Our company helps migrate client software from other languages to C++. We provide them C++ source code for their application along with header files and compiled libraries for runtime support functions. We charge for both the migration as well as the runtime. Recently a potential client asked to migrate one of a number of systems they have. This system contains 7 programs and we would like to limit the runtime so only these 7 programs can acess it. We can time limit the runtime by putting an encrypted expiration date in the object library but, since we have to provide the source code for the converted programs, we are having difficult coming up with a way to limit the access to a specific set of programs. Obviously, anything we put into the source code to identify the program could be copied to any other program so the only hope seems to be having the run time library discover some set of characteristics about the programs and then validating them against a set of characteristics embedded in the run time library. As I understand it, C++ has very little reflection capability (RTTI is all I could find) so I wanted to ask if anyone has faced a similar problem and found a way to solve it. Thanks in advance for any suggestions.
Based on the two answers a little clarification seems in order. We fully expect the client to modify the source code and normally we provide them an unrestricted version of the runtime libraries. This particular client requested a version that was limited to a single system and is happy to enter into a license that restricts the use of the runtime library to that system. Therefore a discussion of the legal issues isn't relevant. The issue is a technical one -- given a license that is limited to a single system and given that the client has the source to the calling programs but not the runtime, is there a way to limit access to the runtime to the set of programs comprising that system thus enforcing the terms of the license.
If they're not supposed to make further changes to the programs, why did you give them the source code? And if they are expected to continue changing the programs (i.e. maintenance), who decides whether a change constitutes a new program that's not allowed to use the library?
There's no technical way to enforce that licensing model.
There's possibly a legal way -- in the code that loads/enables the library, write a comment "This is a copy protection measure". Then DMCA forbids them from including that code into other programs (in the USA). But IANAL, and I don't think DMCA is valid anyway.
Consult a lawyer to find out what rights you have under the contract/bill of sale to restrict their use.
The most obvious answer I could think of is to get the name and/or path of the calling process-- simply compare this name to the 7 "allowed" programs in your support library. Certainly, they could create a new process with the same name, but they might not know to do so.
Another level could be to further compare the executable size against the known size for that application. (You'll likely want to allow a reasonably wide range around the expected size, in case they make changes to the source code, and/or compile with different options.)
As another thought, you might try adding some seemingly benign strings into the app's resources. ("Copyright 2011 ~Your Corporation Name~")-- You can then scan the parent executable for the magic strings. If they create a new product, they might not think to create this resource.
Finally, as already noted by Ben, if you are giving them the source code, there are likely no foolproof solutions to this problem. (As he said, at what point does "modified" code become a new application?) The best you will likely be able to do is to add enough small roadblocks that they won't bother trying to use that lib for another product. It likely depends on how determined and/or lucky they are.
Why not just technically limit the use of the runtime to one system? There are many software protection solutions out there, one that comes to my mind is SmartDongle.
Now the runtime could still be used by any other program on that machine, but I think this should be a minor concern, no?
Related
Is there any way in C++ or Java or Python that would allow me to save the state of my program, no questions asked? For example, I've spent an hour learning how to save a tree-like structure into a file. Very educative but I feel I could just do:
saveState(file);
And the "file" would contain whole memory my program uses. Just like operating system's "hibernate" or "suspend-to-disk" feature. I know about boost serialization, this is probably not what I'm looking for.
What you most likely want is what we call serialization or object marshalling. There are a whole butt load of academic problems with data/object serialization that you can easily google.
That being said given the right library (probably very native) you could do a true snapshot of your running program similarly what "OS specific hibernate" does. Here is an SO answer for doing that on Linux: https://stackoverflow.com/a/12190830/318174
To do the above snapshot-ing though you will most likely need an external process from the process you want to save. I highly recommend you don't that. Instead read/lookup in your language of choice (btw welcome to SO, don't tag every language... that pisses people off) how to do serialization or object marshalling... hint... most people these days pick JSON.
