From 985f4d1f53bd4d028e3f99a5434eeaf1a2276530 Mon Sep 17 00:00:00 2001
From: David Timber <dxdt@dev.snart.me>
Date: Mon, 6 Jan 2025 03:46:13 +0100
Subject: Add writeups/pthread-timedwait

---
 writeups/pthread-timedwait/.gitignore          |   1 +
 writeups/pthread-timedwait/Makefile            |   2 +
 writeups/pthread-timedwait/pthread-timedwait.c | 150 +++++++++++++++++++++++++
 3 files changed, 153 insertions(+)
 create mode 100644 writeups/pthread-timedwait/.gitignore
 create mode 100644 writeups/pthread-timedwait/Makefile
 create mode 100644 writeups/pthread-timedwait/pthread-timedwait.c

diff --git a/writeups/pthread-timedwait/.gitignore b/writeups/pthread-timedwait/.gitignore
new file mode 100644
index 0000000..67eb7e0
--- /dev/null
+++ b/writeups/pthread-timedwait/.gitignore
@@ -0,0 +1 @@
+pthread-timedwait
\ No newline at end of file
diff --git a/writeups/pthread-timedwait/Makefile b/writeups/pthread-timedwait/Makefile
new file mode 100644
index 0000000..03d0b53
--- /dev/null
+++ b/writeups/pthread-timedwait/Makefile
@@ -0,0 +1,2 @@
+pthread-timedwait: pthread-timedwait.c
+	cc -std=c11 -Wall -Wextra -O0 -g -opthread-timedwait pthread-timedwait.c
diff --git a/writeups/pthread-timedwait/pthread-timedwait.c b/writeups/pthread-timedwait/pthread-timedwait.c
new file mode 100644
index 0000000..9b6e7b5
--- /dev/null
+++ b/writeups/pthread-timedwait/pthread-timedwait.c
@@ -0,0 +1,150 @@
+/**
+ * @file pthread-timedwait.c
+ * @brief Demonstrates the effects on pthread_timedwait upon system wall clock
+ * change
+ */
+#define _POSIX_C_SOURCE 199309L
+#include <stdio.h>
+#include <stdbool.h>
+#include <string.h>
+#include <time.h>
+#include <errno.h>
+#include <pthread.h>
+
+#define ARG0 "pthread_timedwait"
+
+
+struct {
+	// desired sleep time
+	struct timespec t;
+} opts;
+
+/**
+ * @brief Raise nanosecond fraction part
+ * @details "05" -> 50000000, "123" -> 123000000, "005" -> 5000000
+ * @note used to preserve precision
+ *
+ * @param str fraction part, excluding the leading decimal point
+ * @param len length of \param str
+ * @return long the fraction part raised to nanosecond scale
+ */
+long raise_nsec_frac (const char *str, const size_t len) {
+	char m[] = { '0', '0', '0', '0', '0', '0', '0', '0', '0', 0 };
+	char *p;
+	long ret = -1;
+
+	memcpy(m, str, len > 9 ? 9 : len);
+
+	for (p = m; *p != 0 && *p == '0'; p += 1);
+
+	if (*p == 0 && m[0] == '0') {
+		return 0;
+	}
+	if (sscanf(p, "%ld", &ret) != 1) {
+		return -1;
+	}
+
+	return ret;
+}
+
+/**
+ * @brief Parse timespec from string, preserving precision
+ *
+ * @param str string to parse
+ * @param ts_out (optional) pointer to timespec struct
+ * @return true if parsed successfully and \param ts_out is set if non-null
+ * @return false otherwise. errno set to EINVAL if \param str is NULL. errno set
+ * to EBADMSG on format error
+ */
+bool parse_ts (const char *str, struct timespec *ts_out) {
+	long long sec = 0;
+	long nsec = 0;
+	int fr;
+	const char *frac, *ipart;
+
+	if (str == NULL) {
+		errno = EINVAL;
+		return false;
+	}
+
+	if (str[0] == '.' || str[0] == ',') {
+		ipart = NULL;
+		frac = str + 1;
+	}
+	else {
+		ipart = str;
+		frac = strchr(str, '.');
+		if (frac == NULL) {
+			frac = strchr(str, ',');
+		}
+	}
+
+	if (ipart != NULL) {
+		fr = sscanf(ipart, "%lld", &sec);
+		if (fr < 1) {
+			errno = EBADMSG;
+			return false;
+		}
+	}
+	if (frac != NULL) {
+		frac += 1;
+		nsec = raise_nsec_frac(frac, strlen(frac));
+		if (nsec < 0) {
+			errno = EBADMSG;
+			return false;
+		}
+	}
+
+	if (ts_out != NULL) {
+		ts_out->tv_sec = sec;
+		ts_out->tv_nsec = nsec;
+	}
+
+	return true;
+}
+
+bool parse_args (const int argc, const char **argv) {
+	if (argc <= 1) {
+		fprintf(stderr, ARG0" <TIME>\n");
+		return false;
+	}
+
+	if (!parse_ts(argv[1], &opts.t)) {
+		perror(ARG0);
+		return false;
+	}
+
+	return true;
+}
+
+int main (const int argc, const char **argv) {
+	pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
+	pthread_mutex_t mtx = PTHREAD_MUTEX_INITIALIZER;
+	struct timespec ts[2];
+	struct timespec tp_deadline;
+
+	if (!parse_args(argc, argv)) {
+		return 1;
+	}
+
+	clock_gettime(CLOCK_REALTIME, &tp_deadline);
+	tp_deadline.tv_nsec += opts.t.tv_nsec;
+	tp_deadline.tv_sec += opts.t.tv_sec + (tp_deadline.tv_nsec / 1000000000);
+	tp_deadline.tv_nsec %= 1000000000;
+
+	clock_gettime(CLOCK_MONOTONIC, ts);
+	pthread_mutex_lock(&mtx);
+	pthread_cond_timedwait(&cond, &mtx, &tp_deadline);
+	clock_gettime(CLOCK_MONOTONIC, ts + 1);
+
+	ts[1].tv_nsec -= ts[0].tv_nsec;
+	ts[1].tv_sec -= ts[0].tv_sec;
+	if (ts[1].tv_nsec < 0) {
+		ts[1].tv_sec -= 1;
+		ts[1].tv_nsec += 1000000000;
+	}
+
+	printf("%lld.%03ld\n", (long long)ts[1].tv_sec, ts[1].tv_nsec / 1000000);
+
+	return 0;
+}
-- 
cgit