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nbase_winunix.c

/***************************************************************************
 * nbase_winunix.h -- Background code that allows checking for input on    *
 * stdin on Windows without blocking.                                      *
 *                                                                         *
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 ***************************************************************************/

/* $Id$ */

#include <assert.h>

#include "nbase.h"

#include "nbase_winunix.h"

/*
This code makes it possible to check for input on stdin on Windows without
blocking. There are two obstacles that need to be overcome. The first is that
select on Windows works for sockets only, not stdin. The other is that the
Windows command shell doesn't echo typed characters to the screen unless the
program is actively reading from stdin (which would normally mean blocking).

The strategy is to create a background thread that constantly reads from stdin.
The thread blocks while reading, which lets characters be echoed. The thread
writes each block of data into an anonymous pipe. We juggle file descriptors and
Windows file handles to make the rest of the program think that the other end of
the pipe is stdin. Only the thread keeps a reference to the real stdin. Windows
has a PeekNamedPipe function that we use to check for input in the pipe without
blocking.

Call win_stdin_start_thread to start the thread and win_stdin_ready for the
non-blocking input check. Any other operations on stdin (read, scanf, etc.)
should be transparent, except I noticed that eof(0) returns 1 when there is
nothing in the pipe, but will return 0 again if more is written to the pipe. Any
data buffered but not delivered to the program before starting the background
thread may be lost when the thread is started.
*/

/* The background thread that reads and buffers the true stdin. */
static HANDLE stdin_thread = NULL;

/* This is a copy of the true stdin file handle before any redirection. It is
   read by the thread. */
static HANDLE thread_stdin_handle = NULL;
/* The thread writes to this pipe and standard input is reassigned to be the
   read end of it. */
static HANDLE stdin_pipe_r = NULL, stdin_pipe_w = NULL;

/* This is the thread that reads from the true stdin (thread_stdin_handle) and
   writes to stdin_pipe_w, which is reassigned to be the stdin that the rest of
   the program sees. Once started, it never finishes except in case of error.
   win_stdin_start_thread is responsible for setting up thread_stdin_handle. */
static DWORD WINAPI win_stdin_thread_func(void *data) {
    DWORD n, nwritten;
    char buffer[BUFSIZ];

    for (;;) {
        if (ReadFile(thread_stdin_handle, buffer, sizeof(buffer), &n, NULL) == 0)
            break;
        if (n == -1 || n == 0)
            break;

        if (WriteFile(stdin_pipe_w, buffer, n, &nwritten, NULL) == 0)
            break;
        if (nwritten != n)
            break;
    }
    CloseHandle(thread_stdin_handle);
    CloseHandle(stdin_pipe_w);

    return 0;
}

/* Get the newline translation mode (_O_TEXT or _O_BINARY) of a file
   descriptor. _O_TEXT does CRLF-LF translation and _O_BINARY does none.
   Complementary to _setmode. */
static int _getmode(int fd)
{
    int mode;

    /* There is no standard _getmode function, but _setmode returns the
       previous value. Set it to a dummy value and set it back. */
    mode = _setmode(fd, _O_BINARY);
    _setmode(fd, mode);

    return mode;
}

/* Start the reader thread and do all the file handle/descriptor redirection.
   Returns nonzero on success, zero on error. */
int win_stdin_start_thread(void) {
    int stdin_fd;
    int stdin_fmode;

    assert(stdin_thread == NULL);
    assert(stdin_pipe_r == NULL);
    assert(stdin_pipe_w == NULL);
    assert(thread_stdin_handle == NULL);

    /* Create the pipe that win_stdin_thread_func writes to. We reassign the
       read end to be the new stdin that the rest of the program sees. */
    if (CreatePipe(&stdin_pipe_r, &stdin_pipe_w, NULL, 0) == 0)
        return 0;

    /* Make a copy of the stdin handle to be used by win_stdin_thread_func.  It
       will remain a reference to the the true stdin after we fake stdin to read
       from the pipe instead. */
    if (DuplicateHandle(GetCurrentProcess(), GetStdHandle(STD_INPUT_HANDLE),
                        GetCurrentProcess(), &thread_stdin_handle,
                        0, FALSE, DUPLICATE_SAME_ACCESS) == 0) {
        CloseHandle(stdin_pipe_r);
        CloseHandle(stdin_pipe_w);
        return 0;
    }

    /* Set the stdin handle to read from the pipe. */
    if (SetStdHandle(STD_INPUT_HANDLE, stdin_pipe_r) == 0) {
        CloseHandle(stdin_pipe_r);
        CloseHandle(stdin_pipe_w);
        CloseHandle(thread_stdin_handle);
        return 0;
    }
    /* Need to redirect file descriptor 0 also. _open_osfhandle makes a new file
       descriptor from an existing handle. */
    /* Remember the newline translation mode (_O_TEXT or _O_BINARY), and
       restore it in the new file descriptor. */
    stdin_fmode = _getmode(STDIN_FILENO);
    stdin_fd = _open_osfhandle((intptr_t) GetStdHandle(STD_INPUT_HANDLE), _O_RDONLY | stdin_fmode);
    if (stdin_fd == -1) {
        CloseHandle(stdin_pipe_r);
        CloseHandle(stdin_pipe_w);
        CloseHandle(thread_stdin_handle);
        return 0;
    }
    dup2(stdin_fd, STDIN_FILENO);

    /* Finally, start up the thread. We don't bother keeping a reference to it
       because it runs until program termination. From here on out all reads
       from the stdin handle or file descriptor 0 will be reading from the
       anonymous pipe that is fed by the thread. */
    stdin_thread = CreateThread(NULL, 0, win_stdin_thread_func, NULL, 0, NULL);
    if (stdin_thread == NULL) {
        CloseHandle(stdin_pipe_r);
        CloseHandle(stdin_pipe_w);
        CloseHandle(thread_stdin_handle);
        return 0;
    }

    return 1;
}

/* Check if input is available on stdin, once all the above has taken place. */
int win_stdin_ready(void) {
    DWORD n;

    assert(stdin_pipe_r != NULL);

    PeekNamedPipe(stdin_pipe_r, NULL, 0, NULL, &n, NULL);

    return n > 0;
}

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