I am trying to limit the ABI of a shared library using the gcc's fvisibility feature. However I am confused what is the correct way to do it.
My makefile organizes the build process in two stages. At the first step all .cpp files are built to object files using some gcc options. Then all the object files are linked together using another set of gcc and ld options. From what I have read fvisibility is relevant to the second step. However this contradicts with the results I observer. If I add fvisibility=hidden to the compile time options the result is as expected, nm -D reporting a much smaller set of exported symbols. On the contrary if I add it to the link time options it does not seem to affect the build.
While looking for an explanation I have compared the object files produced with and without fvisibility. The difference seems to be in the addresses of the symbols inside the object file. However I am not aware how that difference in addresses carries the message to the linker so that it is able to hide the symbols in one of the cases and expose them in the other.
Could anyone please explain to me that. Thank you for your time.
Compile time, as the visibility is placed in the object (.o) files, and then used by the linker when creating the complete executable/shared object. When using it at link time, but not compile time, it will have no effect, as the visibility in the object files is still default. There's also no need to use it at link time at all I've found.
In the case of how the visibility is stored, the different symbols are probably in different segments, and they get their visibility from the options of the segment.
You may find http://gcc.gnu.org/wiki/Visibility to be helpful
Related
How does the compilation and linking process work?
(Note: This is meant to be an entry to Stack Overflow's C++ FAQ. If you want to critique the idea of providing an FAQ in this form, then the posting on meta that started all this would be the place to do that. Answers to that question are monitored in the C++ chatroom, where the FAQ idea started out in the first place, so your answer is very likely to get read by those who came up with the idea.)
The compilation of a C++ program involves three steps:
Preprocessing: the preprocessor takes a C++ source code file and deals with the #includes, #defines and other preprocessor directives. The output of this step is a "pure" C++ file without pre-processor directives.
Compilation: the compiler takes the pre-processor's output and produces an object file from it.
Linking: the linker takes the object files produced by the compiler and produces either a library or an executable file.
Preprocessing
The preprocessor handles the preprocessor directives, like #include and #define. It is agnostic of the syntax of C++, which is why it must be used with care.
It works on one C++ source file at a time by replacing #include directives with the content of the respective files (which is usually just declarations), doing replacement of macros (#define), and selecting different portions of text depending of #if, #ifdef and #ifndef directives.
The preprocessor works on a stream of preprocessing tokens. Macro substitution is defined as replacing tokens with other tokens (the operator ## enables merging two tokens when it makes sense).
After all this, the preprocessor produces a single output that is a stream of tokens resulting from the transformations described above. It also adds some special markers that tell the compiler where each line came from so that it can use those to produce sensible error messages.
Some errors can be produced at this stage with clever use of the #if and #error directives.
Compilation
The compilation step is performed on each output of the preprocessor. The compiler parses the pure C++ source code (now without any preprocessor directives) and converts it into assembly code. Then invokes underlying back-end(assembler in toolchain) that assembles that code into machine code producing actual binary file in some format(ELF, COFF, a.out, ...). This object file contains the compiled code (in binary form) of the symbols defined in the input. Symbols in object files are referred to by name.
Object files can refer to symbols that are not defined. This is the case when you use a declaration, and don't provide a definition for it. The compiler doesn't mind this, and will happily produce the object file as long as the source code is well-formed.
Compilers usually let you stop compilation at this point. This is very useful because with it you can compile each source code file separately. The advantage this provides is that you don't need to recompile everything if you only change a single file.
The produced object files can be put in special archives called static libraries, for easier reusing later on.
It's at this stage that "regular" compiler errors, like syntax errors or failed overload resolution errors, are reported.
Linking
The linker is what produces the final compilation output from the object files the compiler produced. This output can be either a shared (or dynamic) library (and while the name is similar, they haven't got much in common with static libraries mentioned earlier) or an executable.
It links all the object files by replacing the references to undefined symbols with the correct addresses. Each of these symbols can be defined in other object files or in libraries. If they are defined in libraries other than the standard library, you need to tell the linker about them.
At this stage the most common errors are missing definitions or duplicate definitions. The former means that either the definitions don't exist (i.e. they are not written), or that the object files or libraries where they reside were not given to the linker. The latter is obvious: the same symbol was defined in two different object files or libraries.
This topic is discussed at CProgramming.com:
https://www.cprogramming.com/compilingandlinking.html
Here is what the author there wrote:
Compiling isn't quite the same as creating an executable file!
Instead, creating an executable is a multistage process divided into
two components: compilation and linking. In reality, even if a program
"compiles fine" it might not actually work because of errors during
the linking phase. The total process of going from source code files
to an executable might better be referred to as a build.
