diff --git a/lib/freadahead.c b/lib/freadahead.c
index 01b0810..f1b6d3e 100644
--- a/lib/freadahead.c
+++ b/lib/freadahead.c
@@ -22,10 +22,14 @@
 #include <stdlib.h>
 #include "stdio-impl.h"
 
+#ifndef _IO_IN_BACKUP
+#define _IO_IN_BACKUP 0x0100
+#endif
+
 size_t
 freadahead (FILE *fp)
 {
-#if defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
+#if defined _IO_EOF_SEEN || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
   if (fp->_IO_write_ptr > fp->_IO_write_base)
     return 0;
   return (fp->_IO_read_end - fp->_IO_read_ptr)
diff --git a/lib/freadptr.c b/lib/freadptr.c
index 26c66ed..5051843 100644
--- a/lib/freadptr.c
+++ b/lib/freadptr.c
@@ -29,7 +29,7 @@ freadptr (FILE *fp, size_t *sizep)
   size_t size;
 
   /* Keep this code in sync with freadahead!  */
-#if defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
+#if defined _IO_EOF_SEEN || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
   if (fp->_IO_write_ptr > fp->_IO_write_base)
     return NULL;
   size = fp->_IO_read_end - fp->_IO_read_ptr;
diff --git a/lib/freadseek.c b/lib/freadseek.c
index 5301794..a423913 100644
--- a/lib/freadseek.c
+++ b/lib/freadseek.c
@@ -26,6 +26,9 @@
 #include "freadptr.h"
 
 #include "stdio-impl.h"
+#ifndef _IO_IN_BACKUP
+#define _IO_IN_BACKUP 0x0100
+#endif
 
 /* Increment the in-memory pointer.  INCREMENT must be at most the buffer size
    returned by freadptr().
@@ -36,7 +39,7 @@ freadptrinc (FILE *fp, size_t increment)
   /* Keep this code in sync with freadptr!  */
 #if HAVE___FREADPTRINC              /* musl libc */
   __freadptrinc (fp, increment);
-#elif defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
+#elif defined _IO_EOF_SEEN || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
   fp->_IO_read_ptr += increment;
 #elif defined __sferror || defined __DragonFly__ /* FreeBSD, NetBSD, OpenBSD, DragonFly, Mac OS X, Cygwin */
   fp_->_p += increment;
diff --git a/lib/fseterr.c b/lib/fseterr.c
index 1e212e4..913ed88 100644
--- a/lib/fseterr.c
+++ b/lib/fseterr.c
@@ -29,7 +29,7 @@ fseterr (FILE *fp)
   /* Most systems provide FILE as a struct and the necessary bitmask in
      <stdio.h>, because they need it for implementing getc() and putc() as
      fast macros.  */
-#if defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
+#if defined _IO_ERR_SEEN || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
   fp->_flags |= _IO_ERR_SEEN;
 #elif defined __sferror || defined __DragonFly__ /* FreeBSD, NetBSD, OpenBSD, DragonFly, Mac OS X, Cygwin */
   fp_->_flags |= __SERR;