diff --git a/include/vips/vips.h b/include/vips/vips.h
index 1b57da74..e1df633f 100644
--- a/include/vips/vips.h
+++ b/include/vips/vips.h
@@ -360,10 +360,11 @@ typedef struct im__IMAGE {
 	GSList *preclosefns; 	/* list of pre-close callbacks */
 	GSList *invalidatefns; 	/* list of invalidate callbacks */
 
-	/* Set this to indicate a truncated file. We've been able to read the
-	 * header, but don't try reading outside the file size.
+	/* Record the file length here. We use this to stop ourselves mapping
+	 * things beyond the end of the file in the case that the file has
+	 * been truncated.
 	 */
-	int nodata;
+	size_t file_length;
 } IMAGE;
 
 /* Only define if IM_ENABLE_DEPRECATED is set.
diff --git a/libsrc/iofuncs/im_binfile.c b/libsrc/iofuncs/im_binfile.c
index 2702f3be..3271342d 100644
--- a/libsrc/iofuncs/im_binfile.c
+++ b/libsrc/iofuncs/im_binfile.c
@@ -83,7 +83,6 @@ IMAGE *
 im_binfile( const char *name, int xs, int ys, int bands, int offset )
 {
 	IMAGE *im;
-	gint64 length;
 	gint64 psize;
 
 	/* Check parameters.
@@ -112,14 +111,14 @@ im_binfile( const char *name, int xs, int ys, int bands, int offset )
 	/* Read the file length and check against what we think 
 	 * the size should be.
 	 */
-	if( (length = im_file_length( im->fd )) == -1 ) {
+	if( (im->file_length = im_file_length( im->fd )) == -1 ) {
 		im_close( im );
 		return( NULL );
 	}
 
 	/* Very common, so special message.
 	 */
-	if( psize > length ) {
+	if( psize > im->file_length ) {
 		im_error( "im_binfile", _( "unable to open %s: "
 			"file has been truncated" ), im->filename );
 		im_close( im );
@@ -129,7 +128,7 @@ im_binfile( const char *name, int xs, int ys, int bands, int offset )
 	/* Just wierd. Only print a warning for this, since we should
 	 * still be able to process it without coredumps.
 	 */
-	if( psize < length )
+	if( psize < im->file_length )
 		im_warn( "im_binfile", _( "%s is longer than expected" ),
 			im->filename );
 
diff --git a/libsrc/iofuncs/im_init.c b/libsrc/iofuncs/im_init.c
index 75e92951..b181e8d9 100644
--- a/libsrc/iofuncs/im_init.c
+++ b/libsrc/iofuncs/im_init.c
@@ -32,7 +32,7 @@
  * 9/8/08
  * 	- lock global image list (thanks lee)
  * 19/3/09
- *	- add nodata 
+ *	- add file_length 
  */
 
 /*
@@ -170,7 +170,7 @@ im_init( const char *filename )
 	im->preclosefns = NULL;
 	im->invalidatefns = NULL;
 
-	im->nodata = 0;
+	im->file_length = 0;
 
 	if( !(im->filename = im_strdup( NULL, filename )) ) {
 		im_close( im );
diff --git a/libsrc/iofuncs/im_mapfile.c b/libsrc/iofuncs/im_mapfile.c
index 889c0ac6..f7bb389a 100644
--- a/libsrc/iofuncs/im_mapfile.c
+++ b/libsrc/iofuncs/im_mapfile.c
@@ -210,7 +210,6 @@ im__munmap( void *start, size_t length )
 int
 im_mapfile( IMAGE *im )
 {
-	gint64 length;
 	struct stat st;
 	mode_t m;
 
@@ -219,15 +218,14 @@ im_mapfile( IMAGE *im )
 	/* Check the size of the file; if it is less than 64 bytes, then flag
 	 * an error.
 	 */
-	if( (length = im_file_length( im->fd )) == -1 ) 
-		return( -1 );
+	g_assert( im->file_length > 0 );
 	if( fstat( im->fd, &st ) == -1 ) {
 		im_error( "im_mapfile", 
 			"%s", _( "unable to get file status" ) );
 		return( -1 );
 	}
 	m = (mode_t) st.st_mode;
-	if( length < 64 ) {
+	if( im->file_length < 64 ) {
 		im_error( "im_mapfile", 
 			"%s", _( "file is less than 64 bytes" ) );
 		return( -1 ); 
@@ -238,13 +236,10 @@ im_mapfile( IMAGE *im )
 		return( -1 ); 
 	}
 
-	if( !(im->baseaddr = im__mmap( im->fd, 0, length, 0 )) )
+	if( !(im->baseaddr = im__mmap( im->fd, 0, im->file_length, 0 )) )
 		return( -1 );
 
-	/* im__mmap() will fail for >2GB, so this is safe even for large
-	 * files.
-	 */
-	im->length = length;
+	im->length = im->file_length;
 
 	return( 0 );
 }
@@ -254,7 +249,6 @@ im_mapfile( IMAGE *im )
 int
 im_mapfilerw( IMAGE *im )
 {
-	gint64 length;
 	struct stat st;
 	mode_t m;
 
@@ -263,27 +257,23 @@ im_mapfilerw( IMAGE *im )
 	/* Check the size of the file if it is less than 64 bytes return
 	 * make also sure that it is a regular file
 	 */
-	if( (length = im_file_length( im->fd )) == -1 ) 
-		return( -1 );
+	g_assert( im->file_length > 0 );
 	if( fstat( im->fd, &st ) == -1 ) {
 		im_error( "im_mapfilerw", 
 			"%s", _( "unable to get file status" ) );
 		return( -1 );
 	}
 	m = (mode_t) st.st_mode;
-	if( length < 64 || !S_ISREG( m ) ) {
+	if( im->file_length < 64 || !S_ISREG( m ) ) {
 		im_error( "im_mapfile", 
 			"%s", _( "unable to read data" ) ); 
 		return( -1 ); 
 	}
 
