diff --git a/pthread_trace.cc b/pthread_trace.cc
index 53b7a2d..565dabd 100644
--- a/pthread_trace.cc
+++ b/pthread_trace.cc
@@ -133,13 +133,16 @@ class circular_file {
       buffer_ = nullptr;
     }
     if (fd >= 0) {
-      if (next_.load() < size_) {
+      size_t size = next_.load();
+      if (size < size_) {
         // Remove blocks we didn't write anything to.
-        int result = ftruncate(fd, next_.load());
+        int result = ftruncate(fd, size);
         (void)result;
       }
       ::close(fd);
       fd = -1;
+
+      fprintf(stderr, "pthread_trace: Recorded %zu KB trace\n", size / 1024);
     }
   }
 
diff --git a/test/BUILD b/test/BUILD
index 31d5ae3..8cf4cbe 100644
--- a/test/BUILD
+++ b/test/BUILD
@@ -13,11 +13,31 @@ cc_test(
     size = "small",
 )
 
+# We don't have good tests of tracing itself. It's hard to test:
+# - Traces are nondeterministic (they contain pointers and timestamps)
+# - I haven't figured out how to use it without LD_PRELOAD
+# The best we have for now is just running one of the benchmarks that use pthreads with tracing,
+# and verifying it doesn't crash and we see expected pthread_trace output.
+sh_test(
+    name = "trace_thread_benchmark",
+    srcs = ["trace_test.sh"],
+    data = [
+        ":thread_benchmark",
+        "//:pthread_trace.so",
+    ],
+    args = [
+        "$(location //:pthread_trace.so)", 
+        "$(location :thread_benchmark)",
+        "--benchmark_min_time=1x",
+    ],
+    size = "small",
+)
+
 cc_test(
-    name = "benchmark",
-    srcs = ["benchmark.cc"],
+    name = "thread_benchmark",
+    srcs = ["thread_benchmark.cc"],
     deps = ["@google_benchmark//:benchmark_main"],
-    args=["--benchmark_min_time=1x"],
+    args = ["--benchmark_min_time=1x"],
     size = "small",
 )
 
@@ -28,6 +48,6 @@ cc_test(
         "//:proto",
         "@google_benchmark//:benchmark_main"
     ],
-    args=["--benchmark_min_time=1x"],
+    args = ["--benchmark_min_time=1x"],
     size = "small",
 )
diff --git a/test/benchmark.cc b/test/thread_benchmark.cc
similarity index 100%
rename from test/benchmark.cc
rename to test/thread_benchmark.cc
diff --git a/test/trace_test.sh b/test/trace_test.sh
new file mode 100755
index 0000000..9a99cea
--- /dev/null
+++ b/test/trace_test.sh
@@ -0,0 +1,16 @@
+#!/bin/bash
+
+pthread_trace="$1"
+target="$2"
+
+shift
+shift
+
+fail() { rc=$?; (( $# )) && printf '%s\n' "$*" >&2; exit $(( rc == 0 ? 1 : rc )); }
+
+trace=$(mktemp)
+
+exec 5>&1
+output=$(PTHREAD_TRACE_BUFFER_SIZE_KB=1 PTHREAD_TRACE_PATH=$trace LD_PRELOAD="$pthread_trace" "$target" "$@" 2>&1 | tee >(cat - >&5))
+
+echo "$output" | grep "pthread_trace: Recorded.*KB trace" > /dev/null || fail "Did not find pthread_trace report in output"
\ No newline at end of file