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...
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...