diff --git a/src/stdio/open_memstream.c b/src/stdio/open_memstream.c
index 5773709866d2166fb8b8db42fa311ae4e532d5c6..7fc16204daad7551d83ecf0e83b3383230cc1e00 100644
--- a/src/stdio/open_memstream.c
+++ b/src/stdio/open_memstream.c
@@ -13,21 +13,12 @@ static off_t ms_seek(FILE *f, off_t off, int whence)
 {
 	ssize_t base;
 	struct cookie *c = f->cookie;
-	switch (whence) {
-	case SEEK_SET:
-		base = 0;
-		break;
-	case SEEK_CUR:
-		base = c->pos;
-		break;
-	case SEEK_END:
-		base = c->len;
-		break;
-	default:
-	fail:
+	if (whence>2U) {
+fail:
 		errno = EINVAL;
 		return -1;
 	}
+	base = (size_t [3]){0, c->pos, c->len}[whence];
 	if (off < -base || off > SSIZE_MAX-base) goto fail;
 	return c->pos = base+off;
 }
diff --git a/src/stdio/open_wmemstream.c b/src/stdio/open_wmemstream.c
index 41b92d2152f58a402f5d4e965ab4e9cc8be841c2..0db77416cec819ce1cc7782bf68fd31c8955eb4a 100644
--- a/src/stdio/open_wmemstream.c
+++ b/src/stdio/open_wmemstream.c
@@ -14,21 +14,12 @@ static off_t wms_seek(FILE *f, off_t off, int whence)
 {
 	ssize_t base;
 	struct cookie *c = f->cookie;
-	switch (whence) {
-	case SEEK_SET:
-		base = 0;
-		break;
-	case SEEK_CUR:
-		base = c->pos;
-		break;
-	case SEEK_END:
-		base = c->len;
-		break;
-	default:
-	fail:
+	if (whence>2U) {
+fail:
 		errno = EINVAL;
 		return -1;
 	}
+	base = (size_t [3]){0, c->pos, c->len}[whence];
 	if (off < -base || off > SSIZE_MAX/4-base) goto fail;
 	memset(&c->mbs, 0, sizeof c->mbs);
 	return c->pos = base+off;