass)
 {
     int master_fd, slave_fd;
     char slavename[DEVICELEN];
-    VALUE master_io, slave_file;
-    rb_io_t *master_fptr, *slave_fptr;
-    VALUE assoc;
 
     getDevice(&master_fd, &slave_fd, slavename, 1);
 
-    master_io = rb_obj_alloc(rb_cIO);
-    MakeOpenFile(master_io, master_fptr);
-    master_fptr->mode = FMODE_READWRITE | FMODE_SYNC | FMODE_DUPLEX;
-    master_fptr->fd = master_fd;
-    master_fptr->pathv = rb_obj_freeze(rb_sprintf("masterpty:%s", slavename));
+    VALUE master_path = rb_obj_freeze(rb_sprintf("masterpty:%s", slavename));
+    VALUE master_io = rb_io_open_descriptor(rb_cIO, master_fd, FMODE_READWRITE | FMODE_SYNC | FMODE_DUPLEX, master_path, RUBY_IO_TIMEOUT_DEFAULT, NULL);
+
+    VALUE slave_path = rb_obj_freeze(rb_str_new_cstr(slavename));
+    VALUE slave_file = rb_io_open_descriptor(rb_cFile, slave_fd, FMODE_READWRITE | FMODE_SYNC | FMODE_DUPLEX | FMODE_TTY, slave_path, RUBY_IO_TIMEOUT_DEFAULT, NULL);
 
-    slave_file = rb_obj_alloc(rb_cFile);
-    MakeOpenFile(slave_file, slave_fptr);
-    slave_fptr->mode = FMODE_READWRITE | FMODE_SYNC | FMODE_DUPLEX | FMODE_TTY;
-    slave_fptr->fd = slave_fd;
-    slave_fptr->pathv = rb_obj_freeze(rb_str_new_cstr(slavename));
+    VALUE assoc = rb_assoc_new(master_io, slave_file);
 
-    assoc = rb_assoc_new(master_io, slave_file);
     if (rb_block_given_p()) {
         return rb_ensure(rb_yield, assoc, pty_close_pty, assoc);
     }
+
     return assoc;
 }
 
@@ -577,30 +572,27 @@ pty_getpty(int argc, VALUE *argv, VALUE self)
 {
     VALUE res;
     struct pty_info info;
-    rb_io_t *wfptr,*rfptr;
-    VALUE rport = rb_obj_alloc(rb_cFile);
-    VALUE wport = rb_obj_alloc(rb_cFile);
     char SlaveName[DEVICELEN];
 
-    MakeOpenFile(rport, rfptr);
-    MakeOpenFile(wport, wfptr);
-
     establishShell(argc, argv, &info, SlaveName);
 
-    rfptr->mode = rb_io_modestr_fmode("r");
-    rfptr->fd = info.fd;
-    rfptr->pathv = rb_obj_freeze(rb_str_new_cstr(SlaveName));
+    VALUE pty_path = rb_obj_freeze(rb_str_new_cstr(SlaveName));
+    VALUE rport = rb_io_open_descriptor(
+        rb_cFile, info.fd, FMODE_READABLE, pty_path, RUBY_IO_TIMEOUT_DEFAULT, NULL
+    );
 
-    wfptr->mode = rb_io_modestr_fmode("w") | FMODE_SYNC;
-    wfptr->fd = rb_cloexec_dup(info.fd);
-    if (wfptr->fd == -1)
+    int wpty_fd = rb_cloexec_dup(info.fd);
+    if (wpty_fd == -1) {
         rb_sys_fail("dup()");
-    rb_update_max_fd(wfptr->fd);
-    wfptr->pathv = rfptr->pathv;
+    }
+    VALUE wport = rb_io_open_descriptor(
+        rb_cFile, wpty_fd, FMODE_WRITABLE | FMODE_TRUNC | FMODE_CREATE | FMODE_SYNC,
+        pty_path, RUBY_IO_TIMEOUT_DEFAULT, NULL
+    );
 
     res = rb_ary_new2(3);
-    rb_ary_store(res,0,(VALUE)rport);
-    rb_ary_store(res,1,(VALUE)wport);
+    rb_ary_store(res, 0, rport);
+    rb_ary_store(res, 1, wport);
     rb_ary_store(res,2,PIDT2NUM(info.child_pid));
 
