timespect_get: move from cpp-cheat

README.adoc

@@ -17107,6 +17107,10 @@ Good sanity check for user mode: <<qemu-user-mode-getting-started>>
 *** File IO
 **** link:userland/c/file_write_read.c[]
 **** link:userland/linux/open_o_tmpfile.c[]: https://stackoverflow.com/questions/4508998/what-is-an-anonymous-inode-in-linux/44388030#44388030
+** `time.h`
+*** link:userland/c/timespec_get.c[] `timespec_get` is a C11 for `clock_gettime` http://stackoverflow.com/questions/361363/how-to-measure-time-in-milliseconds-using-ansi-c/36095407#36095407
++
+Vs `clock()`: http://stackoverflow.com/questions/12392278/measure-time-in-linux-getrusage-vs-clock-gettime-vs-clock-vs-gettimeofday
 * Fun
 ** link:userland/c/loop.c[]
 

userland/c/timespec_get.c

@@ -0,0 +1,34 @@
+/* https://cirosantilli.com/linux-kernel-module-cheat#c
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <time.h>
+
+/* We use double because time_t can be either floating point or integer.
+ * POSIX guarantees that it is an integer, we could use uintmax_t then. */
+static long double get_nanos(void) {
+    struct timespec ts;
+    timespec_get(&ts, TIME_UTC);
+    return ts.tv_sec * 1e9 + ts.tv_nsec;
+}
+
+int main(int argc, char **argv) {
+    long double start;
+    unsigned long long int i, niters;
+
+    if (argc > 1) {
+        niters = strtoull(argv[1], NULL, 0);
+    } else {
+        niters = 3;
+    }
+    i = 0;
+    start = get_nanos();
+    while (1) {
+        printf("%Lf\n", get_nanos() - start);
+        i++;
+        if (i == niters)
+            break;
+    }
+    return EXIT_SUCCESS;
+}