Is it possible to write a libPOSIX for Windows (Win32) without requiring a background service or DLL that's always loaded?

Question

I know about Cygwin, and I know of its shortcomings. I also know about the slowness of fork, but not why on Earth it's not possible to work around that. I also know Cygwin requires a DLL. I also understand POSIX defines a whole environment (shell, etc...), that's not really what I care about here.

My question is asking if there is another way to tackle the problem. I see more and more of POSIX functionality being implemented by the MinGW projects, but there's no complete solution providing a full-blown (comparable to Linux/Mac/BSD implementation status) POSIX functionality.

The question really boils down to: Can the Win32 API (as of MSVC20??) be efficiently used to provide a complete POSIX layer over the Windows API?

Perhaps this will turn out to be a full libc that only taps into the OS library for low-level things like filesystem access, threads, and process control. But I don't know exactly what else POSIX consists of. I doubt a library can turn Win32 into a POSIX compliant entiity.

Answer 1

POSIX <> Win32.

If you're trying to write apps that target POSIX, why are you not using some variant of *N*X? If you prefer to run Windows, you can run Linux/BSD/whatever inside Hyper-V/VMWare/Parallels/VirtualBox on your PC/laptop/etc.

Windows used to have a POSIX compliant environment that ran alongside the Win32 subsystem, but was discontinued after NT4 due to lack of demand. Microsoft bought Interix and released Services For Unix (SFU). While it's still available for download, SFU 3.5 is now deprecated and no longer developed or supported.

As to why fork is so slow, you need to understand that fork isn't just "Create a new process", it's "create a new process (itself an expensive operation) which is a duplicate of the calling process along with all memory".

In *N*X, the forked process is mapped to the same memory pages as the parent (i.e. is pretty quick) and is only given new pages as and when the forked process tried to modify any shared pages. This is known as copy on write. This is largely achievable because in UNIX, there is no hard barrier between the parent and forked processes.

In NT, on the other hand, all processes are separated by a barrier enforced by CPU hardware. In NT, the easiest way to spawn a parallel activity which has access to your process' memory and resources, is to create a thread. Threads run within the memory space of the creating process and have access to all of the process' memory and resources.

You can also share data between processes via various forms of IPC, RPC, Named Pipes, mailslots, memory-mapped files but each technique has its own complexities, performance characteristics, etc. Read this for more details.

Because it tries to mimic UNIX, CygWin's 'fork' operation creates a new child process (in its own isolated memory space) and has to duplicate every page of memory in the parent process within the newly forked child. This can be a very costly operation.

Again, if you want to write POSIX code, do so in *N*X, not NT.

Answer 2

How about this

Most of the Unix API is implemented by the POSIX.DLL dynamically loaded (shared) library. Programs linked with POSIX.DLL run under the Win32 subsystem instead of the POSIX subsystem, so programs can freely intermix Unix and Win32 library calls.

From http://en.wikipedia.org/wiki/UWIN

The UWIN environment may be what you're looking for, but note that it is hosted at research.att.com, while UWIN is distributed under a liberal license it is not the GNU license. Also, as it is research for att, and only 2ndarily something that they are distributing for use, there are a lot of issues with documentation.

See more info see my write-up as the last answer for Regarding 'for' loop in KornShell

Hmm main UWIN link is bad link in that post, try

http://www2.research.att.com/sw/download/

Also, You can look at

• https://mailman.research.att.com/pipermail/uwin-users/ OR
• https://mailman.research.att.com/pipermail/uwin-developers/

To get a sense of the features vs issues.

I hope this helps.