diff --git a/include/stdio.h b/include/stdio.h
index a5de1de13530..a8aaf6da41cb 100644
--- a/include/stdio.h
+++ b/include/stdio.h
@@ -70,6 +70,12 @@ struct __sbuf {
 
 struct __file_lock;
 
+/* hold a buncha junk that would grow the ABI */
+struct __sFILEX {
+	struct __file_lock *_mtlock;	/* used for MT-safety */
+	unsigned char *_up;	/* saved _p when _p is doing ungetc data */
+};
+
 /*
  * stdio state variables.
  *
@@ -114,7 +120,7 @@ typedef	struct __sFILE {
 
 	/* separate buffer for long sequences of ungetc() */
 	struct	__sbuf _ub;	/* ungetc buffer */
-	unsigned char *_up;	/* saved _p when _p is doing ungetc data */
+	struct __sFILEX *_extra; /* additions to FILE to not break ABI */
 	int	_ur;		/* saved _r when _r is counting ungetc data */
 
 	/* tricks to meet minimum requirements even when malloc() fails */
@@ -127,13 +133,10 @@ typedef	struct __sFILE {
 	/* Unix stdio files get aligned to block boundaries on fseek() */
 	int	_blksize;	/* stat.st_blksize (may be != _bf._size) */
 	fpos_t	_offset;	/* current lseek offset (see WARNING) */
-	struct __file_lock *_lock;	/* used for MT-safety */
 } FILE;
 
 __BEGIN_DECLS
-extern FILE __stdin;
-extern FILE __stdout;
-extern FILE __stderr;
+extern FILE __sF[];
 __END_DECLS
 
 #define	__SLBF	0x0001		/* line buffered */
@@ -196,9 +199,9 @@ __END_DECLS
 #define	SEEK_END	2	/* set file offset to EOF plus offset */
 #endif
 
-#define	stdin	(&__stdin)
-#define	stdout	(&__stdout)
-#define	stderr	(&__stderr)
+#define	stdin	(&__sF[0])
+#define	stdout	(&__sF[1])
+#define	stderr	(&__sF[2])
 
 /*
  * Functions defined in ANSI C standard.
diff --git a/lib/libc/Makefile b/lib/libc/Makefile
index 3b6ea2ae02be..cdb5011c3571 100644
--- a/lib/libc/Makefile
+++ b/lib/libc/Makefile
@@ -7,7 +7,7 @@
 # from CFLAGS below.  To remove these strings from just the system call
 # stubs, remove just -DSYSLIBC_RCS from CFLAGS.
 LIB=c
-SHLIB_MAJOR= 5.20010213
+SHLIB_MAJOR= 5
 SHLIB_MINOR= 0
 CFLAGS+=-DLIBC_RCS -DSYSLIBC_RCS -I${.CURDIR}/include
 AINC=	-I${.CURDIR}/${MACHINE_ARCH}
diff --git a/lib/libc/stdio/_flock_stub.c b/lib/libc/stdio/_flock_stub.c
index 8bea66de5724..a77fad982242 100644
--- a/lib/libc/stdio/_flock_stub.c
+++ b/lib/libc/stdio/_flock_stub.c
@@ -67,6 +67,16 @@ struct	__file_lock {
 	int		fl_count;	/* recursive lock count */
 };
 
+/*
+ * We need to retain binary compatibility for a while.  So pretend
+ * that _lock is part of FILE * even though it is dereferenced off
+ * _extra now.  When we stop encoding the size of FILE into binaries
+ * this can be changed in stdio.h.  This will reduce the amount of
+ * code that has to change in the future (just remove this comment
+ * and #define).
+ */
+#define _lock _extra->_mtlock
+
 /*
  * Allocate and initialize a file lock.
  */
diff --git a/lib/libc/stdio/findfp.c b/lib/libc/stdio/findfp.c
index 48eb5b525997..e0f0e296508e 100644
--- a/lib/libc/stdio/findfp.c
+++ b/lib/libc/stdio/findfp.c
@@ -59,21 +59,24 @@ int	__sdidinit;
 