I think that what you describe would be a feature that few people would actually want to use for a real system. Usually you want to save something so it can be transmitted, or so you can stop running the program, or guard against the possibility that the program quits (or power fails).
In most production systems one wants to make the writes to disk small and incremental so that the system can remain responsive, and writing inconsistent data can be avoided. Writing ALL memory to disk on a regular basis would probably result in lots of non-responsive time. You would need to lock the entire system to avoid inconsistent state.
Writing your own persistence is tedious and error prone however so you may find this SO question of interest: Persisting graph data (Java)
There are a couple of frameworks around this. Check out Google Protocol Buffers if you need support for Java, Python, and C++ https://developers.google.com/protocol-buffers/ I've used it in some projects and it works well.
There's also Thrift (orginally from Facebook) http://thrift.apache.org/ I don't have any experience with it though.
Another option is what #QuentinUK suggests. Use a class that inherits from something streamable and/or make streamable operators/functions.
I'd use a framework.
Here's your problem:
http://en.wikipedia.org/wiki/Address_space_layout_randomization
Back in ancient history (16-bit DOS programs with extenders), compilers used to support "based" pointers which stored relative addresses. These were safe to serialize en masse. And applications did so, saving both code and data, the serialized modules were called "overlays".
Today, you'd need based pointer support in your toolchain (resulting in every pointer access requiring an extra adjustment), or else to go through all the data, distinguishing the pointers from the other data (how?) and adjusting them to their new storage location, in case the OS already loaded some library at the same address your old program had used for its heap. In modern "managed" environments, where pointers already have to be identified for the garbage collector, this is feasible even if not commonly done. In native code, it's very difficult, although that metadata is created to enable relocation of shared libraries.
So instead people end up walking their entire data structures manually, and converting object links (pointers) into something that can be restored on the other end, even though the object has a new address (again, because the old address may have been used for a shared library).
Note that many processors have features to support based addressing... and that since based addressing is no longer common, compilers went ahead and used those pointer arithmetic features to speed up user code.
Yes, derive objects from a streamable class and add the streaming functions. Then you can stream everything to disk. You will need a library for this such as MFC.
Rationale: In my day-to-day C++ code development, I frequently need to
answer basic questions such as who calls what in a very large C++ code
base that is frequently changing. But, I also need to have some
automated way to exactly identify what the code is doing around a
particular area of code. "grep" tools such as Cscope are useful (and
I use them heavily already), but are not C++-language-aware: They
don't give any way to identify the types and kinds of lexical
environment of a given use of a type or function a such way that is
conducive to automation (even if said automation is limited to
"read-only" operations such as code browsing and navigation, but I'm
asking for much more than that below).
Question: Does there exist already an open-source C/C++-based library
(native, not managed, not Microsoft- or Linux-specific) that can
statically scan or analyze a large tree of C++ code, and can produce
result sets that answer detailed questions such as:
What functions are called by some supplied function?
What functions make use of this supplied type?
Ditto the above questions if C++ classes or class templates are involved.
The result set should provide some sort of "handle". I should be able
to feed that handle back to the library to perform the following types
of introspection:
What is the byte offset into the file where the reference was made?
What is the reference into the abstract syntax tree (AST) of that
reference, so that I can inspect surrounding code constructs? And
each AST entity would also have file path, byte-offset, and
type-info data associated with it, so that I could recursively walk
up the graph of callers or referrers to do useful operations.
The answer should meet the following requirements:
API: The API exposed must be one of the following:
C or C++ and probably is "C handle" or C++-class-instance-based
(and if it is, must be generic C o C++ code and not Microsoft- or
Linux-specific code constructs unless it is to meet specifics of
the given platform), or
Command-line standard input and standard output based.
C++ aware: Is not limited to C code, but understands C++ language
constructs in minute detail including awareness of inter-class
inheritance relationships and C++ templates.
