Revert "Hide most of the implementation of `struct rb_io`. (#6511)"

This reverts commit 18e55fc1e1ec20e8f3166e3059e76c885fc9f8f2.

fix [Bug #19704]
https://bugs.ruby-lang.org/issues/19704
This breaks compatibility for extension libraries. Such changes
need a discussion.

14 files changed, 211 insertions, 298 deletions

diff --git a/common.mk b/common.mk
index 71ae5eb6e6..ec13d25ca2 100644
--- a/common.mk
+++ b/common.mk
@@ -10997,7 +10997,6 @@ 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 b5cba590ba..d3f41f1a3a 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_EXTERNAL		= 0x00010000,
+        RUBY_FMODE_PREP 		= 0x00010000,
         RUBY_FMODE_SETENC_BY_BOM	= FMODE_SETENC_BY_BOM,
         RUBY_FMODE_UNIX 		= 0x00200000,
         RUBY_FMODE_INET 		= 0x00400000,
diff --git a/ext/objspace/depend b/ext/objspace/depend
index 3cdfaffa66..91ae569d67 100644
--- a/ext/objspace/depend
+++ b/ext/objspace/depend
@@ -580,7 +580,6 @@ objspace_dump.o: $(top_srcdir)/internal/compilers.h
 objspace_dump.o: $(top_srcdir)/internal/gc.h
 objspace_dump.o: $(top_srcdir)/internal/hash.h
 objspace_dump.o: $(top_srcdir)/internal/imemo.h
-objspace_dump.o: $(top_srcdir)/internal/io.h
 objspace_dump.o: $(top_srcdir)/internal/sanitizers.h
 objspace_dump.o: $(top_srcdir)/internal/serial.h
 objspace_dump.o: $(top_srcdir)/internal/static_assert.h
diff --git a/ext/objspace/objspace_dump.c b/ext/objspace/objspace_dump.c
index 39584e03b8..d8a11083d6 100644
--- a/ext/objspace/objspace_dump.c
+++ b/ext/objspace/objspace_dump.c
@@ -18,7 +18,6 @@
 #include "internal/class.h"
 #include "internal/gc.h"
 #include "internal/hash.h"
-#include "internal/io.h"
 #include "internal/string.h"
 #include "internal/sanitizers.h"
 #include "symbol.h"
diff --git a/ext/pty/pty.c b/ext/pty/pty.c
index 0aca10bfa0..acec33f9bf 100644
--- a/ext/pty/pty.c
+++ b/ext/pty/pty.c
@@ -448,10 +448,8 @@ pty_close_pty(VALUE assoc)
 
     for (i = 0; i < 2; i++) {
         io = rb_ary_entry(assoc, i);
-        if (RB_TYPE_P(io, T_FILE)) {
-            /* it's OK to call rb_io_close again even if it's already closed */
+        if (RB_TYPE_P(io, T_FILE) && 0 <= RFILE(io)->fptr->fd)
             rb_io_close(io);
-        }
     }
     return Qnil;
 }
@@ -501,21 +499,28 @@ 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);
 
-    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);
+    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 assoc = rb_assoc_new(master_io, slave_file);
+    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));
 
+    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;
 }
 
@@ -572,27 +577,30 @@ 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);
 
-    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
-    );
+    rfptr->mode = rb_io_modestr_fmode("r");
+    rfptr->fd = info.fd;
+    rfptr->pathv = rb_obj_freeze(rb_str_new_cstr(SlaveName));
 
-    int wpty_fd = rb_cloexec_dup(info.fd);
-    if (wpty_fd == -1) {
+    wfptr->mode = rb_io_modestr_fmode("w") | FMODE_SYNC;
+    wfptr->fd = rb_cloexec_dup(info.fd);
+    if (wfptr->fd == -1)
         rb_sys_fail("dup()");
-    }
-    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
-    );
+    rb_update_max_fd(wfptr->fd);
+    wfptr->pathv = rfptr->pathv;
 
