This fails on OpenBSD 5.2

ptycheck.c

/**
 * ptycheck.c - Demonstrate possible problem in OpenBSD's ptcwrite
 *
 * Opens a pseudoterminal pair in "raw" mode, then forks off a child
 * process which attempts to write MESSAGE_LEN random bytes to the pty
 * master; meanwhile the parent process tries to read the same number of
 * bytes from the slave.  Once the parent has received the expected number
 * of bytes, it compares the message received against the original to
 * ensure that all data shuffled across the pseudoterminal successfully.
 *
 * On OpenBSD 5.2 i386 and amd64, this fails with the parent process
 * blocking indefinitely after reading only 4088 bytes (out of 4096
 * expected) from the pty slave.  The same behavior is observed on 4.7 and
 * 4.8 i386, and can be seen when MESSAGE_LEN is 1024 or larger.
 *
 * Compile with `gcc -o ptycheck ptycheck.c -lutil`.
 *
 * Mark Shroyer
 * Tue Dec 25 20:04:48 EST 2012
 */

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <termios.h>
#include <unistd.h>
#include <util.h>

/* Number of pseudorandom bytes to try sending */
#define MESSAGE_LEN 4096

int
main(int argc, char *argv)
{
        int fd, sfd, pid;
        char *send_buf, *recv_buf;
        ssize_t n;
        size_t i;
        struct termios t;

        if ((send_buf = malloc(MESSAGE_LEN)) == NULL)
                return 1;

        if ((recv_buf = malloc(MESSAGE_LEN)) == NULL)
                return 1;

        for (i = 0; i < MESSAGE_LEN; i++)
                send_buf[i] = random() % 256;

        if (openpty(&fd, &sfd, NULL, NULL, NULL) < 0) {
                perror("Failed to open PTY pair");
                return 1;
        }

        if (tcgetattr(sfd, &t) < 0)
                return 1;
        cfmakeraw(&t);
        if (tcsetattr(sfd, TCSAFLUSH, &t) < 0)
                return 1;

        i = 0;
        switch (pid = fork()) {
        case -1:
                perror("Unable to fork child process");
                return 1;

        case 0:
                /* Child process writes to pty master */

                while (i < MESSAGE_LEN) {
                        if ((n = write(fd, send_buf + i, MESSAGE_LEN - i)) < 0) {
                                perror("Couldn't write to pty master");
                                return 1;
                        }
                        i += n;
                        printf("-> master: %zd bytes\n", i);
                }
                printf("Done writing.\n");
                return 0;

        default:
                /* Parent process reads from pty slave */

                while (i < MESSAGE_LEN) {
                        if ((n = read(sfd, recv_buf + i, MESSAGE_LEN - i)) < 0) {
                                perror("Couldn't read from pty slave");
                                return 1;
                        }
                        i += n;
                        printf("<- slave: %zd bytes\n", i);
                }
                printf("Done reading.\n");
                if (memcmp(send_buf, recv_buf, MESSAGE_LEN) != 0) {
                        printf("Message altered in transit!\n");
                        return 1;
                }
                printf("Message transferred successfully.\n");
                waitpid(pid, NULL, 0);
                return 0;
        }
}