macOS: DNSSD crash after calling fd_set on its socket - c++

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);
}