     res = rb_ary_new2(3);
-    rb_ary_store(res, 0, rport);
-    rb_ary_store(res, 1, wport);
+    rb_ary_store(res,0,(VALUE)rport);
+    rb_ary_store(res,1,(VALUE)wport);
     rb_ary_store(res,2,PIDT2NUM(info.child_pid));
 
     if (rb_block_given_p()) {
diff --git a/ext/stringio/stringio.c b/ext/stringio/stringio.c
index cb9dbece1c..e69afc31f1 100644
--- a/ext/stringio/stringio.c
+++ b/ext/stringio/stringio.c
@@ -276,7 +276,7 @@ strio_init(int argc, VALUE *argv, struct StringIO *ptr, VALUE self)
 {
     VALUE string, vmode, opt;
     int oflags;
-    struct rb_io_encoding convconfig;
+    struct rb_io_enc_t convconfig;
 
     argc = rb_scan_args(argc, argv, "02:", &string, &vmode, &opt);
     rb_io_extract_modeenc(&vmode, 0, opt, &oflags, &ptr->flags, &convconfig);
@@ -1743,7 +1743,7 @@ strio_set_encoding(int argc, VALUE *argv, VALUE self)
     else {
 	enc = rb_find_encoding(ext_enc);
 	if (!enc) {
-	    struct rb_io_encoding convconfig;
+	    struct rb_io_enc_t convconfig;
 	    int oflags, fmode;
 	    VALUE vmode = rb_str_append(rb_str_new_cstr("r:"), ext_enc);
 	    rb_io_extract_modeenc(&vmode, 0, Qnil, &oflags, &fmode, &convconfig);
diff --git a/file.c b/file.c
index 5f59373a45..2c74f3eff5 100644
--- a/file.c
+++ b/file.c
@@ -169,6 +169,7 @@ 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/internal/core/rfile.h b/include/ruby/internal/core/rfile.h
index 8cf2c4db5b..f8dddde9e5 100644
--- a/include/ruby/internal/core/rfile.h
+++ b/include/ruby/internal/core/rfile.h
@@ -23,10 +23,9 @@
 #include "ruby/internal/core/rbasic.h"
 #include "ruby/internal/cast.h"
 
-/* rb_io is in ruby/io.h and internal/io.h.  The header file has historically
- * not been included into ruby/ruby.h.  We follow that tradition.
- */
-struct rb_io;
+/* rb_io_t is in ruby/io.h.  The header file has historically not been included
+ * into ruby/ruby.h.  We follow that tradition. */
+struct rb_io_t;
 
 /**
  * Ruby's File  and IO.  Ruby's  IO are not  just file descriptors.   They have
@@ -39,7 +38,7 @@ struct RFile {
     struct RBasic basic;
 
     /** IO's specific fields. */
-    struct rb_io *fptr;
+    struct rb_io_t *fptr;
 };
 
 /**
diff --git a/include/ruby/io.h b/include/ruby/io.h
index 881bac03a7..8be83a215c 100644
--- a/include/ruby/io.h
+++ b/include/ruby/io.h
@@ -84,6 +84,29 @@ typedef enum {
     RUBY_IO_PRIORITY = RB_WAITFD_PRI, /**< `IO::PRIORITY` */
 } rb_io_event_t;
 
+/**
+ * IO  buffers.   This  is  an implementation  detail  of  ::rb_io_t::wbuf  and
+ * ::rb_io_t::rbuf.  People don't manipulate it directly.
+ */
+RBIMPL_ATTR_PACKED_STRUCT_UNALIGNED_BEGIN()
+struct rb_io_buffer_t {
+
+    /** Pointer to the underlying memory region, of at least `capa` bytes. */
+    char *ptr;                  /* off + len <= capa */
+
+    /** Offset inside of `ptr`. */
+    int off;
+
+    /** Length of the buffer. */
+    int len;
+
+    /** Designed capacity of the buffer. */
+    int capa;
+} RBIMPL_ATTR_PACKED_STRUCT_UNALIGNED_END();
+
+/** @alias{rb_io_buffer_t} */
+typedef struct rb_io_buffer_t rb_io_buffer_t;
+
 /** Decomposed encoding flags (e.g. `"enc:enc2""`). */
 /*
  * enc  enc2 read action                      write action
@@ -91,7 +114,7 @@ typedef enum {
  * e1   NULL force_encoding(e1)               convert str.encoding to e1
  * e1   e2   convert from e2 to e1            convert str.encoding to e2
  */
-struct rb_io_encoding {
+struct rb_io_enc_t {
     /** Internal encoding. */
     rb_encoding *enc;
     /** External encoding. */
@@ -112,10 +135,103 @@ struct rb_io_encoding {
     VALUE ecopts;
 };
 
