wasi-libc/libc-bottom-half/clocks/clock.c

#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);