Are there any compelling performance reasons to choose static linking over dynamic linking or vice versa in certain situations? I've heard or read the following, but I don't know enough on the subject to vouch for its veracity.
1) The difference in runtime performance between static linking and dynamic linking is usually negligible.
2) (1) is not true if using a profiling compiler that uses profile data to optimize program hotpaths because with static linking, the compiler can optimize both your code and the library code. With dynamic linking only your code can be optimized. If most of the time is spent running library code, this can make a big difference. Otherwise, (1) still applies.
Dynamic linking can reduce total resource consumption (if more than one process shares the same library (including the version in "the same", of course)). I believe this is the argument that drives its presence in most environments. Here "resources" include disk space, RAM, and cache space. Of course, if your dynamic linker is insufficiently flexible there is a risk of DLL hell.
Dynamic linking means that bug fixes and upgrades to libraries propagate to improve your product without requiring you to ship anything.
Plugins always call for dynamic linking.
Static linking, means that you can know the code will run in very limited environments (early in the boot process, or in rescue mode).
Static linking can make binaries easier to distribute to diverse user environments (at the cost of sending a larger and more resource-hungry program).
Static linking may allow slightly faster startup times, but this depends to some degree on both the size and complexity of your program and on the details of the OS's loading strategy.
Some edits to include the very relevant suggestions in the comments and in other answers. I'd like to note that the way you break on this depends a lot on what environment you plan to run in. Minimal embedded systems may not have enough resources to support dynamic linking. Slightly larger small systems may well support dynamic linking because their memory is small enough to make the RAM savings from dynamic linking very attractive. Full-blown consumer PCs have, as Mark notes, enormous resources, and you can probably let the convenience issues drive your thinking on this matter.
To address the performance and efficiency issues: it depends.
Classically, dynamic libraries require some kind of glue layer which often means double dispatch or an extra layer of indirection in function addressing and can cost a little speed (but is the function calling time actually a big part of your running time???).
However, if you are running multiple processes which all call the same library a lot, you can end up saving cache lines (and thus winning on running performance) when using dynamic linking relative to using static linking. (Unless modern OS's are smart enough to notice identical segments in statically linked binaries. Seems hard, does anyone know?)
Another issue: loading time. You pay loading costs at some point. When you pay this cost depends on how the OS works as well as what linking you use. Maybe you'd rather put off paying it until you know you need it.
Note that static-vs-dynamic linking is traditionally not an optimization issue, because they both involve separate compilation down to object files. However, this is not required: a compiler can in principle, "compile" "static libraries" to a digested AST form initially, and "link" them by adding those ASTs to the ones generated for the main code, thus empowering global optimization. None of the systems I use do this, so I can't comment on how well it works.
The way to answer performance questions is always by testing (and use a test environment as much like the deployment environment as possible).
1) is based on the fact that calling a DLL function is always using an extra indirect jump. Today, this is usually negligible. Inside the DLL there is some more overhead on i386 CPU's, because they can't generate position independent code. On amd64, jumps can be relative to the program counter, so this is a huge improvement.
2) This is correct. With optimizations guided by profiling you can usually win about 10-15 percent performance. Now that CPU speed has reached its limits it might be worth doing it.
I would add: (3) the linker can arrange functions in a more cache efficient grouping, so that expensive cache level misses are minimised. It also might especially effect the startup time of applications (based on results i have seen with the Sun C++ compiler)
And don't forget that with DLLs no dead code elimination can be performed. Depending on the language, the DLL code might not be optimal either. Virtual functions are always virtual because the compiler doesn't know whether a client is overwriting it.
For these reasons, in case there is no real need for DLLs, then just use static compilation.
EDIT (to answer the comment, by user underscore)
Here is a good resource about the position independent code problem http://eli.thegreenplace.net/2011/11/03/position-independent-code-pic-in-shared-libraries/
As explained x86 does not have them AFAIK for anything else then 15 bit jump ranges and not for unconditional jumps and calls. That's why functions (from generators) having more then 32K have always been a problem and needed embedded trampolines.
But on popular x86 OS like Linux you do not need to care if the .so/DLL file is not generated with the gcc switch -fpic (which enforces the use of the indirect jump tables). Because if you don't, the code is just fixed like a normal linker would relocate it. But while doing this it makes the code segment non shareable and it would need a full mapping of the code from disk into memory and touching it all before it can be used (emptying most of the caches, hitting TLBs) etc. There was a time when this was considered slow.
So you would not have any benefit anymore.
I do not recall what OS (Solaris or FreeBSD) gave me problems with my Unix build system because I just wasn't doing this and wondered why it crashed until I applied -fPIC to gcc.
Dynamic linking is the only practical way to meet some license requirements such as the LGPL.