Fast: Should scan large code bases significantly faster than
compiling the entire code base from scratch. This probably needs to
be relaxed, but only if Incremental result retrieval and Resilient
to small code changes requirements are fully met below.
Provide Result counts: I should be able to ask "How many results
would you provide to some request (and no don't send me all of the
results)?" that responds on the order of less than 3 seconds versus
having to retrieve all results for any given question. If it takes
too long to get that answer, then wastes development time. This is
coupled with the next requirement.
Incremental result retrieval: I should be able to then ask "Give me
just the next N results of this request", and then a handle to the
result set so that I can ask the question repeatedly, thus
incrementally pulling out the results in stages. This means I
should not have to wait for the entire result set before seeing
some subset of all of the results. And that I can cancel the
operation safely if I have seen enough results. Reason: I need to
answer the question: "What is the build or development impact of
changing some particular function signature?"
Resilient to small code changes: If I change a header or source
file, I should not have to wait for the entire code base to be
rescanned, but only that header or source file
rescanned. Rescanning should be quick. E.g., don't do what cscope
requires you to do, which is to rescan the entire code base for
small changes. It is understood that if you change a header, then
scanning can take longer since other files that include that header
would have to be rescanned.
IDE Agnostic: Is text editor agnostic (don't make me use a specific
text editor; I've made my choice already, thank you!)
Platform Agnostic: Is platform-agnostic (don't make me only use it
on Linux or only on Windows, as I have to use both of those
platforms in my daily grind, but I need the tool to be useful on
both as I have code sandboxes on both platforms).
Non-binary: Should not cost me anything other than time to download
and compile the library and all of its dependencies.
Not trial-ware.
Actively Supported: It is likely that sending help requests to mailing lists
or associated forums is likely to get a response in less than 2
days.
Network agnostic: Databases the library builds should be able to be used directly on
a network from 32-bit and 64-bit systems, both Linux and Windows
interchangeably, at the same time, and do not embed hardcoded paths
to filesystems that would otherwise "root" the database to a
particular network.
Build environment agnostic: Does not require intimate knowledge of my build environment, with
the notable exception of possibly requiring knowledge of compiler
supplied CPP macro definitions (e.g. -Dmacro=value).
I would say that CLang Index is a close fit. However I don't think that it stores data in a database.
Anyway the CLang framework offer what you actually need to build a tool tailored to your needs, if only because of its C, C++ and Objective-C parsing / indexing capabitilies. And since it's provided as a set of reusable libraries... it was crafted for being developed on!
I have to admit that I haven't used either because I work with a lot of Microsoft-specific code that uses Microsoft compiler extensions that i don't expect them to understand, but the two open source analyzers I'm aware of are Mozilla Pork and the Clang Analyzer.
If you are looking for results of code analysis (metrics, graphs, ...) why not use a tool (instead of API) to do that? If you can, I suggest you to take a look at Understand.
It's not free (there's a trial version) but I found it very useful.
Maybe Doxygen with GraphViz could be the answer of some of your constraints but not all,for example the analysis of Doxygen is not incremental.
I know that E&C is a controversial subject and some say that it encourages a wrong approach to debugging, but still - I think we can agree that there are numerous cases when it is clearly useful - experimenting with different values of some constants, redesigning GUI parameters on-the-fly to find a good look... You name it.
My question is: Are we ever going to have E&C on GDB? I understand that it is a platform-specific feature and needs some serious cooperation with the compiler, the debugger and the OS (MSVC has this one easy as the compiler and debugger always come in one package), but... It still should be doable. I've even heard something about Apple having it implemented in their version of GCC [citation needed]. And I'd say it is indeed feasible.
Knowing all the hype about MSVC's E&C (my experience says it's the first thing MSVC users mention when asked "why not switch to Eclipse and gcc/gdb"), I'm seriously surprised that after quite some years GCC/GDB still doesn't have such feature. Are there any good reasons for that? Is someone working on it as we speak?