-struct rb_io;
-typedef struct rb_io rb_io_t;
+/** Ruby's IO, metadata and buffers. */
+typedef struct rb_io_t {
+
+    /** The IO's Ruby level counterpart. */
+    VALUE self;
+
+    /** stdio ptr for read/write, if available. */
+    FILE *stdio_file;
+
+    /** file descriptor. */
+    int fd;
+
+    /** mode flags: FMODE_XXXs */
+    int mode;
+
+    /** child's pid (for pipes) */
+    rb_pid_t pid;
+
+    /** number of lines read */
+    int lineno;
+
+    /** pathname for file */
+    VALUE pathv;
+
+    /** finalize proc */
+    void (*finalize)(struct rb_io_t*,int);
+
+    /** Write buffer. */
+    rb_io_buffer_t wbuf;
+
+    /**
+     * (Byte)  read   buffer.   Note  also   that  there  is  a   field  called
+     * ::rb_io_t::cbuf, which also concerns read IO.
+     */
+    rb_io_buffer_t rbuf;
+
+    /**
+     * Duplex IO object, if set.
+     *
+     * @see rb_io_set_write_io()
+     */
+    VALUE tied_io_for_writing;
+
+    struct rb_io_enc_t encs; /**< Decomposed encoding flags. */
+
+    /** Encoding converter used when reading from this IO. */
+    rb_econv_t *readconv;
+
+    /**
+     * rb_io_ungetc()  destination.   This  buffer   is  read  before  checking
+     * ::rb_io_t::rbuf
+     */
+    rb_io_buffer_t cbuf;
+
+    /** Encoding converter used when writing to this IO. */
+    rb_econv_t *writeconv;
+
+    /**
+     * This is, when set, an instance  of ::rb_cString which holds the "common"
+     * encoding.   Write  conversion  can  convert strings  twice...   In  case
+     * conversion from encoding  X to encoding Y does not  exist, Ruby finds an
+     * encoding Z that bridges the two, so that X to Z to Y conversion happens.
+     */
+    VALUE writeconv_asciicompat;
+
+    /** Whether ::rb_io_t::writeconv is already set up. */
+    int writeconv_initialized;
 
-typedef struct rb_io_encoding rb_io_enc_t;
+    /**
+     * Value   of    ::rb_io_t::rb_io_enc_t::ecflags   stored    right   before
+     * initialising ::rb_io_t::writeconv.
+     */
+    int writeconv_pre_ecflags;
+
+    /**
+     * Value of ::rb_io_t::rb_io_enc_t::ecopts stored right before initialising
+     * ::rb_io_t::writeconv.
+     */
+    VALUE writeconv_pre_ecopts;
+
+    /**
+     * This is a Ruby  level mutex.  It avoids multiple threads  to write to an
+     * IO at  once; helps  for instance rb_io_puts()  to ensure  newlines right
+     * next to its arguments.
+     *
+     * This of course doesn't help inter-process IO interleaves, though.
+     */
+    VALUE write_lock;
+
+    /**
+     * The timeout associated with this IO when performing blocking operations.
+     */
+    VALUE timeout;
+} rb_io_t;
+
+/** @alias{rb_io_enc_t} */
+typedef struct rb_io_enc_t rb_io_enc_t;
 
 /**
  * @private
@@ -215,16 +331,7 @@ typedef struct rb_io_encoding rb_io_enc_t;
  * Setting this one and #FMODE_BINMODE at the same time is a contradiction.
  */
 #define FMODE_TEXTMODE              0x00001000
-/**
- * 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_PREP               0x00010000 */
 /* #define FMODE_SIGNAL_ON_EPIPE    0x00020000 */
 