Compilation
Compilation refers to the processing of source code files (.c, .cc, or
.cpp) and the creation of an 'object' file. This step doesn't create
anything the user can actually run. Instead, the compiler merely
produces the machine language instructions that correspond to the
source code file that was compiled. For instance, if you compile (but
don't link) three separate files, you will have three object files
created as output, each with the name .o or .obj
(the extension will depend on your compiler). Each of these files
contains a translation of your source code file into a machine
language file -- but you can't run them yet! You need to turn them
into executables your operating system can use. That's where the
linker comes in.
Linking
Linking refers to the creation of a single executable file from
multiple object files. In this step, it is common that the linker will
complain about undefined functions (commonly, main itself). During
compilation, if the compiler could not find the definition for a
particular function, it would just assume that the function was
defined in another file. If this isn't the case, there's no way the
compiler would know -- it doesn't look at the contents of more than
one file at a time. The linker, on the other hand, may look at
multiple files and try to find references for the functions that
weren't mentioned.
You might ask why there are separate compilation and linking steps.
First, it's probably easier to implement things that way. The compiler
does its thing, and the linker does its thing -- by keeping the
functions separate, the complexity of the program is reduced. Another
(more obvious) advantage is that this allows the creation of large
programs without having to redo the compilation step every time a file
is changed. Instead, using so called "conditional compilation", it is
necessary to compile only those source files that have changed; for
the rest, the object files are sufficient input for the linker.
Finally, this makes it simple to implement libraries of pre-compiled
code: just create object files and link them just like any other
object file. (The fact that each file is compiled separately from
information contained in other files, incidentally, is called the
"separate compilation model".)
To get the full benefits of condition compilation, it's probably
easier to get a program to help you than to try and remember which
files you've changed since you last compiled. (You could, of course,
just recompile every file that has a timestamp greater than the
timestamp of the corresponding object file.) If you're working with an
integrated development environment (IDE) it may already take care of
this for you. If you're using command line tools, there's a nifty
utility called make that comes with most *nix distributions. Along
with conditional compilation, it has several other nice features for
programming, such as allowing different compilations of your program
-- for instance, if you have a version producing verbose output for debugging.
Knowing the difference between the compilation phase and the link
phase can make it easier to hunt for bugs. Compiler errors are usually
syntactic in nature -- a missing semicolon, an extra parenthesis.
Linking errors usually have to do with missing or multiple
definitions. If you get an error that a function or variable is
defined multiple times from the linker, that's a good indication that
the error is that two of your source code files have the same function
or variable.
GCC compiles a C/C++ program into executable in 4 steps.
For example, gcc -o hello hello.c is carried out as follows:
1. Pre-processing
Preprocessing via the GNU C Preprocessor (cpp.exe), which includes
the headers (#include) and expands the macros (#define).
cpp hello.c > hello.i
The resultant intermediate file "hello.i" contains the expanded source code.
2. Compilation
The compiler compiles the pre-processed source code into assembly code for a specific processor.
gcc -S hello.i
The -S option specifies to produce assembly code, instead of object code. The resultant assembly file is "hello.s".
3. Assembly
The assembler (as.exe) converts the assembly code into machine code in the object file "hello.o".
as -o hello.o hello.s
4. Linker
Finally, the linker (ld.exe) links the object code with the library code to produce an executable file "hello".
ld -o hello hello.o ...libraries...
On the standard front:
a translation unit is the combination of a source files, included headers and source files less any source lines skipped by conditional inclusion preprocessor directive.
the standard defines 9 phases in the translation. The first four correspond to preprocessing, the next three are the compilation, the next one is the instantiation of templates (producing instantiation units) and the last one is the linking.
In practice the eighth phase (the instantiation of templates) is often done during the compilation process but some compilers delay it to the linking phase and some spread it in the two.
The skinny is that a CPU loads data from memory addresses, stores data to memory addresses, and execute instructions sequentially out of memory addresses, with some conditional jumps in the sequence of instructions processed. Each of these three categories of instructions involves computing an address to a memory cell to be used in the machine instruction. Because machine instructions are of a variable length depending on the particular instruction involved, and because we string a variable length of them together as we build our machine code, there is a two step process involved in calculating and building any addresses.
First we laying out the allocation of memory as best we can before we can know what exactly goes in each cell. We figure out the bytes, or words, or whatever that form the instructions and literals and any data. We just start allocating memory and building the values that will create the program as we go, and note down anyplace we need to go back and fix an address. In that place we put a dummy to just pad the location so we can continue to calculate memory size. For example our first machine code might take one cell. The next machine code might take 3 cells, involving one machine code cell and two address cells. Now our address pointer is 4. We know what goes in the machine cell, which is the op code, but we have to wait to calculate what goes in the address cells till we know where that data will be located, i.e. what will be the machine address of that data.
