posix: clock: implement clock_getres()

doc/services/portability/posix/option_groups/index.rst

@@ -332,7 +332,7 @@ POSIX_TIMERS
    :header: API, Supported
    :widths: 50,10
 
-    clock_getres(),
+    clock_getres(),yes
     clock_gettime(),yes
     clock_settime(),yes
     nanosleep(),yes

include/zephyr/posix/time.h

@@ -87,6 +87,7 @@ static inline int32_t _ts_to_ms(const struct timespec *to)
 }
 
 int clock_gettime(clockid_t clock_id, struct timespec *ts);
+int clock_getres(clockid_t clock_id, struct timespec *ts);
 int clock_settime(clockid_t clock_id, const struct timespec *ts);
 int clock_getcpuclockid(pid_t pid, clockid_t *clock_id);
 /* Timer APIs */

lib/posix/options/clock.c

@@ -97,6 +97,28 @@ int clock_gettime(clockid_t clock_id, struct timespec *ts)
 	return 0;
 }
 
+int clock_getres(clockid_t clock_id, struct timespec *res)
+{
+	BUILD_ASSERT(CONFIG_SYS_CLOCK_TICKS_PER_SEC > 0 &&
+			     CONFIG_SYS_CLOCK_TICKS_PER_SEC <= NSEC_PER_SEC,
+		     "CONFIG_SYS_CLOCK_TICKS_PER_SEC must be > 0 and <= NSEC_PER_SEC");
+
+	if (!(clock_id == CLOCK_MONOTONIC || clock_id == CLOCK_REALTIME ||
+	      clock_id == CLOCK_PROCESS_CPUTIME_ID)) {
+		errno = EINVAL;
+		return -1;
+	}
+
+	if (res != NULL) {
+		*res = (struct timespec){
+			.tv_sec = 0,
+			.tv_nsec = NSEC_PER_SEC / CONFIG_SYS_CLOCK_TICKS_PER_SEC,
+		};
+	}
+
+	return 0;
+}
+
 /**
  * @brief Set the time of the specified clock.
  *

tests/posix/common/src/clock.c

@@ -227,4 +227,48 @@ ZTEST(clock, test_clock_getcpuclockid)
 	zassert_equal(ret, EPERM, "POSIX clock_getcpuclock id failed");
 }
 
+ZTEST(clock, test_clock_getres)
+{
+	int ret;
+	struct timespec res;
+	const struct timespec one_ns = {
+		.tv_sec = 0,
+		.tv_nsec = 1,
+	};
+
+	struct arg {
+		clockid_t clock_id;
+		struct timespec *res;
+		int expect;
+	};
+
+	const struct arg args[] = {
+		/* permuting over "invalid" inputs */
+		{CLOCK_INVALID, NULL, -1},
+		{CLOCK_INVALID, &res, -1},
+		{CLOCK_REALTIME, NULL, 0},
+		{CLOCK_MONOTONIC, NULL, 0},
+		{CLOCK_PROCESS_CPUTIME_ID, NULL, 0},
+
+		/* all valid inputs */
+		{CLOCK_REALTIME, &res, 0},
+		{CLOCK_MONOTONIC, &res, 0},
+		{CLOCK_PROCESS_CPUTIME_ID, &res, 0},
+	};
+
+	ARRAY_FOR_EACH_PTR(args, arg) {
+		errno = 0;
+		res = (struct timespec){0};
+		ret = clock_getres(arg->clock_id, arg->res);
+		zassert_equal(ret, arg->expect);
+		if (ret != 0) {
+			zassert_equal(errno, EINVAL);
+			continue;
+		}
+		if (arg->res != NULL) {
+			zassert_true(tp_ge(arg->res, &one_ns));
+		}
+	}
+}
+
 ZTEST_SUITE(clock, NULL, NULL, NULL, NULL, NULL);