 /**
@@ -239,18 +346,6 @@ typedef struct rb_io_encoding 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_encoding *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.
  *
  * @param[in]   obj              An IO object.
@@ -609,12 +704,6 @@ 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>.
  *
@@ -624,12 +713,6 @@ VALUE rb_io_path(VALUE io);
 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()
  * today.  People prefer that API instead.
diff --git a/internal/io.h b/internal/io.h
index c97fd1e616..b5f15499d7 100644
--- a/internal/io.h
+++ b/internal/io.h
@@ -8,125 +8,8 @@
  *             file COPYING are met.  Consult the file for details.
  * @brief      Internal header for IO.
  */
-
-#define RB_IO_T
-
 #include "ruby/ruby.h"          /* for VALUE */
-#include "ruby/io.h"
-
-/**
- * IO  buffers.   This  is  an implementation  detail  of  ::rb_io_t::wbuf  and
- * ::rb_io_t::rbuf.  People don't manipulate it directly.
- */
-RBIMPL_ATTR_PACKED_STRUCT_UNALIGNED_BEGIN()
-struct rb_io_internal_buffer {
-
-    /** Pointer to the underlying memory region, of at least `capa` bytes. */
-    char *ptr;                  /* off + len <= capa */
-
-    /** Offset inside of `ptr`. */
-    int off;
-
-    /** Length of the buffer. */
-    int len;
-
-    /** Designed capacity of the buffer. */
-    int capa;
-} RBIMPL_ATTR_PACKED_STRUCT_UNALIGNED_END();
-
-/** Ruby's IO, metadata and buffers. */
-struct rb_io {
-    /** The IO's Ruby level counterpart. */
-    VALUE self;
-
-    /** stdio ptr for read/write, if available. */
-    FILE *stdio_file;
-
-    /** file descriptor. */
-    int fd;
-
-    /** mode flags: FMODE_XXXs */
-    int mode;
-
-    /** child's pid (for pipes) */
-    rb_pid_t pid;
-
-    /** number of lines read */
-    int lineno;
-
-    /** pathname for file */
-    VALUE pathv;
-
-    /**
-     * Duplex IO object, if set.
-     *
-     * @see rb_io_set_write_io()
-     */
-    VALUE tied_io_for_writing;
-
-    /** finalize proc */
-    void (*finalize)(struct rb_io*,int);
-
-    /** Write buffer. */
-    struct rb_io_internal_buffer wbuf;
-
-    /**
-     * (Byte)  read   buffer.   Note  also   that  there  is  a   field  called
-     * ::rb_io_t::cbuf, which also concerns read IO.
-     */
-    struct rb_io_internal_buffer rbuf;
-
-    struct rb_io_encoding encs; /**< Decomposed encoding flags. */
-
-    /** Encoding converter used when reading from this IO. */
-    rb_econv_t *readconv;
-
-    /**
-     * rb_io_ungetc()  destination.   This  buffer   is  read  before  checking
-     * ::rb_io_t::rbuf
-     */
-    struct rb_io_internal_buffer cbuf;
-
-    /** Encoding converter used when writing to this IO. */
-    rb_econv_t *writeconv;
-
-    /**
-     * This is, when set, an instance  of ::rb_cString which holds the "common"
-     * encoding.   Write  conversion  can  convert strings  twice...   In  case
-     * conversion from encoding  X to encoding Y does not  exist, Ruby finds an
-     * encoding Z that bridges the two, so that X to Z to Y conversion happens.
-     */
-    VALUE writeconv_asciicompat;
-
-    /** Whether ::rb_io_t::writeconv is already set up. */
-    int writeconv_initialized;
-
-    /**
-     * Value   of    ::rb_io_t::rb_io_enc_t::ecflags   stored    right   before
-     * initialising ::rb_io_t::writeconv.
-     */
-    int writeconv_pre_ecflags;
-
-    /**
-     * Value of ::rb_io_t::rb_io_enc_t::ecopts stored right before initialising
-     * ::rb_io_t::writeconv.
-     */
-    VALUE writeconv_pre_ecopts;
-
-    /**
-     * This is a Ruby  level mutex.  It avoids multiple threads  to write to an
-     * IO at  once; helps  for instance rb_io_puts()  to ensure  newlines right
-     * next to its arguments.
-     *
-     * This of course doesn't help inter-process IO interleaves, though.
-     */
-    VALUE write_lock;
-
-    /**
-     * The timeout associated with this IO when performing blocking operations.
-     */
-    VALUE timeout;
-};
+#include "ruby/io.h"            /* for rb_io_t */
 