-	if( !(im->baseaddr = im__mmap( im->fd, 1, length, 0 )) )
+	if( !(im->baseaddr = im__mmap( im->fd, 1, im->file_length, 0 )) )
 		return( -1 );
 
-	/* im__mmap() will fail for >2GB, so this is safe even for large
-	 * files.
-	 */
-	im->length = length;
+	im->length = im->file_length;
 
 	return( 0 );
 }
diff --git a/libsrc/iofuncs/im_open_vips.c b/libsrc/iofuncs/im_open_vips.c
index 8ff551a5..075d0fb7 100644
--- a/libsrc/iofuncs/im_open_vips.c
+++ b/libsrc/iofuncs/im_open_vips.c
@@ -325,14 +325,12 @@ read_chunk( int fd, gint64 offset, size_t length )
 int
 im__has_extension_block( IMAGE *im )
 {
-	gint64 length;
 	gint64 psize;
 
 	psize = im__image_pixel_length( im );
-	if( (length = im_file_length( im->fd )) == -1 ) 
-		return( 0 );
+	g_assert( im->file_length > 0 );
 
-	return( length - psize > 0 );
+	return( im->file_length - psize > 0 );
 }
 
 /* Read everything after the pixels into memory.
@@ -340,29 +338,27 @@ im__has_extension_block( IMAGE *im )
 void *
 im__read_extension_block( IMAGE *im, int *size )
 {
-	gint64 length;
 	gint64 psize;
 	void *buf;
 
 	psize = im__image_pixel_length( im );
-	if( (length = im_file_length( im->fd )) == -1 ) 
-		return( NULL );
-	if( length - psize > 10 * 1024 * 1024 ) {
+	g_assert( im->file_length > 0 );
+	if( im->file_length - psize > 10 * 1024 * 1024 ) {
 		im_error( "im_readhist",
 			"%s", _( "more than a 10 megabytes of XML? "
 			"sufferin' succotash!" ) );
 		return( NULL );
 	}
-	if( length - psize == 0 )
+	if( im->file_length - psize == 0 )
 		return( NULL );
-	if( !(buf = read_chunk( im->fd, psize, length - psize )) )
+	if( !(buf = read_chunk( im->fd, psize, im->file_length - psize )) )
 		return( NULL );
 	if( size )
-		*size = length - psize;
+		*size = im->file_length - psize;
 
 #ifdef DEBUG
 	printf( "im__read_extension_block: read %d bytes from %s\n",
-		(int) (length - psize), im->filename );
+		(int) (im->file_length - psize), im->filename );
 	printf( "data: \"%s\"\n", (char *) buf );
 #endif /*DEBUG*/
 
@@ -896,7 +892,6 @@ im__read_header( IMAGE *image )
 	 */
 	unsigned char header[IM_SIZEOF_HEADER];
 
-	gint64 length;
 	gint64 psize;
 
 	image->dtype = IM_OPENIN;
@@ -913,13 +908,11 @@ im__read_header( IMAGE *image )
 	/* Predict and check the file size.
 	 */
 	psize = im__image_pixel_length( image );
-	if( (length = im_file_length( image->fd )) == -1 ) 
+	if( (image->file_length = im_file_length( image->fd )) == -1 ) 
 		return( -1 );
-	if( psize > length ) {
+	if( psize > image->file_length ) 
 		im_warn( "im_openin", _( "unable to read data for \"%s\", %s" ),
 			image->filename, _( "file has been truncated" ) );
-		image->nodata = 1;
-	}
 
 	/* Set demand style. Allow the most permissive sort.
 	 */
@@ -960,9 +953,13 @@ im_openin( IMAGE *image )
 	if( im__read_header( image ) )
 		return( -1 );
 
+	/* Make sure we can map the whole thing without running over the VM
+	 * limit or running out of file.
+	 */
 	size = (gint64) IM_IMAGE_SIZEOF_LINE( image ) * image->Ysize + 
 		image->sizeof_header;
-	if( size < im__mmap_limit && !image->nodata ) {
+	if( size < im__mmap_limit && 
+		image->file_length >= size ) {
 		if( im_mapfile( image ) )
 			return( -1 );
 		image->data = image->baseaddr + image->sizeof_header;
diff --git a/libsrc/iofuncs/window.c b/libsrc/iofuncs/window.c
index f7eac7b0..eb036774 100644
--- a/libsrc/iofuncs/window.c
+++ b/libsrc/iofuncs/window.c
@@ -219,15 +219,6 @@ im_window_set( im_window_t *window, int top, int height )
 	gint64 start, end, pagestart;
 	size_t length, pagelength;
 
-	/* Make sure this image has data.
-	 */
-	if( window->im->nodata ) {
-		im_error( "im_window_set", 
-			_( "unable to read data for \"%s\", %s" ),
-			window->im->filename, _( "file has been truncated" ) );
-		return( -1 );
-	}
-
 	/* Calculate start and length for our window.
 	 */
 	start = window->im->sizeof_header + 
@@ -238,6 +229,15 @@ im_window_set( im_window_t *window, int top, int height )
 	end = start + length;
 	pagelength = end - pagestart;
 
+	/* Make sure we have enough file.
+	 */
+	if( end > window->im->file_length ) {
+		im_error( "im_window_set", 
+			_( "unable to read data for \"%s\", %s" ),
+			window->im->filename, _( "file has been truncated" ) );
+		return( -1 );
+	}
+
 	if( !(baseaddr = im__mmap( window->im->fd, 0, pagelength, pagestart )) )
 		return( -1 );