I am looking for a GUI library with some very strict requirements:

- It must run on at least Linux and Windows
- It must be possible to use in C
- It must not interrupt execution of the program
EDIT: - It should be fairly simple to incorporate into an existing program

The third requirement means that the program can continue to handle network traffic whilst asking the user to enter some information.

Allegro's isn't simple for this task since it's not an allegro-based program.
It is currently a text-based tunneling proxy, but I wanted to put a GUI on the client to make it look nicer.

EDIT:
But I would be willing to use it if it's the only option

EDIT2:
I forgot to mention fairly simple as a requirement.

Qt is C++, so I wouldn't consider using it (although I like how it looks). I'll see the GTK docs some time to see if I'd get on with it. Thanks.

wxWidgets.

this is commonly known as threading

I recommend GTK but wxWidgets should fit the bill. I haven't found a wxWidgets bin for windows though. It is not too hard to compile.

GTK is horrid looking. Compiling wx doesn't take too long, it took probably 3 hours on my old 366mhz box. What's wrong with C++?

I think that GTK with the Ubuntu theme looks quite nice.

Didn't you hear what Linus Torvalds said?

But seriously, if you're hellbent on using C, GTK is really your main option.

Let's just leave it at "he wants to use C" and not start yet another C vs. C++ thread...

Well, in that case, you can use the "C++ way" for everything Qt/wx provides, but do everything else the C way (with extra nitpicks provided to you courtesy of C++!) to just stick to the man!

I guess I don't get the whole "I want to use C because the C++ way of doing things is annoying" when "The C++ way of doing things is (almost; there are a handful of corner cases) a strict superset of the C way of doing things."

Seriously, there is no reason why you cannot use malloc/free, no exceptions, only functions, and use the wonky (type_to_cast_to_here) syntax.

Suit yourself. (I'm not trying to be condescending or anything, I'm just saying, in most cases, you can compile C code as C++ code just fine with minimal "fixup" work (or so my experiences seem to indicate)).

Actually, it does things the Qt way. It provides its own sane platform to work from, so you don't even have to touch the stl or stdlibc++.

The only reason I decided to work with C++ more often was because Qt is C++, and has un-stupid classes (like a String class that's actually usefull), and a kick ass GUI component.

Qt 4 is much more than just GUI, it has several modules, including the following:

QtCore      Core non-GUI classes used by other modules
QtGui       Graphical user interface components
QtNetwork   Classes for network programming
QtOpenGL    OpenGL support classes
QtSql       Classes for database integration using SQL
QtScript    Classes for evaluating Qt Scripts
QtSvg       Classes for displaying the contents of SVG files
QtXml       Classes for handling XML
QtDesigner  Classes for extending Qt Designer
QtUiTools   Classes for handling Qt Designer forms in applications
QtAssistant Support for online help
Qt3Support  Qt 3 compatibility classes
QtTest      Tool classes for unit testing

Its quite the inclusive platform. It also supports Windows, X, and MacOS. As well as most Qt code can work under QTopia (mobile device version, ie phones and tablets) possibly with some changes.

Qt4 is like the CPAN for C++

Qt4 actually makes C++ feel a little like Perl for me, in the RAD department that is.