--- /dev/null
+++ Libc/Libc-1725.40.4/tests/open_memstream.c
@@ -0,0 +1,45 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+#include <darwintest.h>
+#include <darwintest_perf.h>
+
+static void
+perf_fixed_size(size_t size_per_write)
+{
+	dt_stat_time_t latency = dt_stat_time_create("write_latency",
+			"adding %zu bytes to a memstream", size_per_write);
+	dt_stat_set_variable_d(latency, "bytes", (double)size_per_write);
+	char *src = calloc(1, size_per_write);
+	T_QUIET; T_ASSERT_NOTNULL(src, "allocated source buffer");
+
+	while (!dt_stat_stable(latency)) {
+		char *buf = NULL;
+		size_t size = 0;
+
+		FILE *victim = open_memstream(&buf, &size);
+		T_QUIET; T_WITH_ERRNO; T_ASSERT_NOTNULL(victim, "opened memstream");
+
+		T_STAT_MEASURE_BATCH(latency) {
+			(void)fwrite(src, size_per_write, 1, victim);
+		}
+
+		fclose(victim);
+		T_QUIET; T_ASSERT_NOTNULL(buf, "buffer was set by open_memstream");
+		T_QUIET; T_ASSERT_GE(size, size_per_write,
+				"memstream added non-zero bytes");
+
+		free(buf);
+	}
+
+	free(src);
+	dt_stat_finalize(latency);
+}
+
+T_DECL(perf_open_memstream, "measure the performance of open_memstream")
+{
+	size_t sizes[] = { 1, 8, 16, 64, 1024, 2048, 4096, 16 * 1024 };
+	for (size_t i = 0; i < sizeof(sizes) / sizeof(sizes[0]); i++) {
+		perf_fixed_size(sizes[i]);
+	}
+}