This isn't as naive as the title may lead you to think.
I receive an "Undefined Reference" Error from the linker/ld for a couple of function symbols in third party shared library, that I'm trying to link with my executable.
The strange part is, that library itself is supposed to contain the definition for the offending symbols in question.
The stranger part is that there are several executables in my project, with some facing this problem and some not.
Have I messed up my gcc/ld flags or is it something else?
Have you checked your link order? This has gotten stricter in recent versions of GCC.
For example a common problem is caused by doing this:
g++ -lX11 -lSuperLibrary awesomeApp.cpp
instead of this:
g++ awesomeApp.cpp -lX11 -lSuperLibrary
It also matters which order the library flags are, if they are interdependent.
There are flags for doing a library multi-pass which might help you solve your problem. To to the same as above but with the linker 'grouping' libraries and doing a recursive link on them (at a linker performance cost) you could do something like the following:
g++ awesomeApp.cpp -Wl,--start-group -lX11 -lSuperLibrary -Wl,--end-group
Where -Wl,<option> passes an option to the linker... in this case --start-group and --end-group .
A great analogy of why link order is important is here
I hope that helps.
Impossible to say from your description, which contains approximately zero useful information. One possibility is that you are not including the correct header file in some source files.
Related
Suppose we call
gcc -Dmyflag -lmylib mycode.c
where mylib contains all of mycode but is compiled without -Dmyflag. So all functions and other entities implemented in mycode are available in two versions to the loader. Empirically, I find that the version from mycode is taken. Can I rely on that? Will mycode always overwrite mylib?
Empirically, I find that the version from mycode is taken.
Read this explanation of how linker works with archive libraries, and possibly this one.
Can I rely on that?
You should rely on understanding how this works.
If you understood material in referenced links, you'll observe that adding main to libmylib.a will invert the answer (and if mycode.c also contains main, you'll get duplicate symbol definition error).
If you are using a dynamic library libmylib.so, the rules are different, and the library will always lose to the main binary, although there are many complications, such as LD_PRELOAD, linking the library with -Bsymbolic, and others.
In short, you should prefer to not do this at all.
Normally, I would compile a program that requires a specific library, e.g. math, by passing the linker flag after the sources that need it like so:
gcc foo.c -lm
However, it seems that older versions of gcc work equally well with the reverse order (let's call this BAD ORDER):
gcc -lm foo.c
I wouldn't worry about it if some popular open-source projects I'm trying to compile didn't use the latter while my version of gcc (or is it ld that's the problem?) work only in the former case (also, the correct one in my opinion).
My question is: when did the BAD ORDER stop working and why? It seems that not supporting it breaks legacy packages.
when did the BAD ORDER stop working and why? It seems that not supporting it breaks legacy packages.
When?
Not dead sure but I think pre-GCC 4.5. Long ago. Subsequently, the --as-needed option is operative for shared libraries by default,
so like static libraries, they must occur in the linkage sequence later than the objects for which they provide definitions.
This is a change in the default options that the gcc/g++/gfortran etc. tool-driver passes to ld.
Why?
It was considered confusing to inexpert users that static libraries by default has to appear
later that the objects to which they provided definitions while shared libraries by default did
not - the difference between the two typically being concealed by the -l<name> convention
for linking either libname.a or libname.so.
It was perhaps an unforeseen consequence that inexpert users who
had formerly had a lot of luck with the mistaken belief that a GCC
[compile and] link command conforms to the normal Unix pattern:
command [OPTION...] FILE [FILE...]
e.g.
gcc -lthis -lthat -o prog foo.o bar.o
now fare much worse with it.
In what cases exactly do you need -all_load flag?
Lets say I have something like
g++ source.cpp -o test libA.a libB.a libC.a
From what i recall if there is some reference to a symbol used in source.cpp that is present
in say libB.a file then that libB.a will be linked (just that symbol or whole code in that library? ) and libA.a and libC.a will be ignored (their code will not be present in final executable).
What happens to other libraries when i use -all_load flag as follows
g++ source.cpp -o test -Wl,-all_load libA.a libB.a libC.a
how does 'strip' command effect the output with all_load flag?