     if (rb_block_given_p()) {
diff --git a/file.c b/file.c
index 2c74f3eff5ef0c..5f59373a451439 100644
--- a/file.c
+++ b/file.c
@@ -169,7 +169,6 @@ int flock(int, int);
 #include "internal/thread.h"
 #include "internal/vm.h"
 #include "ruby/encoding.h"
-#include "ruby/io.h"
 #include "ruby/thread.h"
 #include "ruby/util.h"
 
diff --git a/include/ruby/io.h b/include/ruby/io.h
index 8be83a215ca4b8..9b05c7b16411a2 100644
--- a/include/ruby/io.h
+++ b/include/ruby/io.h
@@ -331,7 +331,16 @@ typedef struct rb_io_enc_t rb_io_enc_t;
  * Setting this one and #FMODE_BINMODE at the same time is a contradiction.
  */
 #define FMODE_TEXTMODE              0x00001000
-/* #define FMODE_PREP               0x00010000 */
+/**
+ * This flag means that an IO object is wrapping an "external" file descriptor,
+ * which is owned by something outside the Ruby interpreter (usually a C extension).
+ * Ruby will not close this file when the IO object is garbage collected.
+ * If this flag is set, then IO#autoclose? is false, and vice-versa.
+ *
+ * This flag was previously called FMODE_PREP internally.
+ */
+#define FMODE_EXTERNAL              0x00010000
+
 /* #define FMODE_SIGNAL_ON_EPIPE    0x00020000 */
 
 /**
@@ -345,6 +354,18 @@ typedef struct rb_io_enc_t rb_io_enc_t;
 
 /** @} */
 
+/**
+ * Allocate a new IO object, with the given file descriptor.
+ */
+VALUE rb_io_open_descriptor(VALUE klass, int descriptor, int mode, VALUE path, VALUE timeout, struct rb_io_enc_t *encoding);
+
+/**
+ * Returns whether or not the underlying IO is closed.
+ *
+ * @return Whether the underlying IO is closed.
+ */
+VALUE rb_io_closed_p(VALUE io);
+
 /**
  * Queries the underlying IO pointer.
  *
@@ -703,6 +724,12 @@ VALUE rb_io_set_write_io(VALUE io, VALUE w);
  */
 void rb_io_set_nonblock(rb_io_t *fptr);
 
+/**
+ * Returns the path for the given IO.
+ *
+ */
+VALUE rb_io_path(VALUE io);
+
 /**
  * Returns an integer representing the numeric file descriptor for
  * <em>io</em>.
@@ -712,6 +739,12 @@ void rb_io_set_nonblock(rb_io_t *fptr);
  */
 int rb_io_descriptor(VALUE io);
 
+/**
+ * Get the mode of the IO.
+ *
+ */
+int rb_io_mode(VALUE io);
+
 /**
  * This function  breaks down the  option hash that `IO#initialize`  takes into
  * components.   This is  an implementation  detail of  rb_io_extract_modeenc()
diff --git a/io.c b/io.c
index 88cb2667f5b579..b99041fbd1caaa 100644
--- a/io.c
+++ b/io.c
@@ -16,11 +16,6 @@
 #include "ruby/fiber/scheduler.h"
 #include "ruby/io/buffer.h"
 
-#ifdef _WIN32
-# include "ruby/ruby.h"
-# include "ruby/io.h"
-#endif
-
 #include <ctype.h>
 #include <errno.h>
 #include <stddef.h>
@@ -525,7 +520,6 @@ rb_cloexec_fcntl_dupfd(int fd, int minfd)
 static int io_fflush(rb_io_t *);
 static rb_io_t *flush_before_seek(rb_io_t *fptr);
 
-#define FMODE_PREP (1<<16)
 #define FMODE_SIGNAL_ON_EPIPE (1<<17)
 
 #define fptr_signal_on_epipe(fptr) \
@@ -1463,7 +1457,7 @@ rb_io_wait(VALUE io, VALUE events, VALUE timeout)
 static VALUE
 io_from_fd(int fd)
 {
-    return prep_io(fd, FMODE_PREP, rb_cIO, NULL);
+    return prep_io(fd, FMODE_EXTERNAL, rb_cIO, NULL);
 }
 
 static int
@@ -2875,6 +2869,13 @@ rb_io_descriptor(VALUE io)
     }
 }
 
+int rb_io_mode(VALUE io)
+{
+    rb_io_t *fptr;
+    GetOpenFile(io, fptr);
+    return fptr->mode;
+}
+
 /*
  *  call-seq:
  *    pid -> integer or nil
@@ -2921,7 +2922,7 @@ rb_io_pid(VALUE io)
  *    File.open("testfile") {|f| f.path} # => "testfile"
  */
 
