libc/src/sys/select/linux/select.cpp

//===-- Linux implementation of select ------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#include "src/sys/select/select.h"

#include "hdr/types/fd_set.h"
#include "hdr/types/struct_timespec.h"
#include "hdr/types/struct_timeval.h"
#include "src/__support/CPP/limits.h"
#include "src/__support/OSUtil/linux/syscall_wrappers/pselect6.h"
#include "src/__support/common.h"
#include "src/__support/libc_errno.h"
#include "src/__support/macros/config.h"

namespace LIBC_NAMESPACE_DECL {

LLVM_LIBC_FUNCTION(int, select,
                   (int nfds, fd_set *__restrict read_set,
                    fd_set *__restrict write_set, fd_set *__restrict error_set,
                    struct timeval *__restrict timeout)) {
  // Linux has a SYS_select syscall but it is not available on all
  // architectures. So, we use the SYS_pselect6 syscall which is more
  // widely available. However, SYS_pselect6 takes a struct timespec argument
  // instead of a struct timeval argument.
  struct timespec ts;
  struct timespec *tsp = nullptr;
  if (timeout != nullptr) {
    // In general, if the tv_sec and tv_usec in |timeout| are correctly set,
    // then converting tv_usec to nanoseconds will not be a problem. However,
    // if tv_usec in |timeout| is more than a second, it can lead to overflows.
    // So, we detect such cases and adjust.
    constexpr time_t TIME_MAX = cpp::numeric_limits<time_t>::max();
    if ((TIME_MAX - timeout->tv_sec) < (timeout->tv_usec / 1000000)) {
      ts.tv_sec = TIME_MAX;
      ts.tv_nsec = 999999999;
    } else {
      ts.tv_sec = timeout->tv_sec + timeout->tv_usec / 1000000;
      ts.tv_nsec = (timeout->tv_usec % 1000000) * 1000;
    }
    tsp = &ts;
  }

  auto result = linux_syscalls::pselect6(nfds, read_set, write_set, error_set,
                                         tsp, nullptr);
  if (!result.has_value()) {
    libc_errno = result.error();
    return -1;
  }
  return result.value();
}

} // namespace LIBC_NAMESPACE_DECL