macOS: DNSSD crash after calling fd_set on its socket

Question

I have the following code, which crashes my program, upon calling FD_SET.

void handleEvents(DNSServiceRef service, const int32_t timeout)
{
    if (!service)
      return;

    const int fd = DNSServiceRefSockFD( service );
    const int nfds = fd + 1;

    if (fd < 0)
      return;

    int32_t result = servus::Result::PENDING;
    while(result == servus::Result::PENDING)
    {
        fd_set fdSet;
        FD_ZERO( &fdSet );
        FD_SET( fd, &fdSet ); /// < The crash occurs here

        const int result = ::select( nfds, &fdSet, 0, 0, 0);
        switch (result)
        {
          case 0: // timeout
            return;

          case -1: // error
            std::cerr << "Select error: " << strerror( errno ) << " (" << errno
                 << ")" << std::endl;
            if( errno != EINTR )
            {
                withdraw();
                return;
            }
            break;

          default:
            if(FD_ISSET( fd, &fdSet ))
            {
                const auto error = DNSServiceProcessResult(service);

                if(error != kDNSServiceErr_NoError)
                {
                    std::cerr << "DNSServiceProcessResult error: " << error << std::endl;
                    withdraw();
                    return;
                }
            }
            break;
        }
    }
}

The crash log is:

Exception Type:        EXC_GUARD
Exception Codes:       0x6000000000000012, 0x0000000000000002
Exception Note:        EXC_CORPSE_NOTIFY

Termination Reason:    LIBSYSTEM, [0x2]

External Modification Warnings:
Debugger attached to process.

Thread 0 Crashed:
0   libsystem_kernel.dylib          0x00007fff6d62d96e os_fault_with_payload + 10
1   libsystem_kernel.dylib          0x00007fff6d62e451 __darwin_check_fd_set_overflow.cold.2 + 31
2   libsystem_kernel.dylib          0x00007fff6d61967c __darwin_check_fd_set_overflow + 68
3   score                           0x00000001004f097e handleEvents(_DNSServiceRef_t*, int) + 302

I really don't understand where are things going wrong - the "critical chain" that leads to the crash seems to be

const int fd = DNSServiceRefSockFD( service );
const int nfds = fd + 1;
int result = 0;

fd_set fdSet;
FD_ZERO( &fdSet );
FD_SET( fd, &fdSet ); /// < The crash occurs here

result = ::select( nfds, &fdSet, 0, 0, 0);
fd_set fdSet;
FD_ZERO( &fdSet );
FD_SET( fd, &fdSet ); /// < or here

Answer 1

select() now returns with errno set to EINVAL when nfds is greater than FD_SETSIZE . Use a smaller value for nfds or compile with -D_DARWIN_UNLIMITED_SELECT .

Answer 2

The actual issue in this case was that I was having too many file descriptors open, which can be solved by increasing the rlimit with the following code:

void setup_min_fd(int min_fds)
{
  struct rlimit rlim;
  if (getrlimit(RLIMIT_NOFILE, &rlim) != 0)
    return;

  if (rlim.rlim_cur > rlim_t(min_fds))
    return;

  rlim.rlim_cur = rlim.min_fds;

  setrlimit(RLIMIT_NOFILE, &rlim);
}