-static VALUE
+VALUE
 rb_io_path(VALUE io)
 {
     rb_io_t *fptr = RFILE(io)->fptr;
@@ -5305,7 +5306,7 @@ rb_io_set_close_on_exec(VALUE io, VALUE arg)
 #define rb_io_set_close_on_exec rb_f_notimplement
 #endif
 
-#define IS_PREP_STDIO(f) ((f)->mode & FMODE_PREP)
+#define RUBY_IO_EXTERNAL_P(f) ((f)->mode & FMODE_EXTERNAL)
 #define PREP_STDIO_NAME(f) (RSTRING_PTR((f)->pathv))
 
 static VALUE
@@ -5463,7 +5464,7 @@ fptr_finalize_flush(rb_io_t *fptr, int noraise, int keepgvl,
 
     int done = 0;
 
-    if (IS_PREP_STDIO(fptr) || fd <= 2) {
+    if (RUBY_IO_EXTERNAL_P(fptr) || fd <= 2) {
         // Need to keep FILE objects of stdin, stdout and stderr, so we are done:
         done = 1;
     }
@@ -5770,10 +5771,8 @@ io_close(VALUE io)
  *
  *  Related: IO#close_read, IO#close_write, IO#close.
  */
-
-
-static VALUE
-rb_io_closed(VALUE io)
+VALUE
+rb_io_closed_p(VALUE io)
 {
     rb_io_t *fptr;
     VALUE write_io;
@@ -8289,7 +8288,7 @@ io_reopen(VALUE io, VALUE nfile)
     GetOpenFile(nfile, orig);
 
     if (fptr == orig) return io;
-    if (IS_PREP_STDIO(fptr)) {
+    if (RUBY_IO_EXTERNAL_P(fptr)) {
         if ((fptr->stdio_file == stdin && !(orig->mode & FMODE_READABLE)) ||
             (fptr->stdio_file == stdout && !(orig->mode & FMODE_WRITABLE)) ||
             (fptr->stdio_file == stderr && !(orig->mode & FMODE_WRITABLE))) {
@@ -8315,17 +8314,17 @@ io_reopen(VALUE io, VALUE nfile)
     }
 
     /* copy rb_io_t structure */
-    fptr->mode = orig->mode | (fptr->mode & FMODE_PREP);
+    fptr->mode = orig->mode | (fptr->mode & FMODE_EXTERNAL);
     fptr->pid = orig->pid;
     fptr->lineno = orig->lineno;
     if (RTEST(orig->pathv)) fptr->pathv = orig->pathv;
-    else if (!IS_PREP_STDIO(fptr)) fptr->pathv = Qnil;
+    else if (!RUBY_IO_EXTERNAL_P(fptr)) fptr->pathv = Qnil;
     fptr_copy_finalizer(fptr, orig);
 
     fd = fptr->fd;
     fd2 = orig->fd;
     if (fd != fd2) {
-        if (IS_PREP_STDIO(fptr) || fd <= 2 || !fptr->stdio_file) {
+        if (RUBY_IO_EXTERNAL_P(fptr) || fd <= 2 || !fptr->stdio_file) {
             /* need to keep FILE objects of stdin, stdout and stderr */
             if (rb_cloexec_dup2(fd2, fd) < 0)
                 rb_sys_fail_path(orig->pathv);
@@ -8433,7 +8432,7 @@ rb_io_reopen(int argc, VALUE *argv, VALUE file)
         convconfig_t convconfig;
 
         rb_io_extract_modeenc(&nmode, 0, opt, &oflags, &fmode, &convconfig);
-        if (IS_PREP_STDIO(fptr) &&
+        if (RUBY_IO_EXTERNAL_P(fptr) &&
             ((fptr->mode & FMODE_READWRITE) & (fmode & FMODE_READWRITE)) !=
             (fptr->mode & FMODE_READWRITE)) {
             rb_raise(rb_eArgError,
@@ -8512,7 +8511,7 @@ rb_io_init_copy(VALUE dest, VALUE io)
     rb_io_flush(io);
 
     /* copy rb_io_t structure */
-    fptr->mode = orig->mode & ~FMODE_PREP;
+    fptr->mode = orig->mode & ~FMODE_EXTERNAL;
     fptr->encs = orig->encs;
     fptr->pid = orig->pid;
     fptr->lineno = orig->lineno;
@@ -9186,27 +9185,79 @@ stderr_getter(ID id, VALUE *ptr)
     return rb_ractor_stderr();
 }
 
