BSD Sockets Compatibility

by Warren Young

So you say you're a long-time Unix hacker who's new to Windows programming? And you've heard of this great API called Winsock that's compatibile with your beloved BSD sockets, but try as you might, you just can't find the readv() call? Well bunky, this is the article for you.

Introduction

In the beginning, there was chaos in the world of Windows TCP/IP APIs. A program written for, say, FTP Software's TCP/IP stack wouldn't run on JSB's stack.

Then, sometime in 1990, a bunch of people got together and decided to make one nice, big, compatible API called Windows Sockets that would allow a single program to run on any vendor's stack. They decided to base this API on the popular BSD sockets model of network programming, but for various reasons, there are still many differences between Winsock and BSD sockets. This article points out how Winsock differs from BSD sockets, and how to translate BSD sockets programs to use similar Winsock functionality.

The Official Word

The Winsock API documentation has a section called Porting Socket Applications to Winsock that covers many of the same issues that this article does, and a few others besides.

#include Differences

Under BSD sockets, there are quite a few different header files you need to include, depending on what sockets calls you use. A typical BSD sockets program has a block of #includes near the top like this:

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>

For Winsock, you don't need any of these. Instead, you just need to include winsock.h. (Or, if you need Winsock 2-specific functionality, winsock2.h.)

errno vs. WSAGetLastError()

WSAGetLastError() is essentially the same thing as Unix's errno global variable. The error constants and their values are different; there's a table in the Winsock spec where it lists all the error constants, one column of which shows the equivalent BSD error constant for a given Winsock error constant. Usually the difference is just the addition of "WSA" to the beginning of the constant name for the Winsock versions. (E.g. WSAEINTR is the Winsock version of BSD's EINTR error constant.)

Another thing to keep in mind is that, although the perror() call exists in most Windows compilers' run-time libraries, it doesn't work for Winsock calls. (This is a consequence of Winsock not returning its error codes in the errno variable.) There is a function called WSAGetLastErrorMessage() in the basic Winsock examples area of the FAQ that you can use to build a perror()-like function. It's in the ws-util.cpp module.

EAGAIN

Many Unix programs, especially those with System V roots, check for the EAGAIN value in the global errno variable when a non-blocking call fails. This is the same thing as BSD's EWOULDBLOCK and Winsock's WSAEWOULDBLOCK errors. You'll have to check your system's header files, but all Unixes I've checked on this matter #define EAGAIN and EWOULDBLOCK to the same value, so you may want to get into the habit of using EWOULDBLOCK instead of EAGAIN under Unix, to make transitions to and from Winsock easier.

Equivalence of File and Socket Handles

Under Unix, the I/O system calls work with file descriptors and socket descriptors equally well. It's common in BSD sockets programs to use read() instead of recv() to read data from a socket, for example. Under Windows 3.1 and Windows 95 derivatives, socket descriptors are completely distinct from file descriptors. Although the run-time library (RTL) supplied with most Windows compilers includes POSIX emulation functions, these are designed to work only with files, not sockets.

Windows NT derivatives are more like Unix, in that its native file I/O functions also work with sockets: ReadFile() is roughly equivalent to Winsock's recv() function, for example.

The Visual C++ RTL emulates POSIX functions, except that they're named with a leading underscore: for example, _read() instead of read(). The _read() function uses ReadFile() internally, so you'd think it would work with sockets. The problem is, the first argument is an RTL-specific handle, not an operating system file handle. If you pass a socket handle to _read() or _write(), the RTL will realize that it isn't an RTL handle and the call will fail.

Fortunately, there is a bridge function in Visual C++'s RTL: _open_osfhandle(). (If you're not using Visual C++, you'll have to check its RTL source for a similar function.) I've not tried it, but it appears to take an operating system file handle (including socket handles) and return a handle you can use with the POSIX emulation functions in the RTL. I'm told that this will work with sanely-coded non-Microsoft Winsock stacks, but since I haven't tried it, you should if you want to support these alternate stacks.

Again, the _open_osfhandle() workaround only works on Windows NT derivatives, because the file I/O functions don't work with sockets on the Windows 95 derivatives, so fooling the RTL into accepting one doesn't help you.

If these limitations are too much for your program, you may want to give one of the Unix emulation systems a try. I've personally used Cygwin for this purpose, and it works beautifully. Others include Microsoft's Services for Unix and AT&T's UWIN, but I've not used either of them.

All that aside, it's usually much easier to rewrite your program to use portable functions like recv() than it is to arm-twist the Windows port to work with Unix idioms.

