1 files changed, 23 insertions, 0 deletions
diff --git a/tests/clock-back.c b/tests/clock-back.c
new file mode 100644
index 0000000..50870ca
--- /dev/null
+++ b/tests/clock-back.c
@@ -0,0 +1,23 @@
+#define _GNU_SOURCE
+/* LD_PRELOAD: the wall clock (CLOCK_REALTIME) set back an hour from
+ * CLOCK_BACK_MS milliseconds (200 unless given) after the process first
+ * asks for it, as a person or NTP may set it. A program's timers must go on
+ * all the same: they wait on the monotonic clock (tests/check.py). */
+#include <dlfcn.h>
+#include <stdlib.h>
+#include <time.h>
+
+int clock_gettime(clockid_t id, struct timespec *t) {
+    static int (*real)(clockid_t, struct timespec *);
+    static long long began = -1;
+    if (!real) *(void **)&real = dlsym(RTLD_NEXT, "clock_gettime"); /* as POSIX's dlsym example does */
+    int result = real(id, t);
+    if (result || id != CLOCK_REALTIME) return result;
+    struct timespec now;
+    real(CLOCK_MONOTONIC, &now);
+    long long ms = (long long)now.tv_sec * 1000 + now.tv_nsec / 1000000;
+    if (began < 0) began = ms;
+    const char *after = getenv("CLOCK_BACK_MS");
+    if (ms - began >= (after ? atoll(after) : 200)) t->tv_sec -= 3600;
+    return result;
+}