It is a surprisingly non-trivial amount of work, encompassing many design decisions and feature tradeoffs. Consider: you are debugging. The debugee is suspended. Its image in memory contains the object code of the source, and the binary layout of objects, the heap, the stacks. The debugger is inspecting its memory image. It has loaded debug information about the symbols, types, address mappings, pc (ip) to source correspondences. It displays the call stack, data values.
Now you want to allow a particular set of possible edits to the code and/or data, without stopping the debuggee and restarting. The simplest might be to change one line of code to another. Perhaps you recompile that file or just that function or just that line. Now you have to patch the debuggee image to execute that new line of code the next time you step over it or otherwise run through it. How does that work under the hood? What happens if the code is larger than the line of code it replaced? How does it interact with compiler optimizations? Perhaps you can only do this on a specially compiled for EnC debugging target. Perhaps you will constrain possible sites it is legal to EnC. Consider: what happens if you edit a line of code in a function suspended down in the call stack. When the code returns there does it run the original version of the function or the version with your line changed? If the original version, where does that source come from?
Can you add or remove locals? What does that do to the call stack of suspended frames? Of the current function?
Can you change function signatures? Add fields to / remove fields from objects? What about existing instances? What about pending destructors or finalizers? Etc.
There are many, many functionality details to attend to to make any kind of usuable EnC work. Then there are many cross-tools integration issues necessary to provide the infrastructure to power EnC. In particular, it helps to have some kind of repository of debug information that can make available the before- and after-edit debug information and object code to the debugger. For C++, the incrementally updatable debug information in PDBs helps. Incremental linking may help too.
Looking from the MS ecosystem over into the GCC ecosystem, it is easy to imagine the complexity and integration issues across GDB/GCC/binutils, the myriad of targets, some needed EnC specific target abstractions, and the "nice to have but inessential" nature of EnC, are why it has not appeared yet in GDB/GCC.
Happy hacking!
(p.s. It is instructive and inspiring to look at what the Smalltalk-80 interactive programming environment could do. In St80 there was no concept of "restart" -- the image and its object memory were always live, if you edited any aspect of a class you still had to keep running. In such environments object versioning was not a hypothetical.)
I'm not familiar with MSVC's E&C, but GDB has some of the things you've mentioned:
http://sourceware.org/gdb/current/onlinedocs/gdb/Altering.html#Altering
17. Altering Execution
Once you think you have found an error in your program, you might want to find out for certain whether correcting the apparent error would lead to correct results in the rest of the run. You can find the answer by experiment, using the gdb features for altering execution of the program.
For example, you can store new values into variables or memory locations, give your program a signal, restart it at a different address, or even return prematurely from a function.
Assignment: Assignment to variables
Jumping: Continuing at a different address
Signaling: Giving your program a signal
Returning: Returning from a function
Calling: Calling your program's functions
Patching: Patching your program
Compiling and Injecting Code: Compiling and injecting code in GDB
This is a pretty good reference to the old Apple implementation of "fix and continue". It also references other working implementations.
http://sources.redhat.com/ml/gdb/2003-06/msg00500.html
Here is a snippet:
Fix and continue is a feature implemented by many other debuggers,
which we added to our gdb for this release. Sun Workshop, SGI ProDev
WorkShop, Microsoft's Visual Studio, HP's wdb, and Sun's Hotspot Java
VM all provide this feature in one way or another. I based our
implementation on the HP wdb Fix and Continue feature, which they
added a few years back. Although my final implementation follows the
general outlines of the approach they took, there is almost no shared
code between them. Some of this is because of the architectual
differences (both the processor and the ABI), but even more of it is
due to implementation design differences.
Note that this capability may have been removed in a later version of their toolchain.
UPDATE: Dec-21-2012
There is a GDB Roadmap PDF presentation that includes a slide describing "Fix and Continue" among other bullet points. The presentation is dated July-9-2012 so maybe there is hope to have this added at some point. The presentation was part of the GNU Tools Cauldron 2012.