 #define	NDYNAMIC 10		/* add ten more whenever necessary */
 
-#define	std(handle, flags, file) \
-FILE handle = {0,0,0,flags,file,{0},0,&handle,__sclose,__sread,__sseek,__swrite}
-/*	       p r w flags file _bf z  cookie    close    read    seek    write */
-
+#define	std(flags, file) \
+  	{0,0,0,flags,file,{0},0,__sF+file,__sclose,__sread,__sseek,__swrite, \
+	 {0}, __sFX + file}
+  /*	 p r w flags file _bf z  cookie      close    read    seek    write */
+  /*     _ub _extra */
 				/* the usual - (stdin + stdout + stderr) */
 static FILE usual[FOPEN_MAX - 3];
 static struct glue uglue = { NULL, FOPEN_MAX - 3, usual };
 
-std(__stdin, __SRD, STDIN_FILENO);
-std(__stdout, __SWR, STDOUT_FILENO);
-std(__stderr, __SWR|__SNBF, STDERR_FILENO);
+static struct __sFILEX __sFX[3];
 
-static struct glue sglue2 = { &uglue, 1, &__stderr };
-static struct glue sglue1 = { &sglue2, 1, &__stdout };
-struct glue __sglue = { &sglue1, 1, &__stdin };
+FILE __sF[3] = {
+	std(__SRD, STDIN_FILENO),
+	std(__SWR, STDOUT_FILENO),
+	std(__SWR|__SNBF, STDERR_FILENO)
+};
+
+struct glue __sglue = { &uglue, 3, __sF };
 static struct glue *lastglue = &uglue;
 
 static struct glue *	moreglue __P((int));
@@ -93,18 +96,26 @@ moreglue(n)
 	int n;
 {
 	struct glue *g;
-	FILE *p;
 	static FILE empty;
+	static struct __sFILEX emptyx;
+	FILE *p;
+	struct __sFILEX *fx;
 
-	g = (struct glue *)malloc(sizeof(*g) + ALIGNBYTES + n * sizeof(FILE));
+	g = (struct glue *)malloc(sizeof(*g) + ALIGNBYTES + n * sizeof(FILE) +
+	    n * sizeof(struct __sFILEX));
 	if (g == NULL)
 		return (NULL);
 	p = (FILE *)ALIGN(g + 1);
+	fx = (struct __sFILEX *)&p[n];
 	g->next = NULL;
 	g->niobs = n;
 	g->iobs = p;
-	while (--n >= 0)
-		*p++ = empty;
+	while (--n >= 0) {
+		*p = empty;
+		p->_extra = fx;
+		*p->_extra = emptyx;
+		p++, fx++;
+	}
 	return (g);
 }
 
diff --git a/lib/libc/stdio/fseek.c b/lib/libc/stdio/fseek.c
index f87397552a75..b4343501e6db 100644
--- a/lib/libc/stdio/fseek.c
+++ b/lib/libc/stdio/fseek.c
@@ -204,7 +204,7 @@ _fseeko(fp, offset, whence)
 	 */
 	if (HASUB(fp)) {
 		curoff += fp->_r;	/* kill off ungetc */
-		n = fp->_up - fp->_bf._base;
+		n = fp->_extra->_up - fp->_bf._base;
 		curoff -= n;
 		n += fp->_ur;
 	} else {
diff --git a/lib/libc/stdio/refill.c b/lib/libc/stdio/refill.c
index b597f97cd919..2adb3a73bf49 100644
--- a/lib/libc/stdio/refill.c
+++ b/lib/libc/stdio/refill.c
@@ -109,7 +109,7 @@ __srefill(FILE *fp)
 		if (HASUB(fp)) {
 			FREEUB(fp);
 			if ((fp->_r = fp->_ur) != 0) {
-				fp->_p = fp->_up;
+				fp->_p = fp->_extra->_up;
 				return (0);
 			}
 		}
diff --git a/lib/libc/stdio/ungetc.c b/lib/libc/stdio/ungetc.c
index 872abadc6817..f70fb425bb17 100644
--- a/lib/libc/stdio/ungetc.c
+++ b/lib/libc/stdio/ungetc.c
@@ -164,7 +164,7 @@ __ungetc(int c, FILE *fp)
 	 * Initially, we will use the `reserve' buffer.
 	 */
 	fp->_ur = fp->_r;
-	fp->_up = fp->_p;
+	fp->_extra->_up = fp->_p;
 	fp->_ub._base = fp->_ubuf;
 	fp->_ub._size = sizeof(fp->_ubuf);
 	fp->_ubuf[sizeof(fp->_ubuf) - 1] = c;