[LTP] [PATCH] clock_settime: use POSIX runtime detection for CLOCK_MONOTONIC

Andrea Cervesato
2026-04-16T13:01:24Z

From: Andrea Cervesato <andrea.cervesato@suse.com>

The clock_settime helpers.h used a compile-time #ifdef on
_POSIX_MONOTONIC_CLOCK to select between real and stub
implementations of pts_mono_time_start()/pts_mono_time_check().

On Linux/glibc, _POSIX_MONOTONIC_CLOCK is defined as 0, meaning
support is optional and must be verified at runtime via
sysconf(_SC_MONOTONIC_CLOCK). The #ifdef treated 0 the same as
>0 (always available), which is not POSIX-correct.

Replace the compile-time #ifdef/#else split with a runtime
pts_mono_available() helper that performs proper POSIX detection
via sysconf(). Also add a missing #ifndef include guard and
#include <unistd.h> so _POSIX_MONOTONIC_CLOCK is always defined.

Fixes: 9ecb4a004b18 ("clock_settime: Detect external clock adjustments via CLOCK_MONOTONIC")
Signed-off-by: Andrea Cervesato <andrea.cervesato@suse.com>
---
.../conformance/interfaces/clock_settime/helpers.h | 76 +++++++++++++---------
1 file changed, 46 insertions(+), 30 deletions(-)

diff --git a/testcases/open_posix_testsuite/conformance/interfaces/clock_settime/helpers.h b/testcases/open_posix_testsuite/conformance/interfaces/clock_settime/helpers.h
index 37bf30926f039553e9e370751b7938ca5ea1d00a..b568ec8122d3bba8246b09a5b694f1263084a408 100644
--- a/testcases/open_posix_testsuite/conformance/interfaces/clock_settime/helpers.h
+++ b/testcases/open_posix_testsuite/conformance/interfaces/clock_settime/helpers.h
@@ -13,7 +13,11 @@
* by those tests.
*/

+#ifndef CLOCK_SETTIME_HELPERS_H
+#define CLOCK_SETTIME_HELPERS_H
+
#include <stdlib.h>
+#include <unistd.h>

static int getBeforeTime(struct timespec *tpget)
{
@@ -37,53 +41,65 @@ static int setBackTime(struct timespec tpset)

#define PTS_MONO_MAX_RETRIES 3

+static struct timespec pts_mono_start;
+
+static inline int pts_mono_available(void)
+{
#ifdef _POSIX_MONOTONIC_CLOCK
-static struct timespec _pts_mono_start;
+ if (_POSIX_MONOTONIC_CLOCK > 0)
+ return 1;
+
+ if (!_POSIX_MONOTONIC_CLOCK && sysconf(_SC_MONOTONIC_CLOCK) > 0)
+ return 1;
+#endif
+ return 0;
+}

static inline int pts_mono_time_start(void)
{
- if (clock_gettime(CLOCK_MONOTONIC, &_pts_mono_start) != 0) {
+ if (!pts_mono_available()) {
+ static int warned;
+
+ if (!warned) {
+ printf("CLOCK_MONOTONIC unavailable, test may fail due to clock adjustment\n");
+ warned = 1;
+ }
+ return 0;
+ }
+
+#ifdef _POSIX_MONOTONIC_CLOCK
+ if (clock_gettime(CLOCK_MONOTONIC, &pts_mono_start) != 0) {
perror("clock_gettime(CLOCK_MONOTONIC) failed");
return -1;
}
+#endif
return 0;
}

static inline int pts_mono_time_check(unsigned int expected_secs)
{
- struct timespec now;
- long elapsed;
-
- if (clock_gettime(CLOCK_MONOTONIC, &now) != 0) {
- perror("clock_gettime(CLOCK_MONOTONIC) failed");
- return -1;
- }
+#ifdef _POSIX_MONOTONIC_CLOCK
+ if (pts_mono_available()) {
+ struct timespec now;
+ long elapsed;

- elapsed = now.tv_sec - _pts_mono_start.tv_sec;
+ if (clock_gettime(CLOCK_MONOTONIC, &now) != 0) {
+ perror("clock_gettime(CLOCK_MONOTONIC) failed");
+ return -1;
+ }

- if (labs(elapsed - (long)expected_secs) > 1) {
- printf("Clock adjustment detected (elapsed %lds, expected ~%us)\n",
- elapsed, expected_secs);
- return 1;
- }
- return 0;
-}
-#else
-static inline int pts_mono_time_start(void)
-{
- static int warned;
+ elapsed = now.tv_sec - pts_mono_start.tv_sec;

- if (!warned) {
- printf("CLOCK_MONOTONIC unavailable, test may fail due to clock adjustment\n");
- warned = 1;
+ if (labs(elapsed - (long)expected_secs) > 1) {
+ printf("Clock adjustment detected (elapsed %lds, expected ~%us)\n",
+ elapsed, expected_secs);
+ return 1;
+ }
+ return 0;
}
- return 0;
-}
-
-static inline int pts_mono_time_check(unsigned int expected_secs)
-{
+#endif
(void)expected_secs;
return 0;
}
-#endif

+#endif /* CLOCK_SETTIME_HELPERS_H */

---
base-commit: 620d596c79493af089ba6a1c4dc79efcc76a67c7
change-id: 20260416-fix_clock_settime_helper-d47f0db0057e

Best regards,
--
Andrea Cervesato <andrea.cervesato@suse.com>