Also, I get it that adding E&C to GDB or anywhere in Linux land is a tough chore with all the different components.
But I don't see E&C as controversial. I remember using it in VB5 and VB6 and it was probably there before that. Also it's been in Office VBA since way back. And it's been in Visual Studio since VS2005. VS2003 was the only one that didn't have it and I remember devs howling about it. They intended to add it back anyway and they did with VS2005 and it's been there since. It works with C#, VB, and also C and C++. It's been in MS core tools for 20+ years, almost continuous (counting VB when it was standalone), and subtracting VS2003. But you could still say they had it in Office VBA during the VS2003 period ;)
And Jetbrains recently added it too their C# tool Rider. They bragged about it (rightly so imo) in their Rider blog.
I'm just starting to explore C++, so forgive the newbiness of this question. I also beg your indulgence on how open ended this question is. I think it could be broken down, but I think that this information belongs in the same place.
(FYI -- I am working predominantly with the QT SDK and mingw32-make right now and I seem to have configured them correctly for my machine.)
I knew that there was a lot in the language which is compiler-driven -- I've heard about pre-compiler directives, but it seems like someone would be able to write books the different C++ compilers and their respective parameters. In addition, there are commands which apparently precede make (like qmake, for example (is this something only in QT)).
I would like to know if there is any place which gives me an overview of what compilers are out there, and what their different options are. I'd also like to know how each of them views Makefiles (it seems that there is a difference in syntax between them?).
If there is no website regarding, "Everything you need to know about C++ compilers but were afraid to ask," what would be the best way to go about learning the answers to these questions?
Concerning the "numerous options of the various compilers"
A piece of good news: you needn't worry about the detail of most of these options. You will, in due time, delve into this, only for the very compiler you use, and maybe only for the options that pertain to a particular set of features. But as a novice, generally trust the default options or the ones supplied with the make files.
The broad categories of these features (and I may be missing a few) are:
pre-processor defines (now, you may need a few of these)
code generation (target CPU, FPU usage...)
optimization (hints for the compiler to favor speed over size and such)
inclusion of debug info (which is extra data left in the object/binary and which enables the debugger to know where each line of code starts, what the variables names are etc.)
directives for the linker
output type (exe, library, memory maps...)
C/C++ language compliance and warnings (compatibility with previous version of the compiler, compliance to current and past C Standards, warning about common possible bug-indicative patterns...)
compile-time verbosity and help
Concerning an inventory of compilers with their options and features
I know of no such list but I'm sure it probably exists on the web. However, suggest that, as a novice you worry little about these "details", and use whatever free compiler you can find (gcc certainly a great choice), and build experience with the language and the build process. C professionals may likely argue, with good reason and at length on the merits of various compilers and associated runtine etc., but for generic purposes -and then some- the free stuff is all that is needed.
Concerning the build process
The most trivial applications, such these made of a single unit of compilation (read a single C/C++ source file), can be built with a simple batch file where the various compiler and linker options are hardcoded, and where the name of file is specified on the command line.
For all other cases, it is very important to codify the build process so that it can be done
a) automatically and
b) reliably, i.e. with repeatability.
The "recipe" associated with this build process is often encapsulated in a make file or as the complexity grows, possibly several make files, possibly "bundled together in a script/bat file.
This (make file syntax) you need to get familiar with, even if you use alternatives to make/nmake, such as Apache Ant; the reason is that many (most?) source code packages include a make file.
In a nutshell, make files are text files and they allow defining targets, and the associated command to build a target. Each target is associated with its dependencies, which allows the make logic to decide what targets are out of date and should be rebuilt, and, before rebuilding them, what possibly dependencies should also be rebuilt. That way, when you modify say an include file (and if the make file is properly configured) any c file that used this header will be recompiled and any binary which links with the corresponding obj file will be rebuilt as well. make also include options to force all targets to be rebuilt, and this is sometimes handy to be sure that you truly have a current built (for example in the case some dependencies of a given object are not declared in the make).
