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by justin_ 536 days ago
I believe it is a bug in the the emulator's implementation of COMMAND.COM. Often, these DOS "emulators" re-implement the standard commands of DOS, including the shell[1]. This is in addition to emulating weird 16-bit environment stuff and the BIOS.

The bug can pop up in any C program using stdio that assumes it's fine to do `fread` followed immediately by `fwrite`. The spec forbids this. To make matters more confusing, this behavior does _not_ seem to be in modern libc implementations. Or at least, it works on my machine. I bet modern implementations are able to be more sane about managing different buffers for reading and writing.

The original COMMAND.COM from MS-DOS probably did not have this problem, since at least in some versions it was written in assembly[2]. Even for a shell written in C, the fix is pretty easy: seek the file before switching between reading/writing.

The title of this post is confusing, since it clearly _is_ a bug somewhere. But I think the author was excited about possibly finding a bug in libc:

> Sitting down with a debugger, I could just see how the C run-time library (Open Watcom) could be fixed to avoid this problem.

[1] Here's DOSBox, for example: https://github.com/dosbox-staging/dosbox-staging/blob/main/s...

[2] MS-DOS 4.0: https://github.com/microsoft/MS-DOS/tree/main/v4.0/src/CMD/C...
1 comments
rep_lodsb 536 days ago
The article is very vague about which emulator and COMMAND.COM it is about, and if they're integrated with each other. Can't be DOSBox, since it handles it correctly:

    C:\> echo AB> foo.txt
    C:\> echo CD>> foo.txt
    C:\> type foo.txt
    AB
    CD
(Note that echo adds a newline, same as on real DOS, or even UNIX without "-n". This other shell doesn't for some reason.)

The "real" COMMAND.COM, and all other essential parts of MS-/PC-/DR-DOS, have always been written in asm, where none of this libc nonsense matters.

Also it annoys me greatly when people talk about "the C Library" as if it exists in some Platonic realm, and is essential to all software ever written.

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