macOS iostream on a pseudo tty does not work - c++

Question

GCC has a stdio_filebuf extension that lets you wrap an iostream around a file descriptor while clang's basic_filebuf has an __open() method that lets open a file descriptor. I've tested both on regular files on both linux and macos. Works as expected.
However, for a pseudo tty file descriptor, it works for linux but not macos. I've confirmed that the file descriptor on macos does work by using read(2) on it. Here's my code.
#include <fmt/format.h>
#include <getopt.h>
#include <vector>
#include <functional>
#include <future>
#include <fcntl.h>
#include <cstdlib>
#if defined(__linux__)
#include <ext/stdio_filebuf.h>
#elif defined(__APPLE__)
#include <fstream>
#else
#error "unsupported os"
#endif
#include <iostream>
#include <unistd.h>
int get_ptym(int oflags)
{
int fd;
if ((fd = posix_openpt(oflags)) == -1
or grantpt(fd) == -1
or unlockpt(fd) == -1)
return -1;
return fd;
}
ssize_t getdelim(int fd, char *buf, size_t bufsz,
char delim='\n', bool store_delim=false)
{
size_t j = 0;
while (j < bufsz-2) {
char c;
ssize_t rc;
if ((rc = ::read(fd, &c, 1)) == -1) return rc;
if (rc == 0) break;
if (c != delim) { buf[j++] = c; continue; }
// Break if we get the delimiter.
if (store_delim) buf[j++] = c;
break;
}
buf[j] = '\0';
return j+1;
}
std::string m0(int fd)
{
#if defined(__linux__)
__gnu_cxx::stdio_filebuf<char> fb(fd, std::ios::in|std::ios::out);
std::iostream ioob(&fb);
#endif
#if defined(__APPLE__)
std::basic_filebuf<char> fb;
std::iostream ioob(fb.__open(fd, std::ios::in|std::ios::out));
#endif
std::string s;
if (!std::getline(ioob, s, '\n').good())
throw std::runtime_error("m0: getline failed");
return s;
}
std::string m1(int fd)
{
char buf[4096];
ssize_t nrd;
if ((nrd = getdelim(fd, buf, sizeof(buf))) == -1)
throw std::runtime_error("getlim error");
return std::string(buf);
}
void testing(const std::string &foonm, std::function<std::string(int)> foo)
{
using namespace std::chrono_literals;
int mfd, sfd;
// Open pty master and slave.
if ((mfd = get_ptym(O_RDWR)) == -1) return;
if ((sfd = open(ptsname(mfd), O_RDWR)) == -1) return;
#if defined(__linux__)
__gnu_cxx::stdio_filebuf<char> fb(sfd, std::ios::in|std::ios::out);
std::iostream ioob(&fb);
#endif
#if defined(__APPLE__)
std::basic_filebuf<char> fb;
std::iostream ioob(fb.__open(sfd, std::ios::in|std::ios::out));
#endif
fmt::print("testing {}\n", foonm);
std::string s;
if (!std::getline(std::cin, s, '\n').good()) {
fmt::print("{}: getline failed\n", __func__);
return;
}
auto f = std::async(std::launch::async, foo, mfd);
ioob << s << '\n';
ioob.flush();
try {
std::string fget;
std::future_status fstatus;
fstatus = f.wait_for(8s);
switch (fstatus) {
case std::future_status::timeout:
fmt::print("{}: f.wait_for(8s) timed out\n", __func__);
break;
case std::future_status::ready:
fget = f.get();
fmt::print("{}: fstatus ready, f.get={}\n", __func__, fget);
break;
default:
fmt::print("{}: fstatus default??\n", __func__);
}
}
catch (std::exception &e) {
fmt::print("{}: exception \"{}\"\n", __func__, e.what());
}
}
int main(int argc, char *argv[])
{
testing("m1", m1);
testing("m0", m0);
return 0;
}
Any ideas?