On the Pre-processor:
The pre-processor is the first step toward compiling, although it is technically not part of the compilation. The purposes of this step are:
to remove any comment, and extraneous whitespace
to substitute any macro reference with the relevant C/C++ syntax. Some macros for example are used to define constant values such as say some email address used in the program; during per-processing any reference to this constant value (btw by convention such constants are named with ALL_CAPS_AND_UNDERSCORES) is replace by the actual C string literal containing the email address.
to exclude all conditional compiling branches that are not relevant (the #IFDEF and the like)
What's important to know about the pre-processor is that the pre-processor directive are NOT part of the C-Language proper, and they serve several important functions such as the conditional compiling mentionned earlier (used for example to have multiple versions of the program, say for different Operating Systems, or indeed for different compilers)
Taking it from there...
After this manifesto of mine... I encourage to read but little more, and to dive into programming and building binaries. It is a very good idea to try and get a broad picture of the framework etc. but this can be overdone, a bit akin to the exchange student who stays in his/her room reading the Webster dictionary to be "prepared" for meeting native speakers, rather than just "doing it!".
Ideally you shouldn't need to care what C++ compiler you are using. The compatability to the standard has got much better in recent years (even from microsoft)
Compiler flags obviously differ but the same features are generally available, it's just a differently named option to eg. set warning level on GCC and ms-cl
The build system is indepenant of the compiler, you can use any make with any compiler.
That is a lot of questions in one.
C++ compilers are a lot like hammers: They come in all sizes and shapes, with different abilities and features, intended for different types of users, and at different price points; ultimately they all are for doing the same basic task as the others.
Some are intended for highly specialized applications, like high-performance graphics, and have numerous extensions and libraries to assist the engineer with those types of problems. Others are meant for general purpose use, and aren't necessarily always the greatest for extreme work.
The technique for using each type of hammer varies from model to model—and version to version—but they all have a lot in common. The macro preprocessor is a standard part of C and C++ compilers.
A brief comparison of many C++ compilers is here. Also check out the list of C compilers, since many programs don't use any C++ features and can be compiled by ordinary C.
C++ compilers don't "view" makefiles. The rules of a makefile may invoke a C++ compiler, but also may "compile" assembly language modules (assembling), process other languages, build libraries, link modules, and/or post-process object modules. Makefiles often contain rules for cleaning up intermediate files, establishing debug environments, obtaining source code, etc., etc. Compilation is one link in a long chain of steps to develop software.
Also, many development environments abstract the makefile into a "project file" which is used by an integrated development environment (IDE) in an attempt to simplify or automate many programming tasks. See a comparison here.
As for learning: choose a specific problem to solve and dive in. The target platform (Linux/Windows/etc.) and problem space will narrow the choices pretty well. Which you choose is often linked to other considerations, such as working for a particular company, or being part of a team. C++ has something like 95% commonality among all its flavors. Learn any one of them well, and learning the next is a piece of cake.
Summary: I want to take advantage of compiler optimizations and processor instruction sets, but still have a portable application (running on different processors). Normally I could indeed compile 5 times and let the user choose the right one to run.
My question is: how can I can automate this, so that the processor is detected at runtime and the right executable is executed without the user having to chose it?
I have an application with a lot of low level math calculations. These calculations will typically run for a long time.
I would like to take advantage of as much optimization as possible, preferably also of (not always supported) instruction sets. On the other hand I would like my application to be portable and easy to use (so I would not like to compile 5 different versions and let the user choose).
Is there a possibility to compile 5 different versions of my code and run dynamically the most optimized version that's possible at execution time? With 5 different versions I mean with different instruction sets and different optimizations for processors.
I don't care about the size of the application.
At this moment I'm using gcc on Linux (my code is in C++), but I'm also interested in this for the Intel compiler and for the MinGW compiler for compilation to Windows.
The executable doesn't have to be able to run on different OS'es, but ideally there would be something possible with automatically selecting 32 bit and 64 bit as well.