I agree with the points dnmckee mentions, plus:
Statically linked applications might be easier to deploy, since there are fewer or no additional file dependencies (.dll / .so) that might cause problems when they're missing or installed in the wrong place.
One reason to do a statically linked build is to verify that you have full closure for the executable, i.e. that all symbol references are resolved correctly.
As a part of a large system that was being built and tested using continuous integration, the nightly regression tests were run using a statically linked version of the executables. Occasionally, we would see that a symbol would not resolve and the static link would fail even though the dynamically linked executable would link successfully.
This was usually occurring when symbols that were deep seated within the shared libs had a misspelt name and so would not statically link. The dynamic linker does not completely resolve all symbols, irrespective of using depth-first or breadth-first evaluation, so you can finish up with a dynamically linked executable that does not have full closure.
1/ I've been on projects where dynamic linking vs static linking was benchmarked and the difference wasn't determined small enough to switch to dynamic linking (I wasn't part of the test, I just know the conclusion)
2/ Dynamic linking is often associated with PIC (Position Independent Code, code which doesn't need to be modified depending on the address at which it is loaded). Depending on the architecture PIC may bring another slowdown but is needed in order to get benefit of sharing a dynamically linked library between two executable (and even two process of the same executable if the OS use randomization of load address as a security measure). I'm not sure that all OS allow to separate the two concepts, but Solaris and Linux do and ISTR that HP-UX does as well.
3/ I've been on other projects which used dynamic linking for the "easy patch" feature. But this "easy patch" makes the distribution of small fix a little easier and of complicated one a versioning nightmare. We often ended up by having to push everything plus having to track problems at customer site because the wrong version was token.
My conclusion is that I'd used static linking excepted:
for things like plugins which depend on dynamic linking
when sharing is important (big libraries used by multiple processes at the same time like C/C++ runtime, GUI libraries, ... which often are managed independently and for which the ABI is strictly defined)
If one want to use the "easy patch", I'd argue that the libraries have to be managed like the big libraries above: they must be nearly independent with a defined ABI that must not to be changed by fixes.
Static linking is a process in compile time when a linked content is copied into the primary binary and becomes a single binary.
Cons:
compile time is longer
output binary is bigger
Dynamic linking is a process in runtime when a linked content is loaded. This technic allows to:
upgrade linked binary without recompiling a primary one that increase an ABI stability[About]
has a single shared copy
Cons:
start time is slower(linked content should be copied)
linker errors are thrown in runtime
[iOS Static vs Dynamic framework]
It is pretty simple, really. When you make a change in your source code, do you want to wait 10 minutes for it to build or 20 seconds? Twenty seconds is all I can put up with. Beyond that, I either get out the sword or start thinking about how I can use separate compilation and linking to bring it back into the comfort zone.
Best example for dynamic linking is, when the library is dependent on the used hardware. In ancient times the C math library was decided to be dynamic, so that each platform can use all processor capabilities to optimize it.
An even better example might be OpenGL. OpenGl is an API that is implemented differently by AMD and NVidia. And you are not able to use an NVidia implementation on an AMD card, because the hardware is different. You cannot link OpenGL statically into your program, because of that. Dynamic linking is used here to let the API be optimized for all platforms.
Dynamic linking requires extra time for the OS to find the dynamic library and load it. With static linking, everything is together and it is a one-shot load into memory.
Also, see DLL Hell. This is the scenario where the DLL that the OS loads is not the one that came with your application, or the version that your application expects.
On Unix-like systems, dynamic linking can make life difficult for 'root' to use an application with the shared libraries installed in out-of-the-way locations. This is because the dynamic linker generally won't pay attention to LD_LIBRARY_PATH or its equivalent for processes with root privileges. Sometimes, then, static linking saves the day.
Alternatively, the installation process has to locate the libraries, but that can make it difficult for multiple versions of the software to coexist on the machine.
Another issue not yet discussed is fixing bugs in the library.
With static linking, you not only have to rebuild the library, but will have to relink and redestribute the executable. If the library is just used in one executable, this may not be an issue. But the more executables that need to be relinked and redistributed, the bigger the pain is.
With dynamic linking, you just rebuild and redistribute the dynamic library and you are done.
Static linking includes the files that the program needs in a single executable file.
Dynamic linking is what you would consider the usual, it makes an executable that still requires DLLs and such to be in the same directory (or the DLLs could be in the system folder).
(DLL = dynamic link library)
Dynamically linked executables are compiled faster and aren't as resource-heavy.
static linking gives you only a single exe, inorder to make a change you need to recompile your whole program. Whereas in dynamic linking you need to make change only to the dll and when you run your exe, the changes would be picked up at runtime.Its easier to provide updates and bug fixes by dynamic linking (eg: windows).
