1 files changed, 35 insertions, 0 deletions
diff --git a/libc-bottom-half/clocks/clock.c b/libc-bottom-half/clocks/clock.c
new file mode 100644
index 0000000..6767d73
--- /dev/null
+++ b/libc-bottom-half/clocks/clock.c
@@ -0,0 +1,35 @@
+#define _WASI_EMULATED_PROCESS_CLOCKS
+#include <time.h>
+#include <wasi/api.h>
+#include <common/time.h>
+
+_Static_assert(
+    CLOCKS_PER_SEC == NSEC_PER_SEC,
+    "This implementation assumes that `clock` is in nanoseconds"
+);
+
+// Snapshot of the monotonic clock at the start of the program.
+static __wasi_timestamp_t start;
+
+// Use a priority of 10 to run fairly early in the implementation-reserved
+// constructor priority range.
+__attribute__((constructor(10)))
+static void init(void) {
+    (void)__wasi_clock_time_get(__WASI_CLOCKID_MONOTONIC, 0, &start);
+}
+
+// Define the libc symbol as `__clock` so that we can reliably call it
+// from elsewhere in libc.
+clock_t __clock(void) {
+    // Use `MONOTONIC` instead of `PROCESS_CPUTIME_ID` since WASI doesn't have
+    // an inherent concept of a process. Note that this means we'll incorrectly
+    // include time from other processes, so this function is only declared by
+    // the headers if `_WASI_EMULATED_PROCESS_CLOCKS` is defined.
+    __wasi_timestamp_t now = 0;
+    (void)__wasi_clock_time_get(__WASI_CLOCKID_MONOTONIC, 0, &now);
+    return now - start;
+}
+
+// Define a user-visible alias as a weak symbol.
+__attribute__((__weak__, __alias__("__clock")))
+clock_t clock(void);