If there were just one source file a compiler could theoretically produce fully executable machine code without a linker. In a two pass process it could calculate all of the actual addresses to all of the data cells referenced by any machine load or store instructions. And it could calculate all of the absolute addresses referenced by any absolute jump instructions. This is how simpler compilers, like the one in Forth work, with no linker.
A linker is something that allows blocks of code to be compiled separately. This can speed up the overall process of building code, and allows some flexibility with how the blocks are later used, in other words they can be relocated in memory, for example adding 1000 to every address to scoot the block up by 1000 address cells.
So what the compiler outputs is rough machine code that is not yet fully built, but is laid out so we know the size of everything, in other words so we can start to calculate where all of the absolute addresses will be located. the compiler also outputs a list of symbols which are name/address pairs. The symbols relate a memory offset in the machine code in the module with a name. The offset being the absolute distance to the memory location of the symbol in the module.
That's where we get to the linker. The linker first slaps all of these blocks of machine code together end to end and notes down where each one starts. Then it calculates the addresses to be fixed by adding together the relative offset within a module and the absolute position of the module in the bigger layout.
Obviously I've oversimplified this so you can try to grasp it, and I have deliberately not used the jargon of object files, symbol tables, etc. which to me is part of the confusion.
I have a very huge executable built on IBM AIX. When I enable function level linking, the size of the task is 2.8GB, whereas when I disable function level linking task size goes up to 3.50GB.
This would most likely mean that there's additional object files that are pulled in which my application doesnt need, right? If so, how can I find the symbols that are removed with function level linking.
I tried to look at nm output on both tasks, but was clueless on what to look for, and what to diff
You need to add -Wl,--print-gc-sections to LDFLAGS.
Motivation
I have 2 static libraries, libStatic1.a and libStatic2.a. In addition, I have many SOs (Shared Objects) that compile with libStatic1.a.
Up until now, libStatic1.a and libStatic2.a were independent and everything was okay. But now I added to the code that generates libStatic1.a a dependency on the code that generates libStatic2.a. Therefore, any SO that depends on libStatic1.a now needs to be compiled with libStatic2.a. This is undesirable, because it adds a dependency on libStatic2.a to every build targets that depends on libStatic1.a.
only on libStatic1.a now need to compile their code with libStatic2.a in order for the compilation/runtime to succeed/not crash. This creates an unnecessary coupling and I would like to avoid it.
Therefore, I need to somehow "embed" the object code of libStatic2.a in libStatic1.a. If I would just compile libStatic1.a with all the object files of libStatic2.a (in addition to its own), it will basically contain it but this creates another problem- If some user of libStatic1.a will decide to use libStatic2.a and will link it, he will get a weird "multiple definitions" error. If I could somehow tell the compiler to generate the object files of libStatic2.a with weak symbols (only for the use in libStatic1.a) this would solve the problem- no one will get multiple definitions, and no makefile of all the many SOs that use libStatic1.a will need to change.
My thinking: I know that it is possible (using GCC/g++ extensions to the C language) to declare a function with the keyword __attribute__ and the weak attribute as following:
void __attribute__((weak)) foo(int j);
Is there a way to tell the compiler (g++) to compile an entire compilation unit as "weak", meaning all its global symbols in the symbol table will be considered weak when linking?
Alternatively, is there a way to tell the linker (ld) to consider all the symbols of some object file/library as if they are weak?
If your library is small, the simplest way is still to change the declarations by adding manually the __attribute__((weak)).
Another possibility might be to ask g++ to spill the assembly code (with -S) and have some (perhaps awkor ed) script work on it.
You could also code a GCC plugin (assuming your g++ is a 4.6 version) or a GCC MELT extension for that.
Compile it normally and then objcopy the object file with --weaken.
No, there doesn't appear to be; are there so many weak external functions that it's not practical to set their attributes individually?
Prior to today I had always believed that the order that objects and libraries were passed to g++ during the linking stage was unimportant. Then, today, I tried to link from c++ code to c code. I wrapped all the C headers in an extern "C" block but the linker still had difficulties finding symbols which I knew were in the C object archives.
Perplexed, I created a relatively simple example to isolate the linking error but much to my surprise, the simpler example linked without any problems.
After a little trial and error, I found that by emulating the linking pattern used in the simple example, I could get the main code to link OK. The pattern was object code first, object archives second eg:
g++ -o serverCpp serverCpp.o algoC.o libcrypto.a
Can anyone shed some light on why this might be so?. I've never seen this problem when linking ordinary c++ code.