Edit: Please give clear pointers how to do it, preferably with small code examples or links to explanations. From my point of view I need a super generic solution, which is applicable on any random C++ project I have later.
Edit I assigned the bounty to ShuggyCoUk, he had a great number of pointers to look out for. I would have liked to split it between multiple answers but that is not possible. I'm not having this implemented yet, so the question is still 'open'! Please, still add and/or improve answers, even though there is no bounty to be given anymore.
Thanks everybody!
Yes it's possible. Compile all your differently optimised versions as different dynamic libraries with a common entry point, and provide an executable stub that that loads and runs
the correct library at run-time, via the entry point, depending on config file or other information.
Can you use script?
You could detect the CPU using script, and dynamically load the executable that is most optimized for architecture. It can choose 32/64 bit versions too.
If you are using a Linux you can query the cpu with
cat /proc/cpuinfo
You could probably do this with a bash/perl/python script or windows scripting host on windows. You probably don't want to force the user to install a script engine. One that works on the OS out of the box IMHO would be best.
In fact, on windows you probably would want to write a small C# app so you can more easily query the architecture. The C# app could just spawn whatever executable is fastest.
Alternatively you could put your different versions of code in a dll's or shared object's, then dynamically load them based on the detected architecture. As long as they have the same call signature it should work.
If you wish this to cleanly work on Windows and take full advantage in 64bit capable platforms of the additional 1. Addressing space and 2. registers (likely of more use to you) you must have at a minimum a separate process for the 64bit ones.
You can achieve this by having a separate executable with the relevant PE64 header. Simply using CreateProcess will launch this as the relevant bitness (unless the executable launched is in some redirected location there is no need to worry about WoW64 folder redirection
Given this limitation on windows it is likely that simply 'chaining along' to the relevant executable will be the simplest option for all different options, as well as making testing an individual one simpler.
It also means you 'main' executable is free to be totally separate depending on the target operating system (as detecting the cpu/OS capabilities is, by it's nature, very OS specific) and then do most of the rest of your code as shared objects/dlls.
Also you can 'share' the same files for two different architectures if you currently do not feel that there is any point using the differing capabilities.
I would suggest that the main executable is capable of being forced into making a specific choice so you can see what happens with 'lesser' versions on a more capable machine (or what errors come up if you try something different).
Other possibilities given this model are:
Statically linking to different versions of the standard runtimes (for ones with/without thread safety) and using them appropriately if you are running without any SMP/SMT capabilities.
Detect if multiple cores are present and whether they are real or hyper threading (also whether the OS knows how the schedule effectively in those cases)
checking the performance of things like the system timer/high performance timers and using code optimized to this behaviour, say if you do anything where you look for a certain amount of time to expire and thus can know your best possible granularity.
If you wish to optimize you choice of code based on cache sizing/other load on the box. If you are using unrolled loops then more aggressive unrolling options may depend on having a certain amount level 1/2 cache.
Compiling conditionally to use doubles/floats depending on the architecture. Less important on intel hardware but if you are targetting certain ARM cpu's some have actual floating point hardware support and others require emulation. The optimal code would change heavily, even to the extent you just use conditional compilation rather than using the optimizing compiler(1).
Making use of co-processor hardware like CUDA capable graphics cards.
detect virtualization and alter behaviour (perhaps trying to avoid file system writes)
As to doing this check you have a few options, the most useful one on Intel being the the cpuid instruction.
Windows
Use someone else's implementation but you'll have to pay
Use a free open source one
Linux
Use the built in one
You could also look at open source software doing the same thing
Pixman does a fair amount of this and is a permissive licence.
Alternatively re-implement/update an existing one using available documentation on the features you need.
