Index: libucdf/tester.c
===================================================================
--- libucdf/tester.c	(revision 35436)
+++ libucdf/tester.c	(revision 35437)
@@ -66,6 +66,12 @@
 	if (argc > 1)
 		fn = argv[1];
 
+	if (ucdf_test_parse(fn) != 0) {
+		printf("%s doesn't look like a CDF file\n", fn);
+		return -1;
+	}
+
+
 	if (ucdf_open(&ctx, fn) != 0) {
 		printf("error reading %s: %d (%s)\n", fn, ctx.error, ucdf_error_str[ctx.error]);
 		return -1;
Index: libucdf/ucdf.c
===================================================================
--- libucdf/ucdf.c	(revision 35436)
+++ libucdf/ucdf.c	(revision 35437)
@@ -406,7 +406,7 @@
 	return 0;
 }
 
-int ucdf_open(ucdf_ctx_t *ctx, const char *path)
+static int ucdf_open_(ucdf_ctx_t *ctx, const char *path)
 {
 	ctx->f = fopen(path, "rb");
 	if (ctx->f == NULL) {
@@ -414,9 +414,28 @@
 		return -1;
 	}
 
-	if (ucdf_read_hdr(ctx) != 0)
-		goto error;
+	if (ucdf_read_hdr(ctx) != 0) {
+		fclose(ctx->f);
+		return -1;
+	}
+	return 0;
+}
 
+int ucdf_test_parse(const char *path)
+{
+	ucdf_ctx_t ctx;
+	if (ucdf_open_(&ctx, path) == 0) {
+		fclose(ctx.f);
+		return 0;
+	}
+	return -1;
+}
+
+int ucdf_open(ucdf_ctx_t *ctx, const char *path)
+{
+	if (ucdf_open_(ctx, path) != 0)
+		return -1;
+
 	if (ucdf_read_sats(ctx) != 0)
 		goto error;
 
Index: libucdf/ucdf.h
===================================================================
--- libucdf/ucdf.h	(revision 35436)
+++ libucdf/ucdf.h	(revision 35437)
@@ -84,6 +84,10 @@
 	ucdf_file_t ssd_f;
 };
 
+/* Look at a file, try to read some headers (cheap) to decide if path is a
+   valid CDF file. Returns 0 on no error (file is CDF), -1 on error. */
+int ucdf_test_parse(const char *path);
+
 /* Open and map a CDF file. Returns -1 on error, error code is in ctx->error */
 int ucdf_open(ucdf_ctx_t *ctx, const char *path);