The article lists three libcs (Open Watcom, Microsoft Visual C++ 6.0, IBM C/C++ 3.6 for Windows) from the good old times. Does the emulator link to Open Watcom, i.e., does it emulate DOS on machines about as old as DOS itself? What's the point here?
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...
C:\> echo AB> foo.txt
C:\> echo CD>> foo.txt
C:\> type foo.txt
AB
CD
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.