I'm compiling a trivial wxWidgets app on MacOS X 10.6 with XCode 3.2
The linker is return an error about the symbol _main being defined twice:
once in main.mm
once in the test_app.cpp file.
After I commented out the macro:
Implement_App(TestApp)
The error went away, compiled & linked and I was able to run the application.
I haven't found this anywhere so any ideas about this?
IMPLEMENT_APP is a macro used in wxWidgets to create an entry point to the program without worrying about whether the program will be compiled on Windows, Mac, *nix, or whatever. As a result of this, IMPLEMENT_APP has to define main (or its equivalent, such as WinMain).
You might find the IMPLEMENT_APP_NO_MAIN macro to be useful. Check the other IMPLEMENT_APP_XXX functions in wx/app.h, too.
This paragraph from the wxApp overview is a little helpful too:
Note the use of IMPLEMENT_APP(appClass), which allows wxWidgets to dynamically create an instance of the application object at the appropriate point in wxWidgets initialization. Previous versions of wxWidgets used to rely on the creation of a global application object, but this is no longer recommended, because required global initialization may not have been performed at application object construction time.
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I have an application which uses a class Foo from an .so shared library. I've come across a problem where at runtime it prints
<appname>: symbol lookup error: <appname>: undefined symbol: <mangled_Foo_symbol_name>
Now, it turned out that the unmangled symbol was for the constructor of the class Foo, and the problem was simply that an old version of the library was loaded, which didn't contain Foo yet.
My question isn't about resolving the error (that's obviously to use the correct library), but why it appears at runtime instead of at time of load / startup.
The line of code causing the error just instantiates an object of class Foo, so I'm not using anything like dlopen here, at least not explicitly / to my knowledge.
In contrast, if I remove the whole library from the load search path, I get this error at startup:
<appname>: error while loading shared libraries: libname.so.2: cannot open shared object file: No such file or directory
When the wrong version of gcc / libstdc++ is on the load path, an error also appears at starup:
<appname>: /path/to/gcc-4.8.0/lib64/libstdc++.so.6: version `GLIBCXX_3.4.20' not found (required by <appname>)
This "fail fast" behavior is much more desirable, I don't want to run my application for quite awhile first, until I finally realize it's using the wrong library.
What causes the load error to appear at runtime and how can I make it appear immediately?
From the man page of ld.so:
ENVIRONMENT
LD_BIND_NOW (libc5; glibc since 2.1.1) If set to a nonempty string, causes the dynamic linker to resolve all symbols at program startup instead of deferring function call resolution to the point when they are first referenced. This is useful when using a debugger.
LD_WARN (ELF only)(glibc since 2.1.3) If set to a nonempty string, warn about unresolved symbols.
I think you can not statically link .so library. If you want to avoid load/run time errors you have to use all static libraries (.a). If you do not have static version of library and source then try to find some statifier. After googling I find few statifiers but do not know how do they work so leaving that part up to you.
I am attempting to build a project in Visual Studio 2012 that uses GnuTLS. I downloaded the latest official Windows build from the website, and created a link library by running lib /def:libgnutls-28.def in the bin directory form a Visual Studio command prompt.
After adding a typedef long ssize_t, the code compiles fine, but linking fails with the following error:
source_file.obj : error LNK2001: unresolved external symbol _gnutls_free
C:\Path\to\executable.exe : fatal error LNK1120: 1 unresolved externals
I am calling gnutls_free to free some memory allocated and returned by the library. If I remove the call to gnutls_free, the project links successfully. Given that gnutls_free is just a global variable (containing a function pointer) exported by the library, I'm not sure why accessing it results in an unresolved reference to a different symbol. I have verified that gnutls_free is not #defineed to anything.
As a test, I tried doing gnutls_free_function test = gnutls_free; which also resulting in the link error. Running grep -w -r _gnutls_free . on the GnuTLS source code returns nothing, so I am at a loss.
Any ideas for getting this working would be greatly appreciated.
EDIT:
Adding __declspec(dllimport) to the declaration of gnutls_free in gnutls.h allows the link to succeed. Is there any way to accomplish this without maintaining a custom version of the header file?