 /* io.c */
 void ruby_set_inplace_mode(const char *);
diff --git a/io.c b/io.c
index 26c95d2d86..88cb2667f5 100644
--- a/io.c
+++ b/io.c
@@ -16,6 +16,11 @@
 #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>
@@ -216,7 +221,7 @@ struct argf {
     long lineno;
     VALUE argv;
     VALUE inplace;
-    struct rb_io_encoding encs;
+    struct rb_io_enc_t encs;
     int8_t init_p, next_p, binmode;
 };
 
@@ -520,6 +525,7 @@ 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) \
@@ -1457,7 +1463,7 @@ rb_io_wait(VALUE io, VALUE events, VALUE timeout)
 static VALUE
 io_from_fd(int fd)
 {
-    return prep_io(fd, FMODE_EXTERNAL, rb_cIO, NULL);
+    return prep_io(fd, FMODE_PREP, rb_cIO, NULL);
 }
 
 static int
@@ -2869,13 +2875,6 @@ 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
@@ -2922,7 +2921,7 @@ rb_io_pid(VALUE io)
  *    File.open("testfile") {|f| f.path} # => "testfile"
  */
 
-VALUE
+static VALUE
 rb_io_path(VALUE io)
 {
     rb_io_t *fptr = RFILE(io)->fptr;
@@ -5306,7 +5305,7 @@ rb_io_set_close_on_exec(VALUE io, VALUE arg)
 #define rb_io_set_close_on_exec rb_f_notimplement
 #endif
 
-#define RUBY_IO_EXTERNAL_P(f) ((f)->mode & FMODE_EXTERNAL)
+#define IS_PREP_STDIO(f) ((f)->mode & FMODE_PREP)
 #define PREP_STDIO_NAME(f) (RSTRING_PTR((f)->pathv))
 
 static VALUE
@@ -5420,7 +5419,7 @@ maygvl_fclose(FILE *file, int keepgvl)
     return (int)(intptr_t)rb_thread_call_without_gvl(nogvl_fclose, file, RUBY_UBF_IO, 0);
 }
 
-static void free_io_buffer(struct rb_io_internal_buffer *buf);
+static void free_io_buffer(rb_io_buffer_t *buf);
 static void clear_codeconv(rb_io_t *fptr);
 
 static void*
@@ -5464,7 +5463,7 @@ fptr_finalize_flush(rb_io_t *fptr, int noraise, int keepgvl,
 
     int done = 0;
 
-    if (RUBY_IO_EXTERNAL_P(fptr) || fd <= 2) {
+    if (IS_PREP_STDIO(fptr) || fd <= 2) {
         // Need to keep FILE objects of stdin, stdout and stderr, so we are done:
         done = 1;
     }
@@ -5543,7 +5542,7 @@ rb_io_fptr_cleanup(rb_io_t *fptr, int noraise)
 }
 
 static void
-free_io_buffer(struct rb_io_internal_buffer *buf)
+free_io_buffer(rb_io_buffer_t *buf)
 {
     if (buf->ptr) {
         ruby_sized_xfree(buf->ptr, (size_t)buf->capa);
@@ -5771,8 +5770,10 @@ io_close(VALUE io)
  *
  *  Related: IO#close_read, IO#close_write, IO#close.
  */
-VALUE
-rb_io_closed_p(VALUE io)
+
+
+static VALUE
+rb_io_closed(VALUE io)
 {
     rb_io_t *fptr;
     VALUE write_io;
@@ -6723,7 +6724,7 @@ rb_io_extract_encoding_option(VALUE opt, rb_encoding **enc_p, rb_encoding **enc2
     return extracted;
 }
 
