I'm a little confused about the concept "library" in rust, which is mentioned from "A crate is a binary or library".
If I'm right, a binary means an executable program (which can be run from shell, for example), but what is a library?
Are they some sort of object files with symbols like .a or .so, which will be linked to my program (like C/C++)
Or they are pure source codes which will be compiled together with my program?
As described by Masklinn, yes, Rust does have prebuilt library formats. However, these are mostly used internally, are finnicky for different compiler versions, and cargo still lacks support for them. In fact, crates.io requires libraries to be "open-source" (as in, you provide the source code, you could still have the source code load from some closed-source dependency), and it distributes the source code to whoever downloads the crate. Then, the source code is effectively compiled with your program (this is where rlibs come in to play, but cargo doesn't expose this to the user). This is also why you're able to inspect the source code for pretty much every crate.
If I'm right, a binary means an executable program (which can be run from shell, for example), but what is a library?
Yes. Specifically, per the Linkage documentation
A runnable executable will be produced. This requires that there is a main function in the crate which will be run when the program begins executing. This will link in all Rust and native dependencies, producing a single distributable binary. This is the default crate type.
Are they some sort of object files with symbols like .a or .so, which will be linked to my program (like C/C++)
Or they are pure source codes which will be compiled together with my program?
Never strictly the latter, but the exact artefact depends, as per the linkage documentation:
A Rust library will be produced. This is an ambiguous concept as to what exactly is produced because a library can manifest itself in several forms. The purpose of this generic lib option is to generate the "compiler recommended" style of library. The output library will always be usable by rustc, but the actual type of library may change from time-to-time.
The documentation then lists the various types of libraries:
rlib, a static library with rust-specific metadata (an augmented .a)
dylib, a dynamic library with rust-specific metadata (an augmented .so)
staticlib, a system static library (an actual .a)
cdylib, a system dynamic library (an actual .so)
I would think "lib" aliases to "rlib" but frankly I have no idea, and as the quote notes that's neither fixed nor documented by design.
Related
I'm encountering an issue which has been elaborated in a good article Shared Library Symbol Conflicts (on Linux). The problem is that when the execution and .so have defined the same name functions, if the .so calls this function name, it would call into that one in execution rather than this one in .so itself.
Let's talk about the case in this article. I understand the DoLayer() function in layer.o has an external function dependency of DoThing() when compiling layer.o.
But when compiling the libconflict.so, shouldn't the external function dependency be resolved in-place and just replaced with the address of conflict.o/DoThing() statically?
Why does the layer.o/DoLayer() still use dynamic linking to find DoThing()? Is this a designed behavior?
Is this a designed behavior?
Yes.
At the time of introduction of shared libraries on UNIX, the goal was to pretend that they work just as if the code was in a regular (archive) library.
Suppose you have foo() defined in both libfoo and libbar, and bar() in libbar calls foo().
The design goal was that cc main.c -lfoo -lbar works the same regardless of whether libfoo and libbar are archive or a shared libraries. The only way to achieve this is to have libbar.so use dynamic linking to resolve call from bar() to foo(), despite having a local version of foo().
This design makes it impossible to create a self-contained libbar.so -- its behavior (which functions it ends up calling) depends on what other functions are linked into the process. This is also the opposite of how Windows DLLs work.
Creating self-contained DSOs was not a consideration at the time, since UNIX was effectively open-source.
You can change the rules with special linker flags, such as -Bsymbolic. But the rules get complicated very quickly, and (since that isn't the default) you may encounter bugs in the linker or the runtime loader.
Yes, this is a designed behavior. When you link a program into a binary, all the references to named external (non-static) functions are resolved to point into the symbol table for the binary. Any shared libraries that are linked against are specified as DT_NEEDED entries.
Then, when you run the binary, the dynamic linker loads each required shared library to a suitable address and resolves each symbol to an address. Sometimes this is done lazily, and sometimes it is done once at first startup. If there are multiple symbols with the same name, one of them will be chosen by the linker, and your program will likely crash since you may not end up with the right one.