There doesn't seem to be a way to have the linker or import library automatically dereference the IAT's pointer to the data item the same way that is done for functions (via a small trampoline function that is statically linked into the module importing the function). The __declspec(dllimport) attribute tells that compiler that this dereferencing needs to be done so it can insert code to perform the dereferencing of the IAT pointer implicitly. This allows exported data to be accessed and for functions allows the compiler to call the imported function via an indirect call through the IAT pointer rather than by calling the trampoline function.
See a couple of Raymond Chen's articles about dllimport for a good explanation of what goes on for function calls (he didn't discuss importing data, unfortunately):
Calling an imported function, the naive way
How a less naive compiler calls an imported function
The MS linker or import library doesn't have a mechanism to help the compiler get imported data in a 'naive' way - the compiler needs the the __delcspec(dllimport) hint that an extra dereference through the IAT is needed. Anyway, the point of all this is that it seems there's no way to import data except by using the __declspec(dllimport) attribute.
If you want to avoid modifying the gnutls distribution (which I can understand), here's one rather imperfect workaround:
You can create a small object file that contains nothing but a simple wrapper for gnutls_free(); since gnutls_free() has an interface with no real dependencies, you can have the necessary declarations 'hardcoded' instead of including gnutls.h:
typedef void (*gnutls_free_function) (void *);
__declspec(dllimport) extern gnutls_free_function gnutls_free;
void xgnutls_free(void* p)
{
gnutls_free(p);
}
Have your code call xgnutls_free() instead of gnutls_free().
Not a great solution - it requires your code to call a wrapper (so it's particularly not great if you'll be incorporating 3rd party code that might depend on gnutls_free()), but it might be good enough.
I have Core static library, a few Component static libraries that relays on the Core one, and then there is an App that links against both Core and Component libraries. My App can link both against Core and Component as long as Component don't uses classes from Core (App uses classes from Core).
I got the following error in both armv6 and armv7 versions. So my problem is not the very popular linking issue that everyone has.
ld: symbol(s) not found for architecture armv6
clang: error: linker command failed with exit code 1 (use -v to see invocation)
I added reference to Core in Component and even added it in "Link Binary With Libraries" which shouldn't be necessary for static lib.
Since I start having this issue I start doubting my design... It probably makes more sense in dynamically linking environment but still it should be doable in static one, especially since this already works under Windows with MSVC compilers.
Edit:
I made some progress! Although I still don't know where to go with it.
Here is my setup:
Core has a class cResourceManager that has a templated method GetResource<T>(int id)
Core also has class cResource
Component has class cMesh that inherits cResource
Here are some tests:
If I try from App to call rm->GetResource<cMesh>(...) I get the linking error
If I try from App to construct cMesh I get linking the linking error
If I try from App to call static method that will return new instance of cMesh I get the linking error
If I comment out the construction of cMesh but leave other member cMesh function calls the App links fine. I can even call delete mesh.
I have never seen anything like it!
If you remove the cMesh constructor, then you are then using the default (no argument, no body) cMesh constructor that is given to you. It almost sounds like there's a build error or missing code as a result of some code in your cMesh constructor and so the library isn't actually getting generated, and perhaps Xcode isn't reporting the error. Xcode is no good at reporting linker errors.
I would suggest looking at what symbols the linker says are missing and double-check that they are actually defined in your code. My guess is that you're using one of those symbols in your cMesh constructor. A lot of times with virtual base classes, you may forget to define and implement a method or two in a child class. Could be a result of missing a method based on your template, or your template isn't #included correctly. This could compile fine but result in linker errors like you're seeing.
If Xcode isn't showing you the full linker error, show the Log Navigator (Command ⌘+7), double-click the last "Build " entry, select the error, and then press the button on the far-right of the row that appears when selected. The symbols should be listed there. If not, it's time for xcodebuild in the Terminal.
If it's not that case, I'd be interested in seeing the results of whether or not the library is being built for the appropriate architecture, or maybe this can spur some progress:
In the Xcode Organizer Shift ⇧+Command ⌘+2, click Projects and find the path to the DerivedData for your project.