-typedef struct rb_io_encoding convconfig_t;
+typedef struct rb_io_enc_t convconfig_t;
 
 static void
 validate_enc_binmode(int *fmode_p, int ecflags, rb_encoding *enc, rb_encoding *enc2)
@@ -7257,7 +7258,7 @@ static void
 fptr_copy_finalizer(rb_io_t *fptr, const rb_io_t *orig)
 {
 #if defined(__CYGWIN__) || !defined(HAVE_WORKING_FORK)
-    void (*const old_finalize)(rb_io_t*,int) = fptr->finalize;
+    void (*const old_finalize)(struct rb_io_t*,int) = fptr->finalize;
 
     if (old_finalize == orig->finalize) return;
 #endif
@@ -8288,7 +8289,7 @@ io_reopen(VALUE io, VALUE nfile)
     GetOpenFile(nfile, orig);
 
     if (fptr == orig) return io;
-    if (RUBY_IO_EXTERNAL_P(fptr)) {
+    if (IS_PREP_STDIO(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))) {
@@ -8314,17 +8315,17 @@ io_reopen(VALUE io, VALUE nfile)
     }
 
     /* copy rb_io_t structure */
-    fptr->mode = orig->mode | (fptr->mode & FMODE_EXTERNAL);
+    fptr->mode = orig->mode | (fptr->mode & FMODE_PREP);
     fptr->pid = orig->pid;
     fptr->lineno = orig->lineno;
     if (RTEST(orig->pathv)) fptr->pathv = orig->pathv;
-    else if (!RUBY_IO_EXTERNAL_P(fptr)) fptr->pathv = Qnil;
+    else if (!IS_PREP_STDIO(fptr)) fptr->pathv = Qnil;
     fptr_copy_finalizer(fptr, orig);
 
     fd = fptr->fd;
     fd2 = orig->fd;
     if (fd != fd2) {
-        if (RUBY_IO_EXTERNAL_P(fptr) || fd <= 2 || !fptr->stdio_file) {
+        if (IS_PREP_STDIO(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);
@@ -8432,7 +8433,7 @@ rb_io_reopen(int argc, VALUE *argv, VALUE file)
         convconfig_t convconfig;
 
         rb_io_extract_modeenc(&nmode, 0, opt, &oflags, &fmode, &convconfig);
-        if (RUBY_IO_EXTERNAL_P(fptr) &&
+        if (IS_PREP_STDIO(fptr) &&
             ((fptr->mode & FMODE_READWRITE) & (fmode & FMODE_READWRITE)) !=
             (fptr->mode & FMODE_READWRITE)) {
             rb_raise(rb_eArgError,
@@ -8511,7 +8512,7 @@ rb_io_init_copy(VALUE dest, VALUE io)
     rb_io_flush(io);
 
     /* copy rb_io_t structure */
-    fptr->mode = orig->mode & ~FMODE_EXTERNAL;
+    fptr->mode = orig->mode & ~FMODE_PREP;
     fptr->encs = orig->encs;
     fptr->pid = orig->pid;
     fptr->lineno = orig->lineno;
@@ -9186,79 +9187,26 @@ stderr_getter(ID id, VALUE *ptr)
 }
 
 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_encoding *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);
-    }
-
-
-    struct rb_io *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)
 {
-    VALUE path_value = Qnil;
-    if (path) {
-        path_value = rb_obj_freeze(rb_str_new_cstr(path));
-    }
-
-    VALUE self = rb_io_open_descriptor(klass, fd, fmode, path_value, Qnil, NULL);
-    struct rb_io *io = RFILE(self)->fptr;
+    rb_io_t *fp;
+    VALUE io = io_alloc(klass);
 
-    if (!io_check_tty(io)) {
+    MakeOpenFile(io, fp);
+    fp->self = io;
+    fp->fd = fd;
+    fp->mode = fmode;
+    fp->timeout = Qnil;
+    if (!io_check_tty(fp)) {
 #ifdef __CYGWIN__
-        io->mode |= FMODE_BINMODE;
+        fp->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 self;
+    return io;
 }
 