+static VALUE
+allocate_and_open_new_file(VALUE klass)
+{
+    VALUE self = io_alloc(klass);
+    rb_io_make_open_file(self);
+    return self;
+}
+
+VALUE
+rb_io_open_descriptor(VALUE klass, int descriptor, int mode, VALUE path, VALUE timeout, struct rb_io_enc_t *encoding)
+{
+    int state;
+    VALUE self = rb_protect(allocate_and_open_new_file, klass, &state);
+    if (state) {
+        /* if we raised an exception allocating an IO object, but the caller
+           intended to transfer ownership of this FD to us, close the fd before
+           raising the exception. Otherwise, we would leak a FD - the caller
+           expects GC to close the file, but we never got around to assigning
+           it to a rb_io. */
+        if (!(mode & FMODE_EXTERNAL)) {
+            maygvl_close(descriptor, 0);
+        }
+        rb_jump_tag(state);
+    }
+
+
+    rb_io_t *io = RFILE(self)->fptr;
+    io->self = self;
+    io->fd = descriptor;
+    io->mode = mode;
+
+    /* At this point, Ruby fully owns the descriptor, and will close it when
+       the IO gets GC'd (unless FMODE_EXTERNAL was set), no matter what happens
+       in the rest of this method. */
+
+    if (NIL_P(path)) {
+        io->pathv = Qnil;
+    }
+    else {
+        StringValue(path);
+        io->pathv = rb_str_new_frozen(path);
+    }
+
+    io->timeout = timeout;
+
+    if (encoding) {
+        io->encs = *encoding;
+    }
+
+    rb_update_max_fd(descriptor);
+
+    return self;
+}
+
 static VALUE
 prep_io(int fd, int fmode, VALUE klass, const char *path)
 {
-    rb_io_t *fp;
-    VALUE io = io_alloc(klass);
+    VALUE path_value = Qnil;
+    if (path) {
+        path_value = rb_obj_freeze(rb_str_new_cstr(path));
+    }
 
-    MakeOpenFile(io, fp);
-    fp->self = io;
-    fp->fd = fd;
-    fp->mode = fmode;
-    fp->timeout = Qnil;
-    if (!io_check_tty(fp)) {
+    VALUE self = rb_io_open_descriptor(klass, fd, fmode, path_value, Qnil, NULL);
+    rb_io_t*io = RFILE(self)->fptr;
+
+    if (!io_check_tty(io)) {
 #ifdef __CYGWIN__
-        fp->mode |= FMODE_BINMODE;
+        io->mode |= FMODE_BINMODE;
         setmode(fd, O_BINARY);
 #endif
     }
-    if (path) fp->pathv = rb_obj_freeze(rb_str_new_cstr(path));
-    rb_update_max_fd(fd);
 
-    return io;
+    return self;
 }
 
 VALUE
@@ -9222,7 +9273,7 @@ static VALUE
 prep_stdio(FILE *f, int fmode, VALUE klass, const char *path)
 {
     rb_io_t *fptr;
-    VALUE io = prep_io(fileno(f), fmode|FMODE_PREP|DEFAULT_TEXTMODE, klass, path);
+    VALUE io = prep_io(fileno(f), fmode|FMODE_EXTERNAL|DEFAULT_TEXTMODE, klass, path);
 
     GetOpenFile(io, fptr);
     fptr->encs.ecflags |= ECONV_DEFAULT_NEWLINE_DECORATOR;
@@ -9266,7 +9317,7 @@ rb_io_stdio_file(rb_io_t *fptr)
 }
 
 static inline void
-rb_io_buffer_init(rb_io_buffer_t *buf)
+rb_io_buffer_init(struct rb_io_buffer_t *buf)
 {
     buf->ptr = NULL;
     buf->off = 0;
@@ -9406,7 +9457,7 @@ rb_io_initialize(int argc, VALUE *argv, VALUE io)
 
     if (!NIL_P(opt)) {
         if (rb_hash_aref(opt, sym_autoclose) == Qfalse) {
-            fmode |= FMODE_PREP;
+            fmode |= FMODE_EXTERNAL;
         }
 
         path = rb_hash_aref(opt, RB_ID2SYM(idPath));
@@ -9583,7 +9634,7 @@ rb_io_autoclose_p(VALUE io)
 {
     rb_io_t *fptr = RFILE(io)->fptr;
     rb_io_check_closed(fptr);
-    return RBOOL(!(fptr->mode & FMODE_PREP));
+    return RBOOL(!(fptr->mode & FMODE_EXTERNAL));
 }
 
