I am porting a static library project composed entirely of "C" files to iOS and when I try to use the library in an application I get a bunch of undefined symbols.
The problem appears to be name decoration. For example, let's say I have the function:
unsigned int hello_world(int arg1, int arg2);
When I use "nm" to look at the .a file I see the following:
For file1.c where the function is implemented:
0000002c T __Z10hello_worldii
For file2.c that calls the function:
U __Z10hello_worldii
But for file3.c that also calls the function and for which the symbol is undefined I get:
U _hello_world
All three .c files are in the library project. The prototype is in a header file that is included by both file2.c and file3.c. Are the __Z10 and ii C++ name decorations? If so, why are these C files being compiled as C++?
EDIT:
Codo's question in regard to the "Compile Sources As" setting led me to try changing it to "C" from "According to File Type". This forced all files to be compiled as "C" and resolved the function name mangling problem but I still had 42 undefined symbols which almost all turned out to be variables. I noticed that there were no initializers for these variables so I added an initializer to one of them and when I rebuilt the project that uses the library, I only had 3 undefined symbols. It appears that adding just one initializer to a variable enabled the linker to resolve all references to that modules variables. The 3 remaining undefined symbols appear to be yet another problem(s).
EDIT2:
Of the 3 remaining errors, 1 was an uninitialized variable from another module and 2 were in-lined functions that needed to be declared static.
Related
BACKGROUND
I'm building an iOS app (which I'll just call MyApp from here) that will rely on calculations done by several separate static libraries (which I'll call Lib1, Lib2, Lib3,...). Each library is built in it's own project, then imported into a single workspace (so the workspace will contain MyApp, Lib1, Lib2, ...). More details on how this is set up here. The libraries are used by other products that are independent from MyApp, so I want to minimize any changes in the libraries. The libraries are also written in (plain) C, so there are no header files.
Certain function names are used by multiple libraries (so both Lib1 and Lib2 might each have a DoStuff method). Functions with the same name generally do the same thing, but there are some specifics about how that do it that can be different between libraries, so the actual code in DoStuff on Lib1 might be quite different than the code in DoStuff on Lib2. It would be very difficult to write one universal DoStuff that would be exactly the same in each library.
THE ISSUE
While the app is running, it isn't calling the correct DoStuff from the correct library. I found out about this because the wrong function was called during a debug session (which eventually caused the app to crash, due to the subtle differences in the DoStuff functions).
WHAT I'M LOOKING FOR
Each library has only one entry point from MyApp, and each entry point is uniquely named. If DoStuff is called from the entry point method of Lib1 (or any other method on Lib1, for that matter), then I want it to call the DoStuff method on Lib1. What's the best way to make that happen?
Is there any way (maybe through a setting somewhere in XCode) I can make it so that each library is it's own namespace? That would be my preferred way to fix the issue. I guess I could go through and rename the duplicate functions so that they are all unique (so the DoStuff method on Lib1 could be renamed to Lib1DoStuff, or something similar), but there are hundreds of functions that could have duplicate names, and we are going to be adding hundreds of libraries to the project, so having to go in and rename all the functions by hand and fix all the calls to them would take a significant amount of time, and my boss doesn't see that as a viable option.
UPDATE
After looking at the comments from Josh Caswell and some of the links he provided, it looks like it might be possible to automatically rename all the functions when the libraries are compiled, and that would be the best way to try to fix THE ISSUE above. From what I've seen, the objcopy that gets mentioned in a couple of the links in the comments isn't support on iOS. I eventually came across this blog entry, which talks about creating custom build rules for Xcode targets, and this blog that talks about custom build settings and build phases.
Am I right to assume that I can use scripts at some point in the build process to automatically append to the name of all the functions in each of my libraries, instead of doing it manually as I described in the last paragraph of the WHAT I'M LOOKING FOR section above? If so, which is the correct part of the build process to make those changes? Lastly, what would the syntax look like for doing something like that? The 'scripts' used in the different parts of the build processes certainly doesn't look like Obj-C. I've never used these 'scripts' before, so I'm completely in the dark on how I'd use them, and that's what I'm looking for help with.