-all_load is for when you want to link compile units that are (to the linker) unnecessary. For instance, perhaps you will dynamically access functions within the static library at runtime that you know the addresses of, but haven't actually made any explicit function calls to. How would you do that? Well, the compiler could help you by storing a bunch of function pointers in the executable to be read at run time, and then you'd build a lookup system for finding those functions using a string, and you'd call the whole thing Objective-C, which is probably the most common user of -all_load (at least if Google is any guide).
The most common case of this in ObjC is when you have a category in its own compile unit. The complier may not be able to tell that you reference it and so won't link it. So ObjC programmers use -all_load (or -force_load) more often than other C-like programmers. In fact, -all_load is a Darwin-specific extension in gcc.
But there are cases where people might want to use -all_load outside of ObjC. For instance, there might be some inter-dependencies in libA and libB. Consider this case:
source.cpp requires A() and B()
libA defines A() in a.o and Aprime() in aprime.o
libB defines B() in b.o and requires Aprime()
This typically won't link (*). The compiler will start with source.o and make a list of requirements: A() and B(). It'll then look at libA and see that it defines A(), so it'll link a.o (but not aprime.o). Then it will look at libB and see that it defines B() and requires Aprime(). It is now out of libraries, and it hasn't resolved Aprime(). It fails.
(*) Actually, it will with clang because clang is quite smart about this. But it won't with g++ at least up through 4.6.
The best solution would be to reorder it so that libB comes first (**). But if the dependencies were circular, you could get completely stuck. -all_load and -force_load let you work around these situations by turning off the linker's optimization.
(**) The really best solution is usually to redesign your libraries to avoid this kind of interdependency, but that may be hoping too much.
If you want to play around with the issue, see https://gist.github.com/rnapier/5710509.
strip just removes symbols from executables. That's not particularly related to static linking and -all_load (though it does impact dynamic linking). strip(1) has lots of discussion of that.
Is there any way to tell the compiler (gcc/mingw32) when building an object file (lib*.o) to only expose certain functions from the .c file?
The reason I want to do this is that I am statically linking to a 100,000+ line library (SQLite), but am only using a select few of the functions it offers. I am hoping that if I can tell the compiler to only expose those functions, it will optimize out all the code of the functions that are never needed for those few I selected, thus dratically decreasing the size of the library.
I found several possible solutions:
This is what I asked about. It is the gcc equivalent of Windows' dllexpoort:
http://gcc.gnu.org/onlinedocs/gcc-4.6.1/gcc/Code-Gen-Options.html (-fvisibility)
http://gcc.gnu.org/wiki/Visibility
I also discovered link-time code-generation. This allows the linker to see what parts of the code are actually used and get rid of the rest. Using this together with strip and -fwhole-program has given me drastically better results.
http://gcc.gnu.org/onlinedocs/gcc-4.6.1/gcc/Optimize-Options.html (see -flto and -fwhole-program)
Note: This flag only makes sense if you are not compiling the whole program in one call to gcc, which is what I was doing (making a sqlite.o file and then statically linking it in).
The third option which I found but have not yet looked into is mentioned here:
How to remove unused C/C++ symbols with GCC and ld?
That's probably the linkers job, not the compilers. When linking that as a program (.exe), the linker will take care of only importing the relevant symbols, and when linking a DLL, the __dllexport mechanism is probably what you are looking for, or some flags of ld can help you (man ld).
I can never remember what to type when linking include files in GCC, in fact the only one I can remember is -lm for math.h. The one I am specifically concerned with right now is sys/time.h.
This page clears things up some, but I would still like a list.
Does anyone know of a good list of linking options?
EDIT:
Maybe my question was not clear. I want to know what I need to type at the command line (like -lm for math or -lpthread for pthread) for the various libraries I might need to link when making C programs.
The functionality provided in <sys/time.h> is implemented in libc.so (C library). You don't need to link anything else in as gcc should automatically link to libc.so by itself. There is no 'linking of include files', rather you are linking against libraries that contain the symbols defined by code.
The -l flag is one of GCC's linker options and is used to specify additional libraries to link against.
edit because my gcc was performing optimizations on my source code at compile time
Also, the information in that link is a little outdated - you should not need an explicit link to libm (which is what -l m or -lm does) in modern GCC.
I'm not sure i understand your question but -lm is not an ld option, -l is an option and -lx links libx.a (or .so, it depends). you might want to look at the ld manual for a full list of options.
I think all other standard libraries other than math are included in libc.so(.a) (-lc)