Note that this is the behavior on Linux, which has all symbols as a flat namespace. Windows resolves symbols differently, using a tree topology, which has both advantages (fewer conflicts) and disadvantages (the inability to allocate memory in one library and free it in another).
The Linux behavior is very important if you want things like LD_PRELOAD to work. This allows you to use debugging tools like Electric Fence and CPU profiling tools like the Google performance tools, or replace a memory allocator at runtime. None of these things would work if symbols were preferentially resolved to their binary or shared library.
The GNU dynamic linker does support symbol versions, however, so that it's possible to load multiple versions of a shared library into the same program. Oftentimes distros like Debian will do this with libraries they expect to change frequently, like OpenSSL. If the program uses liba which uses OpenSSL 1.0 and libb which uses OpenSSL 1.1, then the program should still function in such a case since OpenSSL has versioned symbols, and each library will use the appropriate version of the relevant symbol.
There's a well-known technique for interposing dynamically linked binaries: creating a shared library and and using LD_PRELOAD variable. But it doesn't work for statically-linked binaries.
One way is to write a static library that interpose the functions and link it with the application at compile time. But this isn't practical because re-compiling isn't always possible (think of third-party binaries, libraries, etc).
So I am wondering if there's a way to interpose statically linked binaries in the same LD_PRELOAD works for dynamically linked binaries i.e., with no code changes or re-compilation of existing binaries.
I am only interested in ELF on Linux. So it's not an issue if a potential solution is not "portable".
One way is to write a static library that interpose the functions and link it with the application at compile time.
One difficulty with such an interposer is that it can't easily call the original function (since it has the same name).
The linker --wrap=<symbol> option can help here.
But this isn't practical because re-compiling
Re-compiling is not necessary here, only re-linking.
isn't always possible (think of third-party binaries, libraries, etc).
Third-party libraries work fine (relinking), but binaries are trickier.
It is still possible to do using displaced execution technique, but the implementation is quite tricky to get right.
I'll assume you want to interpose symbols in main executable which came from a static library which is equivalent to interposing a symbol defined in executable. The question thus reduces to whether it's possible to intercept a function defined in executable.
This is not possible (EDIT: at least not without a lot of work - see comments to this answer) for two reasons:
by default symbols defined in executable are not exported so not accessible to dynamic linker (you can alter this via -export-dynamic or export lists but this has unpleasant performance or maintenance side effects)
even if you export necessary symbols, ELF requires executable's dynamic symtab to be always searched first during symbol resolution (see section 1.5.4 "Lookup Scope" in dsohowto); symtab of LD_PRELOAD-ed library will always follow that of executable and thus won't have a chance to intercept the symbols
What you are looking for is called binary instrumentation (e.g., using Dyninst or ptrace). The idea is you write a mutator program that attaches to (or statically rewrites) your original program (called mutatee) and inserts code of your choice at specific points in the mutatee. The main challenge usually revolves around finding those insertion points using the API provided by the instrumentation engine. In your case, since you are mainly looking for static symbols, this can be quite challenging and would likely require heuristics if the mutatee is stripped of non-dynamic symbols.
This question was asked before Rust officially supported incremental compilation. Rust 1.24.0 and later enable incremental compilation by default for development (debug) builds.
I'm an outsider trying to see if Rust is appropriate for my projects.
I've read that Rust lacks incremental compilation (beta features notwithstanding).
Is this similar to having everything be implemented in the headers in C++ (like in much of Boost)?
If the above is correct, does this limit Rust to rather small projects with small dependencies? (If, say, Qt or KDE were header-only libraries, then programs using them would be extremely painful to develop, since you'd effectively recompile Qt/KDE every time you want to compile your own code.)
In C and C++, a compilation unit is usually a source file and all the header files it transitively includes. An application or library is usually comprised of multiple compilation units that are linked together. An application or library can additionally be linked with other libraries. This means that changing a source file requires recompiling that source file only and then relinking, changing an external library only requires relinking, but changing a header file (whether it's part of the project or external; the compiler can't tell the difference) requires recompiling all source files that use it and then relinking.
