libc/src/time/linux/clock.cpp

   1 //===-- Linux implementation of the clock function ------------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8
   9 #include "src/time/clock.h"
  10
  11 #include "src/__support/CPP/limits.h"
  12 #include "src/__support/OSUtil/syscall.h" // For internal syscall function.
  13 #include "src/__support/common.h"
  14
  15 #include <errno.h>
  16 #include <sys/syscall.h> // For syscall numbers.
  17 #include <time.h>
  18
  19 namespace __llvm_libc {
  20
  21 LLVM_LIBC_FUNCTION(clock_t, clock, ()) {
  22   struct timespec ts;
  23   long ret_val = __llvm_libc::syscall_impl(
  24       SYS_clock_gettime, CLOCK_PROCESS_CPUTIME_ID, reinterpret_cast<long>(&ts));
  25   if (ret_val < 0) {
  26     errno = -ret_val;
  27     return clock_t(-1);
  28   }
  29
  30   // The above syscall gets the CPU time in seconds plus nanoseconds. We should
  31   // make sure that corresponding clocks can actually be represented by clock-t.
  32   // The standard requires that we return clock_t(-1) if we cannot represent
  33   // clocks as a clock_t value.
  34   constexpr clock_t CLOCK_SECS_MAX =
  35       cpp::numeric_limits<clock_t>::max() / CLOCKS_PER_SEC;
  36   if (ts.tv_sec > CLOCK_SECS_MAX)
  37     return clock_t(-1);
  38   if (ts.tv_nsec / 1000000000 > CLOCK_SECS_MAX - ts.tv_sec)
  39     return clock_t(-1);
  40
  41   // For the integer computation converting tv_nsec to clocks to work
  42   // correctly, we want CLOCKS_PER_SEC to be less than 1000000000.
  43   static_assert(1000000000 > CLOCKS_PER_SEC,
  44                 "Expected CLOCKS_PER_SEC to be less than 1000000000.");
  45   return clock_t(ts.tv_sec * CLOCKS_PER_SEC +
  46                  ts.tv_nsec / (1000000000 / CLOCKS_PER_SEC));
  47 }
  48
  49 } // namespace __llvm_libc