Winsock's closesocket() vs. Unix's close()

Winsock defines a different function for closing sockets because not all versions of Windows have file descriptor and socket descriptor equivalency like Unix. See the discussion in the previous item for more on the file/socket handle mismatch issue.

Winsock's ioctlsocket() vs. Unix's ioctl()

Unix provides the ioctl() call to allow you to set and get various bits of info on a file descriptor, which includes socket descriptors. Winsock replicates some common Unix ioctls in the ioctlsocket() call, but much is missing.

If you use the SIOCGIFCONF ioctl on Unix to get information about the system's network interfaces, Winsock 2 provides very similar functionality with its SIO_GET_INTERFACE_LIST option for ioctlsocket().

fcntl()

The Unix fcntl() call has no direct equivalent under Winsock. Where necessary, similar functionality exists in Winsock's ioctlsocket() call. For example, the equivalent of using Unix's fcntl() to set a socket's O_NONBLOCK flag is setting the FIONBIO flag with Winsock's ioctlsocket().

poll()

There are several wrappers for poll() using select() out there. Here's one. It doesn't attempt to implement any of the special poll() features found in a true System V system, such as STREAMS support. Also, the code is rather old, written in a K&R C style that some newer compilers might reject. Finally, since it is built directly on top of select(), it has the same limitations.

Another option is to dig the implementation of poll() out of Jarle Aasa's Win32 port of the adns library. This implementation has three limitations: 1) It's GPL'd, which means you can't use the code in your program unless your program is also licensed under the GPL; 2) it's built on the Win32 event object mechanism, which has a hard 64-object limitation; and 3) it is reportedly not written in a way that is easy for third-party programmers to extract and use.

readv() and writev()

Winsock 2's overlapped I/O mechanism includes scatter/gather functionality similar to that provided by readv() and writev().

dup()

The Unix dup() function duplicates a file handle, and of course also works for sockets. Under Winsock 2, you can do the same thing with WSADuplicateSocket(). It's a bit more involved, but the WSADuplicateSocket() documentation in MSDN has a good step-by-step example showing how to use this mechansim.

dup2()

There is partial support for this feature under Winsock, though the mechanism is dissimilar to the dup2() feature. Under Unix, dup2() takes a handle and duplicates it like dup() does, but with a twist: it assigns the new filehandle a value that you specify. This is usually used to map a socket to the C language's stdin or stdout file descriptors so that you can use standard I/O functions like printf() and fgets() with the socket.

Item Q190351 in the Microsoft Knowledge Base documents a method by which you can redirect a child process's standard descriptors to a socket. The limitations are that you cannot do this to your own process's descriptors, you cannot redirect arbitrary descriptors to a socket (i.e. you can only do it with stdin, stdout and stderr), and not all processes are fully compatible with this API feature. Still, it at least makes an inetd-like program possible under Win32.

Detecting a Dropped Connection

Under BSD Unixes, if the remote peer closes its connection and your program is blocking on recv(), you will get a 0 back from recv(). Winsock behaves the same way, except that it can also return -1, with WSAGetLastError() returning WSAECONNRESET, WSAECONNABORTED or WSAESHUTDOWN, to signal the detectable flavors of abnormal disconnections.

Under Unix, if you're blocking on send() and your program is ignoring the SIGPIPE signal, it will return with a -1 when the remote peer disconnects, and errno will be EPIPE. Otherwise, your program will be sent the SIGPIPE signal, which will terminate your program if you don't handle it. Under Winsock, the SIGPIPE/EPIPE functionality does not exist at all: send() will either return 0 for a normal disconnect or -1 for an abnormal disconnect, with WSAGetLastError() returning the same errors as in the recv() discussion above.

UDP Behavior

According to Ilpo Ruotsalainen, "...most BSD socket implementations do not pass delayed UDP errors (ICMP port unreachable at least, maybe others too) to recvfrom() while Winsock 2 [under Windows 2000 but not Windows 98] does. Linux [behaves like Windows 2000] too, but provives SO_BSDCOMPAT setsockopt() for being compatible with the BSD style."

In other words, a portable program has to be prepared for the possibility of error codes for non-immediate problems from recvfrom(), but it can't depend on receiving them.

A Cry For Updates

This article can always use more work. If you can help me out, please drop me a line! In particular, I'll bet that there are several other BSD-only ioctl() options that we can duplicate in other ways with Win32 facilities.

Copyright © 1998-2005 by Warren Young. All rights reserved.