In the Terminal, navigate to that directory (cd ~/Library/Developer/Xcode/DerivedData/proj-<random value>/)
Remove (or move aside) the Build directory (rm -r Build)
In Xcode, try to build with the cMesh constructor present.
Find the Library product file (cd Build/Products/<scheme>-iphoneos)
Your compiled static libraries (<libname>.a) should be in this directory. If they're not there, they didn't build (unless you put your products elsewhere). If your libraries are there, let's confirm that they actually are getting built for the appropriate architecture. Run otool -vh <library>.a. You should see something like:
$ otool -vh libtesting.a
Archive : libtesting.a
libtesting.a(testing.o):
Mach header
magic cputype cpusubtype caps filetype ncmds sizeofcmds flags
MH_MAGIC ARM V7 0x00 OBJECT 3 1928 SUBSECTIONS_VIA_SYMBOLS
As you can see, my test library was built for ARMv7.
Make sure you are linking them in the correct order.
If Component depends on symbols in Core, then Component needs to be first in the link order, so the linker knows which symbols to look for in Core.
In MSVC the order doesn't matter, but in most other compiler suites it does.
I don't think Clang generates code for armv6, if you're targeting devices that old you still need to use GCC.
If I do a LoadLibrary("msvcrt.dll") do I need to initialize the CRT somehow? Section 2 in the following document seems to say that I do, but I just get an undefined symbol error for _CRT_INIT when I try to call _CRT_INIT:
http://support.microsoft.com/kb/94248
Edit: I should have said that this is for a program that dynamically loads all the dlls that it uses, hence the call to LoadLibrary("msvcrt.dll").
Call DllMain() in it. If it relies on the C runtime, it will call CRT_INIT.
But a far better question is if a program is using something in msvcrt, there's no need to explicitly load the dll and initialize it, so why are you doing this?
If you're working in C++, have you declared _CRT_INIT as extern "C"?
Have you tried using the DUMPBIN utility ( http://support.microsoft.com/kb/177429 -- if you haven't your PATH up yourself, you'll have to use the Visual Studio Command Prompt I think) with the /EXPORTS switch to see which functions are available from the CRT DLL, just to double check?
If you get stuck, VS2005 and earlier (and presumably later...) come supplied with the source code for the runtime library. For VS2005, this is in VC/crt/src, relative to the VS install folder. It looks like _CRT_INIT is the right name -- see crtdll.c and dllcrt0.c, and it's a C function.
You must not call _CRT_INIT() but call CRT_INIT() (if you really must)
The link you referenced refers to using CRT_INIT() only when "Using the CRT Libraries When Building a DLL", and even then it is only one of two alternatives; the first probably being preferable in most cases.
I am converting my project to use DLLs and am trying to break apart my Singleton class to avoid using templates.
My class, LudoMemory, originally inherited from Singleton. I am trying to give it the functions to destroy and create itself now and have my main engine not rely on the Singleton.
I have written a simple destroy method like such:
LudoMemory *memory_Singleton = NULL;
void LudoMemory::Destroy()
{
LUDO_SAFE_DELETE(m_Singleton)
}
and upon running the program (no compiler errors) I recieve this error:
The procedure entry point
?Destroy#LudoMemory##SAXXZ could not
be located in the dynamic link library
LudoCore.dll
LudoCore is the project that LudoMemory belongs to. Why is this happening? How can I solve it?
you don't have multiple versions of ludocore.dll on your system, do you?
Procedure entry points errors usually mean: you compiled your project against ludocore.lib version x, and when running the program, it uses ludocore.dll version y, and version y does not define LudoMemory::Destroy().
Jacob's answer about multiple DLL versions seems likely.
Also, with some build systems, you must explicitly list which functions will be exported in a DLL.
Research your build environment, and see if you must provide a list of methods to be exported as an entry-point.
In Visual Studio build environment, also you could try by disabling the References in Linker Optimization Settings [ No(/OPT:NOREF)]