I tried to be as clear as I could, but if there are any questions on what I'm asking please let me know.
Why isn't xcode calling the correct library function?
Let's say I have 3 C libraries as mentioned by you. Let's say it has the following code.
Library 1 - test1lib.a with code:
#include <stdio.h>
void doStuff()
{
printf("\nDoing stuff for lib1\n");
}
void uniqueEntryPoint1()
{
printf("\nUnique entry point for lib1\n");
doStuff();
}
Library 2 - test2lib.a with code:
#include <stdio.h>
void doStuff()
{
printf("\nDoing stuff for lib2\n");
}
void uniqueEntryPoint2()
{
printf("\nUnique entry point for lib2\n");
doStuff();
}
Library 3 - test3lib.a with code:
#include <stdio.h>
void doStuff()
{
printf("\nDoing stuff for lib3\n");
}
void uniqueEntryPoint3()
{
printf("\nUnique entry point for lib3\n");
doStuff();
}
Here each library has a unique function and one common function doStuff()
When we add these 3 libraries to xcode and link them. xcode links but does not load all the objects files. Let's say the objective C code is like this:
#implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
uniqueEntryPoint1();
}
The output is
Unique entry point for lib1
Doing stuff for lib1
In this case xcode will only load symbols which are referred (library 1 objects) in this case.
If you are read about linker flags/options such as -all_load, -force_load and -objC, you will have better understanding.
If we add -all_load linker option, it will force linker to load all the objects of libraries so we will get the following error in xcode
ld: 2 duplicate symbols for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
The reason this fails as the linker detects that doStuff() is redefined multiple times.
The only way around this problem is change the linker input i.e. symbols present in these 3 libraries. This is already mentioned by Josh in the comments. I will add my $0.02 to it.
Possible Solutions
Solution 1:
The best solution (self explanatory) is to change the source code if you have access to the same.
Solution 2:
Use objcopy to rename or prefix the function as provided in the answer of this How to deal with symbol collisions between statically linked libraries?
Now your doubt on how to find the objcopy.
Option 1:
You can use this project https://github.com/RodAtDISA/llvm-objcopy. This will be tricky to compile as it builds along with llvm. You will have to follow instructions at http://llvm.org/docs/GettingStarted.html and http://llvm.org/docs/CMake.html.
If you rewrite the https://github.com/RodAtDISA/llvm-objcopy/blob/master/llvm-objcopy.cpp, you can probably reuse the parsing and object rewriting logic without depending upon llvm.
Option 2:
Compile and reuse binutils objcopy from https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=blob;f=binutils/objcopy.c;h=2636ab4bcb34cf1e1e54db9933018a805b366727;hb=HEAD
Solution 3:
You can follow the answer provided by Richard in this link Rewriting symbols in static iOS libraries. This is more of a hack but using a hex editor you can rewrite the symbols if their length is kept the same. If you have more symbols and its a big library, you can consider using https://sourceforge.net/projects/bbe-/ and nm to write a script.
All of this is a considerable effort but apparently there is no shortcut.
I'm trying to use a c library in RubyMotion, and in order to call out to functions in the library I need to generate a bridgesupport file. RubyMotion is requesting the generation of this file, but I can see that not a single variadic function from the library appears in the bridgesupport file. I've tried walking through the source of gen_bridge_metadata, but in the end it calls out to a parser in a shared object lib so I can't get much further than that. All I can see is that it's not declaring an AFunctionDecl for that function.
Are variadic functions just not supported full stop, or is there some sort of config that I need to apply somewhere?
So this appears to be caused by not having all the .h and .a files for the library and it's dependencies together in the same directories. Eg. I had:
/vendor/lib1
/vendor/lib2
/vendor/lib3
when I should have had
/vendor/lib3 (containing all of lib1 + lib2 as well)
Something strange is going on with my Xcode 5. All of a sudden I'm getting Undeclared Identifier errors for all the values in my Constants.h file, which is imported in my Prefix.pch file.
Two things are weird here:
This hasn't happened before.