The order you specify object files and libraries is VERY important in GCC - if you haven't been bitten by this before you have lead a charmed life. The linker searches symbols in the order that they appear, so if you have a source file that contains a call to a library function, you need to put it before the library, or the linker won't know that it has to resolve it. Complex use of libraries can mean that you have to specify the library more than once, which is a royal pain to get right.
The library order pass to gcc/g++ does actually matter. If A depends on B, A must be listed first. The reason is that it optimizes out symbols that aren't referenced, so if it sees library B first, and no one has referenced it at that point then it won't link in anything from it at all.
A static library is a collection of object files grouped into an archive. When linking against it, the linker only chooses the objects it needs to resolve any currently undefined symbols. Since the objects are linked in order given on the command line, objects from the library will only be included if the library comes after all the objects that depend on it.
So the link order is very important; if you're going to use static libraries, then you need to be careful to keep track of dependencies, and don't introduce cyclic dependencies between libraries.
You can use --start-group archives --end-group
and write the 2 dependent libraries instead of archives:
gcc main.o -L. -Wl,--start-group -lobj_A -lobj_b -Wl,--end-group
I have an ARM project that I'm building with make. I'm creating the list of object files to link based on the names of all of the .c and .cpp files in my source directory. However, I would like to exclude objects from being linked if they are never used. Will the linker exclude these objects from the .elf file automatically even if I include them in the list of objects to link? If not, is there a way to generate a list of only the objects that need to be linked?
You have to compile your code differently to strip out function and data that isn't used. Usually all the objects are compiled into the same symbol, so they can't be individually omitted if they're not used.
Add the two following switches to your compiler line:
-ffunction-sections -fdata-sections
When you compile, the compiler will now put individual functions and data into their own sections instead of lumping them all in one module section.
Then, in your linker, specify the following:
--gc-sections
This instructs the linker to remove unused sections ("gc" is for garbage collection). It will garbage collect parts of files and entire files. For example, if you're compiling an object, but only use 1 function of 100 in the object, it will toss out the other 99 you're not using.
If you run into issues with functions not found (it happens due to various reasons like externs between libraries), you can use .keep directives in your linker file (*.ld) in order to prevent garbage collection on those individual functions.
If you are using RealView, it seems that it is possible. This section discusses it:
3.3.3 Unused section elimination
Unused section elimination removes code that is never executed, or data that is not
referred to by the code, from the final image. This optimization can be controlled by the
--remove, --no_remove, --first, --last, and --keep linker options. Use the --info unused
linker option to instruct the linker to generate a list of the unused sections that have been
eliminated.
Like many people said, the answer is "depends". In my experience, RVCT is very good about dead code stripping. Unused code and data will almost always be removed in the final link stage. GCC, on the other hand (at least without the LLVM back end), is rather poor at whole image static analysis and will not do a very good job at removing unused code (and woe be it to you if your code is in different sections requiring long jumps). You can take some steps to mitigate it, such as using function-sections, which creates a separate section for each function and enables some better dead code stripping.
Have your linker generate a map file of your binary so you can see what made it in there and what got stripped out.
Depending on the sophistication of the compiler/linker and optimization level, the linker will not link in code that isn't being called.
What compiler/linker are you using? Some linkers do this automatically, and some provide the feature as a command-line option.
In my experience, many compilers will not include unused code on an object file basis. Some may not have this resolution and will include entire libraries ("because this makes the build process faster").
For example, given a file junk.c and it has three functions: Func1, Func2 and Func3. The build process creates an object file, junk.o, which has all three functions in it. If function Func2 is not used, it will be included anyway because the linker can't exclude one function out of an object file.
On the other hand, given files: Func1.c, Func2.c, and Func3.c, with the functions above, one per file. If Func2 in Func2.c is not used, the linker will not include it.
Some linkers are intelligent enough to exclude files out of libraries. However, each linker is different on its granularity of file inclusion (and thus file exclusion). Read your linker's manual or contact their customer support for exact information.
I suggest moving the suspect functions into a separate file (one function per file) and rebuild. Measure the code size before and after. Also, there may be a difference between Debug and Release linking. The Debug linking could be lazy and just throw everything in while the Release linking puts more effort into removing unused code.
Just my thoughts and experience, Your Mileage May Vary (YMMV).
Traditionally linkers link in all object files that are explicity specified in the command line, even if they could be left out and the program would not have any unresolved symbols. This means that you can deliberately change the behaviour of a program by including an object file that does something triggered from static initialization but is not called directly or indirectly from main.
Typically if you place most of your object files in a static library and link this library with a single object file containing your entry point the linker will only pick out members of the library (iteratively) that help resolve an unresolved symbol reference in the original object file or one included subsequently because it resolved a previous unresolved symbol.
In short, place most of your object files in a library and just link this with one object containing your entry point.