diff --git a/src/regression/flockfile-list.c b/src/regression/flockfile-list.c
index 09f18ff..a269cb8 100644
--- a/src/regression/flockfile-list.c
+++ b/src/regression/flockfile-list.c
@@ -6,6 +6,84 @@
 #include <string.h>
 #include "test.h"
 
+#define t_fatal(...) (t_error(__VA_ARGS__), _Exit(t_status))
+#define length(a) (sizeof(a)/sizeof*(a))
+
+// interpose malloc functions
+// freed memory is not reused, it is checked for clobber.
+
+static unsigned char buf[1<<20];
+static size_t pos;
+static struct {
+	size_t pos;
+	size_t n;
+	int freed;
+} alloc[100];
+static int idx;
+
+void *malloc(size_t n)
+{
+	if (n == 0) n++;
+	if (n > sizeof buf - pos)
+		t_fatal("test buffer is small, pos: %zu, need: %zu\n", pos, n);
+	if (idx >= length(alloc))
+		t_fatal("test buffer is small, idx: %d\n", idx);
+	void *p = buf + pos;
+	alloc[idx].pos = pos;
+	alloc[idx].n = n;
+	pos += n;
+	idx++;
+	return p;
+}
+
+void *calloc(size_t n, size_t m)
+{
+	return memset(malloc(n*m), 0, n*m);
+}
+
+void *aligned_alloc(size_t a, size_t n)
+{
+	t_fatal("aligned_alloc is unsupported\n");
+}
+
+static int findidx(void *p)
+{
+	size_t pos = (unsigned char *)p - buf;
+	for (int i=0; i<idx; i++)
+		if (alloc[i].pos == pos)
+			return i;
+	t_fatal("%p is not an allocated pointer\n", p);
+	return -1;
+}
+
+void *realloc(void *p, size_t n)
+{
+	void *q = malloc(n);
+	size_t m = alloc[findidx(p)].n;
+	memcpy(q, p, m < n ? m : n);
+	free(p);
+	return q;
+}
+
+void free(void *p)
+{
+	if (p == 0) return;
+	int i = findidx(p);
+	memset(p, 42, alloc[i].n);
+	alloc[i].freed = 1;
+}
+
+static void checkfreed(void)
+{
+	for (int i=0; i<idx; i++)
+		if (alloc[i].freed)
+			for (size_t j=0; j<alloc[i].n; j++)
+				if (buf[alloc[i].pos + j] != 42) {
+					t_error("freed allocation %d (pos: %zu, len: %zu) is clobbered\n", i, alloc[i].pos, alloc[i].n);
+					break;
+				}
+}
+
 int main()
 {
 	FILE *f = tmpfile();
@@ -14,29 +92,8 @@ int main()
 	flockfile(f);
 	funlockfile(g);
 	fclose(g);
-
-	/* fill memory */
-	if (t_vmfill(0,0,0) < 0)
-		t_error("vmfill failed: %s\n", strerror(errno));
-	size_t i,n;
-	unsigned char *p;
-	for (n = 1; n < 10000; n++) {
-		if (!(p=malloc(n))) break;
-		free(p);
-	}
-	n--;
-	if (!(p=malloc(n))) {
-		t_error("bad malloc fragmentation\n");
-		return t_status;
-	}
-	memset(p, 0xff, n);
-
 	/* may corrupt memory */
 	funlockfile(f);
-	for (i=0; i<n; i++) {
-		if (p[i]!=0xff) {
-			t_error("p[%zu] = %.2x\n", i, p[i]);
-		}
-	}
+	checkfreed();
 	return t_status;
 }