Key Capture

For historical reasons, I/O operations in Fortran are record-based (even if some limited stream-based I/O is possible in modern language versions). Therefore, no intrinsic Fortran routine exists to capture single key-stroke events. The best way seems to be to call external C routines, which works on most Unix-like operating systems. For non-blocking keyboard input, one may interface the C procedures listed on Rosetta Code.

stty

On Unix, the command-line tool stty(1) is used to configure the terminal device interface. The cbreak mode lets us read single characters from terminal. We could either write interfaces to ioctl(2) to modify the device parameters, or, just execute stty through the Fortran intrinsic routine execute_command_line().

For both cases, the C function getchar(3) in libc returns the next input character from the input stream. We simply define an ISO C binding interface to the C function in order to read a single character in Fortran.

! getch.f90
program main
    use, intrinsic :: iso_c_binding, only: c_int
    implicit none

    interface
        ! int getchar(void)
        function c_getchar() bind(c, name='getchar')
            import :: c_int
            implicit none
            integer(kind=c_int) :: c_getchar
        end function c_getchar
    end interface

    integer :: ch

    ! Enable cbreak mode.
    call execute_command_line('stty -echo cbreak </dev/tty >/dev/tty 2>&1')

    print '("Press <q> to quit.")'

    do
        ch = c_getchar()
        print '("Key pressed: ", i0)', ch
        if (ch == ichar('q')) exit
    end do

    ! Disable cbreak mode.
    call execute_command_line('stty echo -cbreak </dev/tty >/dev/tty 2>&1')
end program main

Compile and run the example program with:

$ gfortran12 -o getch getch.f90
$ ./getch
Press <q> to quit.
Key pressed: 70

termios

The termios(4) API lets us control terminal I/O on Unix. We have to define a C routine setmode() to disable echo and line editing mode, and another C routine nextchar() to read a single input character:

/* term.c */
#include <stdio.h>
#include <termios.h>

void setmode(int *mode)
{
    static struct termios termattr, saveattr;

    if (*mode != 0)
    {
        tcgetattr(0, &termattr);

        saveattr = termattr;
        termattr.c_lflag &= ~(ICANON | ECHO);
        termattr.c_cc[VMIN] = 1;
        termattr.c_cc[VTIME] = 0;

        tcsetattr(0, TCSADRAIN, &termattr);
    }
    else
    {
        tcsetattr(0, TCSADRAIN, &saveattr);
    }
}

void nextchar(int *nextch)
{
    *nextch = getchar();
}

In order to access these routines from Fortran, ISO C binding interfaces have to be implemented. The following example prints the code of each pressed key.

! key.f90
program main
    use, intrinsic :: iso_c_binding
    implicit none

    interface
        subroutine c_setmode(mode) bind(c, name='setmode')
            import :: c_int
            implicit none
            integer(kind=c_int) :: mode
        end subroutine c_set_mode

        subroutine c_nextchar(nextch) bind(c, name='nextchar')
            import :: c_int
            implicit none
            integer(kind=c_int) :: nextch
        end subroutine c_next_char
    end interface

    integer :: key

    ! Enable single key capture.
    call c_setmode(1)

    print '("Press <q> to quit.")'

    do
        ! Read single character.
        call c_nextchar(key)
        print '("Key pressed: ", i0)', key
        if (key == ichar('q')) exit
    end do

    ! Revert to default.
    call c_setmode(0)
end program main

Compile the example with GCC:

$ gcc12 -c term.c
$ gfortran12 -o key key.f90 term.o

Or, instead, using Clang/Flang:

$ clang -c term.c
$ flang -o key key.f90 term.o

The code of each pressed key will be printed to screen until the user hits q:

$ ./key
Press <q> to quit.
Key pressed: 70

References

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