poll on FIFO returns immediately in HP-UX

Question

I'm trying to use poll(2) on FIFO on various platforms, but (compared to Linux, Solaris, AIX), HP-UX's behavior is somewhat different to other platforms.

Below code creates a FIFO(named pipe) and poll it with 3s timeout. Since this code doesn't write to the FIFO, I expect poll result would be 0.

#include <fcntl.h>
#include <poll.h>
#include <signal.h>
#include <sys/stat.h>
#include <sys/types.h>

#include <unistd.h>
#include <errno.h>
#include <inttypes.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

#if defined(CLOCK_MONOTONIC)
#define MY_CLOCK_TIME CLOCK_MONOTONIC
#elif defined(CLOCK_REALTIME)
#define MY_CLOCK_TIME CLOCK_REALTIME
#else
#error neither CLOCK_REALTIME nor CLOCK_MONOTONIC defined.
#endif

#define HANDLE_ERROR(expr, msg) \
    do                          \
    {                           \
        if ((expr) < 0)         \
        {                       \
            perror(msg);        \
            exit(EXIT_FAILURE); \
        }                       \
    } while (0);

namespace {
const char pipename[] = "pipe";

template <size_t N>
struct type_of_size
{
    typedef char type[N];
};

template <typename T, size_t Size>
typename type_of_size<Size>::type& sizeof_array_helper(T (&)[Size]);

#define SIZE_OF_ARR(pArray) sizeof(sizeof_array_helper(pArray))

int my_unlink(const char* name)
{
    int result = unlink(name);
    if (result < 0)
    {
        if (errno == ENOENT)
        {
            result = 0;
            errno = 0;
        }
    }
    return result;
}

double timediff(const struct timespec& t1, const struct timespec& t2)
{
    return static_cast<double>(static_cast<int64_t>(
               (t1.tv_sec - t2.tv_sec) * 1000000000LL + (t1.tv_nsec - t2.tv_nsec))) /
           1000000000LL;
}
} // namespace

int main()
{
    HANDLE_ERROR(my_unlink(pipename), "unlink");
    HANDLE_ERROR(mkfifo(pipename, 0600), "mkfifo");

    int result = 0;
    int poll_timeout = 3000; // 3s
    struct timespec before;
    memset(&before, 0, sizeof(struct timespec));
    struct timespec after;
    memset(&after, 0, sizeof(struct timespec));

    int fd = open(pipename, O_RDONLY | O_NONBLOCK);
    HANDLE_ERROR((fd < 0), "open");
    pollfd fds[1];
    memset(fds, 0, sizeof(pollfd) * SIZE_OF_ARR(fds));
    fds[0].fd = fd;
    fds[0].events = POLLIN;

    HANDLE_ERROR(clock_gettime(MY_CLOCK_TIME, &before), "clock_gettime");

#ifdef USE_FAKE_OPEN
    int fakeopen_fd = open(pipename, O_WRONLY | O_NONBLOCK);
    HANDLE_ERROR((fakeopen_fd < 0), "open");
#endif

    result = poll(fds, SIZE_OF_ARR(fds), poll_timeout);
    memset(&after, 0, sizeof(struct timespec));
    HANDLE_ERROR(clock_gettime(MY_CLOCK_TIME, &after), "clock_gettime");
    double interval = timediff(after, before);
    printf("call interval: %.3f, poll result: %d, errr: %s\n", interval, result, strerror(errno));

#ifdef USE_FAKE_OPEN
    close(fakeopen_fd);
#endif

    close(fd);
    return (interval > 2.0) ? EXIT_SUCCESS : EXIT_FAILURE;
}

On Linux, Solaris 10, AIX 7.1, it works as expected(call interval: 3.000, poll result: 0, errr: Error 0), but HP-UX(11.31) doesn't obey timeout parameter and returns immediately(call interval: 0.000, poll result: 1, errr: Error 0).

If I open FIFO for writing before polling for reading it, (define USE_FAKE_OPEN) it works as expected even for HP-UX.

1. What makes this behavior?

2. Is HP-UX's poll POSIX Compliant?

Answer 1

Okay, there is a rule of programming that "It's Always Your Fault (Select Isn't Broken)", but HP says their poll on FIFO is broken in their documentation for HP-UX patch PHKL_41419:

poll(2) sets POLLIN (data ready for reading)event even if there is no writer at the other end when a FIFO is opened with O_RDONLY in non blocking mode(O_NONBLOCK set).

Case closed.