In Rust, the crate is the compilation unit. (A crate can be an application or a library.) Rust doesn't use header files; instead, the equivalent information is stored as metadata in the compiled crates (which is faster to parse, and has the same effect as precompiled headers in C/C++). A crate can additionally be linked with other crates. This means that changing any of the source files for a crate requires recompiling the whole crate, and changing a crate requires recompiling all crates that depend on it (currently, this means recompiling from source, even if the API happens to not have changed).
To answer your questions, no, Rust doesn't recompile all dependencies every time you recompile your project; quite the opposite in fact.
Incremental compilation in Rust is about reusing the work done in previous compilations of a crate to speed up compilation times. For example, if you change a module and it doesn't affect the other modules, the compiler would be able to reuse the data that was generated when the other modules were compiled last time. The lack of incremental compilation is usually only a problem with large or complex crates (e.g. those who make heavy use of macros).
Here is my desired use case:
I have a package with a single module that reads HDF5 files and writes some of their data to Haskell records. To do the work, the library uses the bindings-hdf5 package. Here is my cabal's build-depends. reader-types is a module I wrote that defines the types of the Haskell records that contain the read-in data.
build-depends: base >=4.7 && <4.8
, text
, vector
, containers
, bindings-hdf5
, reader-types
Note that my cabal file does not currently use extra-libraries or ghc-options. I can load my module, src/Mabel.hs in ghci as long as I specify the required hdf5_hl library:
ghci src/Mabel.hs -lhdf5_hl -L/long/nixos/path/lib
and within ghci, I can run my function perfectly fine.
Now, what I want to do is compile this library/module into a single, compiled file that I can later load with the GHC API in a different Haskell program. By single file, I mean that it needs to run even if the hdf5_hl library does not exist on the system. Preferably, it would also run even if text, vector, and/or containers are missing, but this is not essential because reader-types requires those types anyway. When loading the module with the GHC API, I want it to load in already compiled form, and not run interpreted.
My purpose for doing this is that I want the self-contained file to act as a single, pre-compiled plugin file that is later loaded and executed by a different Haskell executable. Other plugins might not use hdf5 at all, and the only package they are guaranteed to use is reader-types, which essentially defines the plugin interface types.
The hdf5 library on my system contains the following files: libhdf5_la.la, libhdf5_hl.so, libhdf5.la, libhdf5.so, and similar files that have the version number in the file name.
I have done a lot of googling, but am getting confused by all the edge cases I am finding. Here are some examples that I'm either sure don't fit my case, or I can't tell.
I do not want to compile a Haskell library to use from C or Python, only a Haskell program using GHC API.
I do not want to compile C wrappers for a C++ library into a Haskell module because the bindings already exist and the library is already a C library.
I do not to want compile a library that is entirely self-contained because, since I am loading it with the GHC API, I don't need the GHC runtime included in the library. (My understanding is that the plugins must be compiled with the same ghc version they will be loaded with in the GHC API).
I do not want to compile C bindings and the C library at the same time because the C library is already compiled and the bindings are specified in separate package (bindings-hdf5).
The closest resource for what I want to do is this exchange on the mailing list from 2009. However, I added extra-libraries: hdf5_hl or extra-libraries: hdf5 to my cabal file, and in both cases the resulting .a, .so, .dyn_hi, .dyn_o, .hi, and .o files in dist/build are all the exact same size as without using extra-libraries, so I'm confident it is not working correctly.
What changes to my cabal file do I need to make to create a self-contained, standalone file that I can later load with the GHC API? If this is not possible, what are the alternatives?
Instead of using the GHC API, I am also open to using the plugins library to load the plugin, but the self-contained requirements are still the same.
EDIT: I do not care what form the compiled "plugin" must take (I assume object file is the right way), but I want to load it dynamically from an separate executable at run time and execute functions it defines with known names and known types. The reason I want a single file is that there will eventually be other different plugins, and I want them all to behave the same way without having to worry about lib paths and dependencies for each one. A compiled, single file is a simpler interface for doing this than zipping/unzipping archives that include Haskell object code and their dependencies.