 VALUE
@@ -9274,7 +9222,7 @@ 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_EXTERNAL|DEFAULT_TEXTMODE, klass, path);
+    VALUE io = prep_io(fileno(f), fmode|FMODE_PREP|DEFAULT_TEXTMODE, klass, path);
 
     GetOpenFile(io, fptr);
     fptr->encs.ecflags |= ECONV_DEFAULT_NEWLINE_DECORATOR;
@@ -9318,7 +9266,7 @@ rb_io_stdio_file(rb_io_t *fptr)
 }
 
 static inline void
-rb_io_buffer_init(struct rb_io_internal_buffer *buf)
+rb_io_buffer_init(rb_io_buffer_t *buf)
 {
     buf->ptr = NULL;
     buf->off = 0;
@@ -9458,7 +9406,7 @@ rb_io_initialize(int argc, VALUE *argv, VALUE io)
 
     if (!NIL_P(opt)) {
         if (rb_hash_aref(opt, sym_autoclose) == Qfalse) {
-            fmode |= FMODE_EXTERNAL;
+            fmode |= FMODE_PREP;
         }
 
         path = rb_hash_aref(opt, RB_ID2SYM(idPath));
@@ -9635,7 +9583,7 @@ rb_io_autoclose_p(VALUE io)
 {
     rb_io_t *fptr = RFILE(io)->fptr;
     rb_io_check_closed(fptr);
-    return RBOOL(!(fptr->mode & FMODE_EXTERNAL));
+    return RBOOL(!(fptr->mode & FMODE_PREP));
 }
 
 /*
@@ -9661,9 +9609,9 @@ rb_io_set_autoclose(VALUE io, VALUE autoclose)
     rb_io_t *fptr;
     GetOpenFile(io, fptr);
     if (!RTEST(autoclose))
-        fptr->mode |= FMODE_EXTERNAL;
+        fptr->mode |= FMODE_PREP;
     else
-        fptr->mode &= ~FMODE_EXTERNAL;
+        fptr->mode &= ~FMODE_PREP;
     return autoclose;
 }
 
@@ -14494,7 +14442,7 @@ argf_closed(VALUE argf)
 {
     next_argv();
     ARGF_FORWARD(0, 0);
-    return rb_io_closed_p(ARGF.current_file);
+    return rb_io_closed(ARGF.current_file);
 }
 
 /*
@@ -15541,7 +15489,7 @@ 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_p, 0);
+    rb_define_method(rb_cIO, "closed?", rb_io_closed, 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 b4435446f0..c54e11860e 100644
--- a/process.c
+++ b/process.c
@@ -103,7 +103,6 @@ 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 1c554a1a31..cd4bc80229 100644
--- a/spec/ruby/optional/capi/ext/io_spec.c
+++ b/spec/ruby/optional/capi/ext/io_spec.c
@@ -28,13 +28,9 @@ static int set_non_blocking(int fd) {
 }
 
 static int io_spec_get_fd(VALUE io) {
-#ifdef RUBY_VERSION_IS_3_1
-  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) {
@@ -307,7 +303,7 @@ VALUE io_spec_rb_io_set_nonblock(VALUE self, VALUE io) {
   GetOpenFile(io, fp);
   rb_io_set_nonblock(fp);
 #ifdef F_GETFL
-  flags = fcntl(io_spec_get_fd(io), F_GETFL, 0);
+  flags = fcntl(fp->fd, F_GETFL, 0);
   return flags & O_NONBLOCK ? Qtrue : Qfalse;
 #else
   return Qfalse;
@@ -326,13 +322,9 @@ 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 c2d12a79f4..ec65f5383b 100644
--- a/win32/win32.c
+++ b/win32/win32.c
@@ -8269,7 +8269,10 @@ 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)) {
-        f = (HANDLE)rb_w32_get_osfhandle(rb_io_descriptor(tmp));
+        rb_io_t *fptr;
+
+        GetOpenFile(tmp, fptr);
+        f = (HANDLE)rb_w32_get_osfhandle(fptr->fd);
         if (f == (HANDLE)-1) return INVALID_HANDLE_VALUE;
     }
     else {