Merge pull request #1899 from kleisauke/byteswap-vipsload

Ensure vipsload only byte swaps if necessary

libvips/iofuncs/image.c

@@ -859,10 +859,7 @@ vips_image_build( VipsObject *object )
 		if( (magic = vips__file_magic( filename )) ) {
 			/* We may need to byteswap.
 			 */
-			guint32 us = vips_amiMSBfirst() ? 
-				VIPS_MAGIC_INTEL : VIPS_MAGIC_SPARC;
-
-			if( magic == us ) {
+			if( GUINT_FROM_BE( magic ) == image->magic ) {
 				/* Native open.
 				 */
 				if( vips_image_open_input( image ) )

libvips/iofuncs/util.c

@@ -1602,16 +1602,13 @@ vips_ispoweroftwo( int p )
 int
 vips_amiMSBfirst( void )
 {
-        int test;
-        unsigned char *p = (unsigned char *) &test;
-
-        test = 0;
-        p[0] = 255;
-
-        if( test == 255 )
-                return( 0 );
-        else
-                return( 1 );
+#if G_BYTE_ORDER == G_BIG_ENDIAN
+	return( 1 );
+#elif G_BYTE_ORDER == G_LITTLE_ENDIAN
+	return( 0 );
+#else
+#error "Byte order not recognised"
+#endif
 }
 
 /* Return the tmp dir. On Windows, GetTempPath() will also check the values of 

libvips/iofuncs/vips.c

@@ -354,7 +354,7 @@ vips__read_header_bytes( VipsImage *im, unsigned char *from )
 	/* We need to swap for other fields if the file byte order is 
 	 * different from ours.
 	 */
-	swap = vips_amiMSBfirst() != (im->magic == VIPS_MAGIC_SPARC);
+	swap = vips_amiMSBfirst() != vips_image_isMSBfirst( im );
 
 	for( i = 0; i < VIPS_NUMBER( fields ); i++ ) {
 		fields[i].copy( swap,
@@ -435,7 +435,7 @@ vips__write_header_bytes( VipsImage *im, unsigned char *to )
 	/* Swap if the byte order we are asked to write the header in is
 	 * different from ours.
 	 */
-	gboolean swap = vips_amiMSBfirst() != (im->magic == VIPS_MAGIC_SPARC);
+	gboolean swap = vips_amiMSBfirst() != vips_image_isMSBfirst( im );
 
 	int i;
 	unsigned char *q;

test/test-suite/test_foreign.py

@@ -128,6 +128,17 @@ class TestForeign:
         for i in range(len(before_exif)):
             assert before_exif[i] == after_exif[i]
 
+        # https://github.com/libvips/libvips/issues/1847
+        filename = temp_filename(self.tempdir, ".v")
+        x = pyvips.Image.black(16, 16) + 128
+        x.write_to_file(filename)
+
+        x = pyvips.Image.new_from_file(filename)
+        assert x.width == 16
+        assert x.height == 16
+        assert x.bands == 1
+        assert x.avg() == 128
+
         x = None
 
     @skip_if_no("jpegload")