There are a vast and increasing number of systems where an extreme level of static linking can have an enormous positive impact on applications and system performance.
I refer to what are often called "embedded systems", many of which are now increasingly using general-purpose operating systems, and these systems are used for everything imaginable.
An extremely common example are devices using GNU/Linux systems using Busybox. I've taken this to the extreme with NetBSD by building a bootable i386 (32-bit) system image that includes both a kernel and its root filesystem, the latter which contains a single static-linked (by crunchgen) binary with hard-links to all programs that itself contains all (well at last count 274) of the standard full-feature system programs (most except the toolchain), and it is less than 20 megabytes in size (and probably runs very comfortably in a system with only 64MB of memory (even with the root filesystem uncompressed and entirely in RAM), though I've been unable to find one so small to test it on).
It has been mentioned in earlier posts that the start-up time of a static-linked binaries is faster (and it can be a lot faster), but that is only part of the picture, especially when all object code is linked into the same file, and even more especially when the operating system supports demand paging of code direct from the executable file. In this ideal scenario the startup time of programs is literally negligible since almost all pages of code will already be in memory and be in use by the shell (and and init any other background processes that might be running), even if the requested program has not ever been run since boot since perhaps only one page of memory need be loaded to fulfill the runtime requirements of the program.
However that's still not the whole story. I also usually build and use the NetBSD operating system installs for my full development systems by static-linking all binaries. Even though this takes a tremendous amount more disk space (~6.6GB total for x86_64 with everything, including toolchain and X11 static-linked) (especially if one keeps full debug symbol tables available for all programs another ~2.5GB), the result still runs faster overall, and for some tasks even uses less memory than a typical dynamic-linked system that purports to share library code pages. Disk is cheap (even fast disk), and memory to cache frequently used disk files is also relatively cheap, but CPU cycles really are not, and paying the ld.so startup cost for every process that starts every time it starts will take hours and hours of CPU cycles away from tasks which require starting many processes, especially when the same programs are used over and over, such as compilers on a development system. Static-linked toolchain programs can reduce whole-OS multi-architecture build times for my systems by hours. I have yet to build the toolchain into my single crunchgen'ed binary, but I suspect when I do there will be more hours of build time saved because of the win for the CPU cache.
Another consideration is the number of object files (translation units) that you actually consume in a library vs the total number available. If a library is built from many object files, but you only use symbols from a few of them, this might be an argument for favoring static linking, since you only link the objects that you use when you static link (typically) and don't normally carry the unused symbols. If you go with a shared lib, that lib contains all translation units and could be much larger than what you want or need.
There are several discussions on forums about shared vs. static libraries regarding performance. But how do those approaches compare to compiling the code altogether?
In my case, I have a class (the evaluation code) that contains a few methods that contain several for loops and that will be called several times by a method from another class (the evaluator code). I have not finished implementing and testing everything yet. But, for the sake of performance, I am wondering if I should compile all the files altogether (compiler optimization advantages?), or compile some files separately to generate static or shared libraries.
These approaches will depend on your compiler and options:
Not using libraries:
A good compiler, and build system will cache results and this should be just as fast as the other two. In practice, many code bases have less than optimal compartmentalization leading to slow compile times, the classic approach is to break the thing apart into libraries.
Static:
This might be slower than dynamic linking because there is an opportunity to run link time optimization (LTO), which could take a while
Dynamic:
Might be slower when you have a small number of functions because of specifics on how dynamic loading is implemented.
In conclusion, unless you're working on some monster project where you're worried about people truking up the build system, keep it all in one project and avoid needlessly complicated debugging.
Suppose now two C++ libraries are available: one library has all the functions that will be needed by the program ( a C++ application program that will invoke the library), and the other one not only has the necessary functions that will be needed by the program but also has other functions that will not not used by the program. We assume that for the common functions in both libraries they are implemented in the same manner. My question is: when the program uses the library to perform a certain task, what's the effect of the library on the performance of the program?
The reason why I asked this question is because when developing a c++ library I often wrote some additional functions, which may not be invoked by the users of the library but are important for debugging. When the library is finished, I have two choices: one is to keep these auxiliary functions and the other is removing them or using other strategies of keep them (for example, define MACRO to disable these functions). If keeping these auxiliaries functions will not deteriorate the performance, I would like to keep them.
Everything else being the same, there will be no performance difference.
In addition, if the library is a static library, the linker will not include the functions that are not used, and the executables will have the same size.
Well if you have written a static library that I guess you have. Then the only difference it will create is that the static library functionality will be part of you executable no matter if you use it or not.
I don't think it will hurt you in terms of speed but yes it will occupy a lot more space since a copy of lib will be created with you executable.
