不同长度的Fortran子串比较问题

Question

I am reading stdin in a loop and do some comparaison with a bunch of "if" to decide what to do according to the input.

Here is a shortened snippet of the code :

CHARACTER (len= :), allocatable :: input
CHARACTER (len=4096) :: inbuffer            ! frustrating but... well, fortran :3

DO

    ! get input
    READ(*, '(a)') inbuffer     ! because apparently you can't have allocation on read so you can't just read "input". meh.
    input = TRIM(inbuffer)
    CALL debug_log(input)

    IF(input .EQ. 'uci') THEN
        CALL debug_log("   printing uci info")

    !isready
    ELSE IF(input .EQ. 'isready') THEN
        CALL debug_log("   isready -> readyok")
        WRITE(*, '(a)') "readyok"

    !ucinewgame
    ELSE IF(input .EQ. 'ucinewgame') THEN
        CALL debug_log("not implemented : reset board and start a new game")

    !position
    ELSE IF(input(1:8) .EQ. 'position') THEN
        CALL debug_log("not implemented : set position")

    !quit -> exit main loop
    ELSE IF(input .EQ. 'quit') THEN
        CALL debug_log("   quit command issued, exiting main loop")
        EXIT

    !non uci command
        !nothing yet

    !unknown command
    ELSE
        CALL debug_log("   ignoring invalid command")
    END IF

end do

The input will expect command like "position 123 23415 etc..."

If I type "posi" it's say it's an invalid command as expected.

If I type "position" it say it's not implemented as expected too.

However :

• If I type "position": I get not implemented
• Followed by "posi": it says "not implemented" instead of "invalid command"

My guess is that it read 8 character even if the input is only 4 and since the previous command was "position" it make posi + tion = position

Here is some log to demonstrate:

** opening debug file : 20181111 / 223418.127
223418.127 : Initializing Fortiche engine
223418.129 : Entering main loop
223420.859 : posi
223420.859 :    ignoring invalid command
223426.467 : xxxxtion
223426.467 :    ignoring invalid command
223430.498 : posi
223430.498 : not implemented : set position
223437.323 : xxxxxxxxx
223437.323 :    ignoring invalid command
223439.418 : posi
223439.418 :    ignoring invalid command
223443.979 : position
223443.979 : not implemented : set position
223447.122 : quit
223447.122 :    quit command issued, exiting main loop
223447.122 : closing, bye

xxxxtion + posi = position

Which is clearly wrong but I can understand how it ended up like this.

Should I use something other than .EQ.? When I print the input it clearly doesn't print the input + whatever garbage was left behind in memory. But it's doing it when comparing string of possibly different length.

What can I do to solve this problem?

I'm not even started with the hardcore parsing and I already have a problem.

I'm using GNU Fortran on Windows.

Yes, it's UCI stuff as Universal Chess Interface.

EDIT : Full source code : https://github.com/ker2x/fortiche (comment the dirty hack at line 107 & 108 to reproduce the problem)

Answer 1

Substring references need to have starting and ending positions that are within the limits of the string.

You don't defend against a string that has a length less than eight prior to the substring reference input(1:8) .eq. 'position' input(1:8) .eq. 'position' .

With input shorter than eight characters, your program is non-conforming, anything can then happen, where anything very reasonably includes the behaviour you see.

Runtime debugging options may help to catch this programming error, depending on the capabilities of your compiler.