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 #include "hdr/time_macros.h"
  11 #include "src/__support/CPP/limits.h"
  12 #include "src/__support/common.h"
  13 #include "src/__support/macros/config.h"
  14 #include "src/__support/time/clock_gettime.h"
  15 #include "src/__support/time/units.h"
  16 #include "src/errno/libc_errno.h"
  17
  18 namespace LIBC_NAMESPACE_DECL {
  19
  20 LLVM_LIBC_FUNCTION(clock_t, clock, ()) {
  21   using namespace time_units;
  22   struct timespec ts;
  23   auto result = internal::clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &ts);
  24   if (!result.has_value()) {
  25     libc_errno = result.error();
  26     return -1;
  27   }
  28
  29   // The above syscall gets the CPU time in seconds plus nanoseconds.
  30   // The standard requires that we return clock_t(-1) if we cannot represent
  31   // clocks as a clock_t value.
  32   constexpr clock_t CLOCK_SECS_MAX =
  33       cpp::numeric_limits<clock_t>::max() / CLOCKS_PER_SEC;
  34   if (ts.tv_sec > CLOCK_SECS_MAX)
  35     return clock_t(-1);
  36   if (ts.tv_nsec / 1_s_ns > CLOCK_SECS_MAX - ts.tv_sec)
  37     return clock_t(-1);
  38
  39   // For the integer computation converting tv_nsec to clocks to work
  40   // correctly, we want CLOCKS_PER_SEC to be less than 1000000000.
  41   static_assert(1_s_ns > CLOCKS_PER_SEC,
  42                 "Expected CLOCKS_PER_SEC to be less than 1'000'000'000.");
  43   return clock_t(ts.tv_sec * CLOCKS_PER_SEC +
  44                  ts.tv_nsec / (1_s_ns / CLOCKS_PER_SEC));
  45 }
  46
  47 } // namespace LIBC_NAMESPACE_DECL