Quite a lot of separate documents to work out how to detect things:
Intel:
SSE 4.1/4.2
SSE3
MMX
A large part of what you would be paying for in the CPU-Z library is someone doing all this (and the nasty little issues involved) for you.
be careful with this - it is hard to beat decent optimizing compilers on this
Have a look at liboil: http://liboil.freedesktop.org/wiki/ . It can dynamically select implementations of multimedia-related computations at run-time. You may find you can liboil itself and not just its techniques.
Since you mention you are using GCC, I'll assume your code is in C (or C++).
Neil Butterworth already suggested making separate dynamic libraries, but that requires some non-trivial cross-platform considerations (manually loading dynamic libraries is different on Linux, Windows, OSX, etc., and getting it right will likely take some time).
A cheap solution is to simply write all of your variants using unique names, and use a function pointer to select the proper one at runtime.
I suspect the extra dereference caused by the function pointer will be amortized by the actual work you are doing (but you'll want to confirm that).
Also, getting different compiler optimizations will likely require different .c/.cpp files, as well as some twiddling of your build tool. But it's probably less overall work than separate libraries (which needed this already in one form or another).
Since you didn't specify whether you have limits on the number of files, I propose another solution: compile 5 executables, and then create a sixth executable that launches the appropriate binary. Here is some pseudocode, for Linux
int main(int argc, char* argv[])
{
char* target_path[MAXPATH];
char* new_argv[];
char* specific_version = determine_name_of_specific_version();
strcpy(target_path, "/usr/lib/myapp/versions");
strcat(target_path, specific_version);
/* append NULL to argv */
new_argv = malloc(sizeof(char*)*(argc+1));
memcpy(new_argv, argv, argc*sizeof(char*));
new_argv[argc] = 0;
/* optionally set new_argv[0] to target_path */
execv(target_path, new_argv);
}
On the plus side, this approach allows to provide the user transparently with both 32-bit and 64-bit binaries, unlike any library methods that have been proposed. On the minus side, there is no execv in Win32 (but a good emulation in cygwin); on Windows, you have to create a new process, rather than re-execing the current one.
Lets break the problem down to its two constituent parts. 1) Creating platform dependent optimized code and 2) building on multiple platforms.
The first problem is pretty straightforward. Encapsulate the platform dependent code in a set of functions. Create a different implementation of each function for each platform. Put each implementation in its own file or set of files. It's easiest for the build system if you put each platform's code in a separate directory.
For part two I suggest you look at Gnu Atuotools (Automake, AutoConf, and Libtool). If you've ever downloaded and built a GNU program from source code you know you have to run ./configure before running make. The purpose of the configure script is to 1) verify that your system has all of the required libraries and utilities need to build and run the program and 2) customize the Makefiles for the target platform. Autotools is the set of utilities for generating the configure script.
Using autoconf, you can create little macros to check that the machine supports all of the CPU instructions your platform dependent code needs. In most cases, the macros already exists, you just have to copy them into your autoconf script. Then, automake and autoconf can set up the Makefiles to pull in the appropriate implementation.
All this is a bit much for creating an example here. It takes a little time to learn. But the documentation is all out there. There is even a free book available online. And the process is applicable to your future projects. For multi-platform support, this is really the most robust and easiest way to go, I think. A lot of the suggestions posted in other answers are things that Autotools deals with (CPU detection, static & shared library support) without you have to think about it too much. The only wrinkle you might have to deal with is finding out if Autotools are available for MinGW. I know they are part of Cygwin if you can go that route instead.
You mentioned the Intel compiler. That is funny, because it can do something like this by default. However, there is a catch. The Intel compiler didn't insert checks for the approopriate SSE functionality. Instead, they checked if you had a particular Intel chip. There would still be a slow default case. As a result, AMD CPUs would not get suitable SSE-optimized versions. There are hacks floating around that will replace the Intel check with a proper SSE check.
The 32/64 bits difference will require two executables. Both the ELF and PE format store this information in the exectuables header. It's not too hard to start the 32 bits version by default, check if you are on a 64 bit system, and then restart the 64 bit version. But it may be easier to create an appropriate symlink at installation time.