 /*
@@ -9609,9 +9660,9 @@ rb_io_set_autoclose(VALUE io, VALUE autoclose)
     rb_io_t *fptr;
     GetOpenFile(io, fptr);
     if (!RTEST(autoclose))
-        fptr->mode |= FMODE_PREP;
+        fptr->mode |= FMODE_EXTERNAL;
     else
-        fptr->mode &= ~FMODE_PREP;
+        fptr->mode &= ~FMODE_EXTERNAL;
     return autoclose;
 }
 
@@ -14442,7 +14493,7 @@ argf_closed(VALUE argf)
 {
     next_argv();
     ARGF_FORWARD(0, 0);
-    return rb_io_closed(ARGF.current_file);
+    return rb_io_closed_p(ARGF.current_file);
 }
 
 /*
@@ -15489,7 +15540,7 @@ Init_IO(void)
     rb_define_method(rb_cIO, "close_on_exec=", rb_io_set_close_on_exec, 1);
 
     rb_define_method(rb_cIO, "close", rb_io_close_m, 0);
-    rb_define_method(rb_cIO, "closed?", rb_io_closed, 0);
+    rb_define_method(rb_cIO, "closed?", rb_io_closed_p, 0);
     rb_define_method(rb_cIO, "close_read", rb_io_close_read, 0);
     rb_define_method(rb_cIO, "close_write", rb_io_close_write, 0);
 
diff --git a/process.c b/process.c
index c54e11860e6248..b4435446f0c4b7 100644
--- a/process.c
+++ b/process.c
@@ -103,6 +103,7 @@ int initgroups(const char *, rb_gid_t);
 #include "internal/error.h"
 #include "internal/eval.h"
 #include "internal/hash.h"
+#include "internal/io.h"
 #include "internal/numeric.h"
 #include "internal/object.h"
 #include "internal/process.h"
diff --git a/spec/ruby/optional/capi/ext/io_spec.c b/spec/ruby/optional/capi/ext/io_spec.c
index cd4bc80229c9b8..c60cd6e2226b07 100644
--- a/spec/ruby/optional/capi/ext/io_spec.c
+++ b/spec/ruby/optional/capi/ext/io_spec.c
@@ -28,9 +28,13 @@ static int set_non_blocking(int fd) {
 }
 
 static int io_spec_get_fd(VALUE io) {
+#ifdef RUBY_VERSION_IS_3_3
+  return rb_io_descriptor(io);
+#else
   rb_io_t* fp;
   GetOpenFile(io, fp);
   return fp->fd;
+#endif
 }
 
 VALUE io_spec_GetOpenFile_fd(VALUE self, VALUE io) {
@@ -303,7 +307,7 @@ VALUE io_spec_rb_io_set_nonblock(VALUE self, VALUE io) {
   GetOpenFile(io, fp);
   rb_io_set_nonblock(fp);
 #ifdef F_GETFL
-  flags = fcntl(fp->fd, F_GETFL, 0);
+  flags = fcntl(io_spec_get_fd(io), F_GETFL, 0);
   return flags & O_NONBLOCK ? Qtrue : Qfalse;
 #else
   return Qfalse;
@@ -322,9 +326,13 @@ static VALUE io_spec_errno_set(VALUE self, VALUE val) {
 }
 
 VALUE io_spec_mode_sync_flag(VALUE self, VALUE io) {
+#ifdef RUBY_VERSION_IS_3_3
+  if (rb_io_mode(io) & FMODE_SYNC) {
+#else
   rb_io_t *fp;
   GetOpenFile(io, fp);
   if (fp->mode & FMODE_SYNC) {
+#endif
     return Qtrue;
   } else {
     return Qfalse;
diff --git a/win32/win32.c b/win32/win32.c
index ec65f5383b6619..c2d12a79f4edbc 100644
--- a/win32/win32.c
+++ b/win32/win32.c
@@ -8269,10 +8269,7 @@ w32_io_info(VALUE *file, w32_io_info_t *st)
 
     tmp = rb_check_convert_type_with_id(*file, T_FILE, "IO", idTo_io);
     if (!NIL_P(tmp)) {
-        rb_io_t *fptr;
-
-        GetOpenFile(tmp, fptr);
-        f = (HANDLE)rb_w32_get_osfhandle(fptr->fd);
+        f = (HANDLE)rb_w32_get_osfhandle(rb_io_descriptor(tmp));
         if (f == (HANDLE)-1) return INVALID_HANDLE_VALUE;
     }
     else {