Why are all libraries in Boost not headers-only?
Saying it differently, what makes the use of .lib/.dll mandatory?
Is it when a class can't be a template or has static fields?
Different points, I guess.
Binary size. Could header-only put a size burden on the client?
Compilation times. Could header-only mean a significant decrease in compilation performance?
Runtime Performance. Could header-only give superior performance?
Restrictions. Does the design require header-only?
About binary size.
and a bit of security
If there's a lot of reachable code in the boost library, or code about which the compiler can't argue whether it is reachable by the client, it has to be put into the final binary. (*)
On operating systems that have package management (e.g. RPM- or .deb-based), shared libraries can mean a big decrease in binary distribution size and have a security advantage: Security fixes are distributed faster and are then automatically used by all .so/.DLL users. So you had one recompile and one redistribution, but N profiteers. With a header-only library, you have N recompiles, N redistributions, always for each fix, and some member of those N are huge in themselves already.
(*) reachable here means "potentially executed"
About compilation times.
Some boost libraries are huge. If you would #include it all, each time you change a bit in your source-file, you have to recompile everything you #included.
This can be counter-measured with cherry picked headers, e.g.
#include <boost/huge-boost-library.hpp> // < BAD
#include <boost/huge-boost-library/just-a-part-of-it.hpp> // < BETTER
but sometimes the stuff you really need to include is already big enough to cripple your recompiles.
The countermeasure is to make it a static or shared library, in turn meaning "compile completely exactly once (until the next boost update)".
About runtime performance.
We are still not in an age were global optimization solves all of our C++ performance problems. To make sure you give the compiler all the information it needs, you can make stuff header-only and let the compiler make inlining decisions.
In that respect, note that inlining gives not always superior performance because of caching and speculation issues on the CPU.
Note also that this argument is mostly with regards to boost libraries that might be used frequently enough, e.g. one could expect boost::shared_ptr<> to be used very often, and thus be a relevant performance factor.
But consider the real and only relevant reason boost::shared_ptr<> is header-only ...
About restrictions.
Some stuff in C++ can not be put into libraries, namely templates and enumerations.
But note that this is only halfway true. You can write typesafe, templated interfaces to your real data structures and algorithms, which in turn have their runtime-generic implementation in a library.
Likewise, some stuff in C++ should be put into source files, and in case of boost, libraries. Basically, this is everything that would give "multiple definition" errors, like static member variables or global variables in general.
Some examples can also be found in the standard library: std::cout is defined in the standard as extern ostream cout;, and so cout basically requires the distribution of something (library or sourcefile) that defines it once and only once.
I've been reading a few gaming books. And they always prefer to create the engine as a static library over dynamic link. I am new to c++ so I am not highly knowledge when it comes to static libraries and dynamic link libraries. All I know is static libraries increase the size of your program, where DLL link libraries are loaded as you need them within your program.
[edit]
I've played games where it almost seemed they used DLL's to load in sound, lighting, and what not all individually. as the level was loading up. cause you don't necessarily need that when your at the game menu.
Dynamic link libraries need to be position independent; this can cause performance inefficiencies on some processor architectures.
Static libraries can be optimized when included in your program, e.g., by stripping dead code. This can improve cache performance.
By position independent, he means that since the game engine and DLL are completely separated, the DLL is stand-alone and cannot be interwoven into the game engine code, whereas statically linking a library allows the compiler to optimize using both your game engine code AND the library code.
For example, say there's a small function that the compiler thinks should be inlined (copied directly in place of a function call). Then with a static library, the compiler would be able to inline this code, since it knows what the code is (you're linking at compile-time). However, with a dynamic library, the compiler would be unable to inline that code, since it does not know what the code is (since it will be linking at run-time).
Another often overlooked reason which deserves mention is that for many games you aren't going to be running lots of other stuff, and many libraries that are used for games aren't going to be used for the other things that you may be running at the same time as a game, so you don't have to worry about one of the major positives you get from using shared libraries, which is that only one copy of (most of) the library needs to loaded at one time while several things can make use of that one copy. When running a game you will probably only have one program that would want to use that library running anyway because you probably aren't going to be running many other programs (particularly other games or 3D programs) at the same time.
You also open up the possibility of global/link time optimization, which is much more difficult with shared libraries.
Another question covers the differences between static and dynamic libraries: When to use dynamic vs. static libraries
As for why they use static libraries, the extra speed may be worth it and you can avoid DLL hell (was a big problem in the past). It's also useful if you want to distribute your program and libraries together, ensuring the recipient has the correct dependencies, though there's nothing stopping you from distributing DLLs together with the executable.
When developing games for a console, often dynamic linking isn't an option. If you want to use the engine for both console and PC development, it would be best to avoid dynamic linking.