How to create a shared object that is statically linked with pthreads and libstdc++ on Linux/gcc?
Before I go to answering your question as it was described, I will note that it is not exactly clear what you are trying to achieve in the end, and there is probably a better solution to your problem.
That said - there are two main problems with trying to do what you described:
One is, that you will need to decompose libpthread and libstdc++ to the object files they are made with. This is because ELF binaries (used on Linux) have two levels of "run time" library loading - even when an executable is statically linked, the loader has to load the statically linked libraries within the binary on execution, and map the right memory addresses. This is done before the shared linkage of libraries that are dynamically loaded (shared objects) and mapped to shared memory. Thus, a shared object cannot be statically linked with such libraries, as at the time the object is loaded, all static linked libraries were loaded already. This is one difference between linking with a static library and a plain object file - a static library is not merely glued like any object file into the executable, but still contains separate tables which are referred to on loading. (I believe that this is in contrast to the much simpler static libraries in MS-DOS and classic Windows, .LIB files, but there may be more to those than I remember).
Of course you do not actually have to decompose libpthread and libstdc++, you can just use the object files generated when building them. Collecting them may be a bit difficult though (look for the objects referred to by the final Makefile rule of those libraries). And you would have to use ld directly and not gcc/g++ to link, to avoid linking with the dynamic versions as well.
The second problem is consequential. If you do the above, you will sure have such a shared object / dynamic library as you asked to build. However, it will not be very useful, as once you try to link a regular executable that uses those libpthread/libstdc++ (the latter being any C++ program) with this shared object, it will fail with symbol conflicts - the symbols of the static libpthread/libstdc++ objects you linked your shared object against will clash with the symbols from the standard libpthread/libstdc++ used by that executable, no matter if it is dynamically or statically linked with the standard libraries.
You could of course then try to either hide all symbols in the static objects from libstdc++/libpthread used by your shared library, make them private in some way, or rename them automatically on linkage so that there will be no conflict. However, even if you get that to work, you will find some undesireable results in runtime, since both libstdc++/libpthread keep quite a bit of state in global variables and structures, which you would now have duplicate and each unaware of the other. This will lead to inconsistencies between these global data and the underlying operating system state such as file descriptors and memory bounds (and perhaps some values from the standard C library such as errno for libstdc++, and signal handlers and timers for libpthread.
To avoid over-broad interpretation, I will add a remark: at times there can be sensible grounds for wanting to statically link against even such basic libraries as libstdc++ and even libc, and even though it is becoming a bit more difficult with recent systems and versions of those libraries (due to a bit of coupling with the loader and special linker tricks used), it is definitely possible - I did it a few times, and know of other cases in which it is still done. However, in that case you need to link a whole executable statically. Static linkage with standard libraries combined with dynamic linkage with other objects is not normally feasible.
Edit: One issue which I forgot to mention but is important to take into account is C++ specific. C++ was unfortunately not designed to work well with the classic model of object linkage and loading (used on Unix and other systems). This makes shared libraries in C++ not really portable as they should be, because a lot of things such as type information and templates are not cleanly separated between objects (often being taken, together with a lot of actual library code at compile time from the headers). libstdc++ for that reason is tightly coupled with GCC, and code compiled with one version of g++ will in general only work with the libstdc++ from with this (or a very similar) version of g++. As you will surely notice if you ever try to build a program with GCC 4 with any non-trivial library on your system that was built with GCC 3, this is not just libstdc++. If your reason for wanting to do that is trying to ensure that your shared object is always linked with the specific versions of libstdc++ and libpthread that it was built against, this would not help because a program that uses a different/incompatible libstdc++ would also be built with an incompatible C++ compiler or version of g++, and would thus fail to link with your shared object anyway, aside from the actual libstdc++ conflicts.
