Non blocking pseudo terminal, recovery after POLLHUP

Question

I create a new pseudo-terminal by opening /dev/ptmx with open() function and O_RDWR | O_NOCTTY | O_NONBLOCK flags. Then I use poll() function to wait for incoming data from the remote end:

struct pollfd pollFileDescriptors[numberOfTerminals];
for (unsigned terminalIndex = 0; terminalIndex < numberOfTerminals; terminalIndex++) {
    pollFileDescriptors[terminalIndex].fd = terminals[terminalIndex].getFileDescriptor();
    pollFileDescriptors[terminalIndex].events = POLLIN | POLLPRI | POLLRDNORM | POLLRDBAND;
}

int ready = poll(pollFileDescriptors, terminals.getNumberOfTerminals(), timeoutMSec);

Everything works like a dream until the remote end closes connection. In such a case the poll() function returns all the time with POLLHUP revents flag. This is by design, however what could I do to make it operating as before i.e. waiting for another process to open and use the pseudo-terminal. I mean it waits, but returns immediately with POLLHUP set. On the other hand, if I close the file descriptor I have no guarantee of receiving the same pseudo-terminal-id that after reopening the /dev/ptmx. Is there any way to remove the POLLHUP revents flag?

I found a similar question: Poll() on Named Pipe returns with POLLHUP constantly and immediately , however I am using already O_RDWR as described there but it doesn't help as in case of named pipes.

score 1 · Accepted Answer · answered Mar 07 '21 at 20:16

The issue can be easily solved by reopening the pseudo-terminal right after it is created. The POLLHUP won't appear as long as at least one writer exists so we can do this by ourselves with open() and ptsname():

// Create a new pseudo terminal
int fileDescriptor = open("/dev/ptmx", O_RDWR | O_NOCTTY | O_NONBLOCK);
grantpt(fileDescriptor);
unlockpt(fileDescriptor);

// Reopen it for write
const char *targetPath = ptsname(fileDescriptor);
int dummyWriterFileDescriptor = open(fileName.c_str(), O_WRONLY | O_NOCTTY | O_NONBLOCK);

Non blocking pseudo terminal, recovery after POLLHUP

1 Answers1