If you wonder "why wasn't this done simpler?", a general rumination worth pondering: For C++ to work nicely with dynamic/shared libraries (meaning compatibility across compilers, and the ability to replace a dynamic library with another version with a compatible interface without rebuilding everything that uses it), not just compiler standartization is needed, but at the level of the operating system's loader, the structure and interface of object and library files and the work of the linker would need to be significantly extended beyond the relatively simple Unix classics used on common operating systems (Microsoft Windows, Mach based systems and NeXTStep relatives such as Mac OS, VMS relatives and some mainframe systems also included) for natively built code today. The linker and dynamic loader would need to be aware of such things as templates and typing, having to some extent functionality of a small compiler to actually adapt the library's code to the type given to it - and (personal subjective observation here) it seems that higher-level intermediate intermediate code (together with higher-level languages and just-in-time compilation) is catching ground faster and likely to be standardized sooner than such extensions to the native object formats and linkers.
You mentioned in a separate comment that you are trying to port a C++ library to an embedded device. (I am adding a new answer here instead of editing my original answer here because I think other StackOverflow users interested in this original question may still be interested in that answer in its context)
Obviously, depending on how stripped down your embedded system is (I have not much embedded Linux experience, so I am not sure what is most likely), you may of course be able to just install the shared libstdc++ on it and dynamically link everything as you would do otherwise.
If dynamically linking with libstdc++ would not be good for you or not work on your system (there are so many different levels of embedded systems that one cannot know), and you need to link against a static libstdc++, then as I said, your only real option is static linking the executable using the library with it and libstdc++. You mentioned porting a library to the embedded device, but if this is for the purpose of using it in some code you write or build on the device and you do not mind a static libstdc++, then linking everything statically (aside from perhaps libc) is probably OK.
If the size of libstdc++ is a problem, and you find that your library is actually only using a small part of its interfaces, then I would nonetheless suggest first trying to determine the actual space you would save by linking against only the parts you need. It may be significant or not, I never looked that deep into libstdc++ and I suspect that it has a lot of internal dependencies, so while you surely do not need some of the interfaces, you may or may not still depend on a big part of its internals - I do not know and did not try, but it may surprise you. You can get an idea by just linking a binary using the library against a static build of it and libstdc++ (not forgetting to strip the binary, of course), and comparing the size of the resulting executable that with the total size of a (stripped) executable dynamically linked together with the full (stripped) shared objects of the library and libstdc++.
If you find that the size difference is significant, but do not want to statically link everything, you try to reduce the size of libstdc++ by rebuilding it without some parts you know that you do not need (there are configure-time options for some parts of it, and you can also try to remove some independent objects at the final creation of libstdc++.so. There are some tools to optimize the size of libraries - search the web (I recall one from a company named MontaVista but do not see it on their web site now, there are some others too).
Other than the straightforward above, some ideas and suggestions to think of:
You mentioned that you use uClibc, which I never fiddled with myself (my experience with embedded programming is a lot more primitive, mostly involving assembly programming for the embedded processor and cross-compiling with minimal embedded libraries). I assume you checked this, and I know that uClibc is intended to be a lightweight but rather full standard C library, but do not forget that C++ code is hardly independent on the C library, and g++ and libstdc++ depend on quite some delicate things (I remember problems with libc on some proprietary Unix versions), so I would not just assume that g++ or the GNU libstdc++ actually works with uClibc without trying - I don't recall seeing it mentioned in the uClibc pages.
Also, if this is an embedded system, think of its performance, compute power, overall complexity, and timing/simplicity/solidity requirements. Take into consideration the complexity involved, and think whether using C++ and threads is appropriate in your embedded system, and if nothing else in the system uses those, whether it is worth introducing for that library. It may be, not knowing the library or system I cannot tell (again, embedded systems being such a wide range nowadays).
And in this case also, just a quick link I stumbled upon looking for uClibc -- if you are working on an embedded system, using uClibc, and want to use C++ code on it -- take a look at uClibc++. I do not know how much of the standard C++ stuff you need and it already supports, and it seems to be an ongoing project, so not clear if it is in a state good enough for you already, but assuming that your work is also under development still, it might be a good alternative to GCC's libstdc++ for your embedded work.
I think this guy explains quite well why that wouldn't make sense. C++ code that uses your shared object but a different libstdc++ would link alright, but wouldn't work.