When I do build and run, the build succeeds and the app runs with no problems.
I tried restarting Xcode and the simulator, and even restarting the whole machine. No luck.
What's going on? How can I get rid of these false errors?
EDIT following rmaddy's request. The error is Use of undeclared identifier kOffsetFromTop (for example, there are other similar errors with different constants.)
I don't really want to post my entire constants file, but the constant in question is defined like this:
static int const kOffsetFromTop = 20;
When this happens I normally do the following
Comment out the import from the .pch
Clean all ⌘⌥⇧K
Delete derived data
Build
Then uncomment the import from the .pch and build again. I'm not sure which step is actually sorting the issue but this normally gets me going again.
Multiple points here :
You have a warning, not an error. A warning is just that, it warns you but does not prevent the code to compile. That is just to warning you that something is odd or unexpected, or to tell you that what you wrote may not be actually what you were intended, because the compiler finds it odd or not standard.
You didn't have the warning before probably because it wasn't activated by default in previous versions of Xcode. The latest version of Xcode5 activates more warnings (which is a good thing, as it warns you about more things that could go wrong in your code and encourage you to fix them), hence this new one you have but didn't have before.
As I understand what you describe, tour usage of a constant is incorrect (and that's probably why Xcode emits a warning).
The correct way to declare a constant that you want to be accessible from multiple files is to:
Declare it (define its existence and type) in a header (or in your pch) like this: extern <type> const <name>;
And define it (give it a value) only in one implementation file (like your main.m, your AppDelegate.m, some Constants.m file, whatever) like this: <type> const <name> = <value>
Some details and reminders about constants declaration (also valid in standard C)
You use static <type> const <name> = <value>; in an implementation file only, when the constant is local to the file and does not need to be used by other files. In that case, you declare it typically in the .m file in which you will use it, and other files won't have access to it (which is quite what the static keyword means, actually (making the constant attached/local to the file).
In that matter, you should never declare a constant that way in a header file (and especially not in your .pch file), because header (and pre-compiled header) files will be included multiple times. If you do that, this would declare as many independent constants as the number of implementation files you include your headers into (this has evil side effects especially for pointers/objects, for exemple declaring an NSString* const that way -- for, say, using it as a notification name of error domain -- will create multiple string constants, with the same value but different addresses, which will probably not behave like you will expect)
When you need to declare a constant that you need to use / be accessible from multiple implementation files, so declaring this constant in a header file so that it is known to all implementation fiels that includes this header, you need to only declare it in the header, and not making it static (at this would run against the purpose of having the same constant for all files instead of multiple independent instances of the constant) but instead indicate extern to let the compiler know that its definition (value) is set elsewhere. Hence the solution given above.
I have a file which looks something like this:
#include <Shlwapi.h>
...
void SomeFunction()
{
UrlCombineW(...)
}
This compiled just fine until I installed another Delphi component in C++ Builder IDE but now reports unresolved external for UrlCombineW. The above call was fine before installing this component.
It seems that the component is overwriting this in some way so I need to explicitly tell the compiler where to look for UrlCombineW. This is a function from Shlwapi.dll.
Compiler does not complain, but how do I explicitly tell the linker where to look for this function and avoid unresolved external error?
Expanding my comment to an answer.
You need to link to Shlwapi.lib in order for the linker to find the functions. (This explanation glosses over a few things, but a .lib, a library file, can be either a static or import library. A static library contains the functions themselves - it's basically a collection of .obj files bundled together; an import library says that functions X, Y and Z are found in a specific DLL.) Either way, if you link the .lib in you will get the functions that you need.
There are a couple of ways to do tell the linker to link in the file:
Use #pragma comment(lib, "Filename.lib") in a .cpp file somewhere. For your case, this is #pragma comment(lib, "Shlwapi.lib").
Add it to the project options, which in turn adds it to the linker command line. In C++ Builder you do this by actually adding the .lib file to the project, ie drag and drop it onto the project in the Project Manager, or use File > Add To Project.
Which you prefer is up to you. I tend to link to localized things locally - so in my code, there's only one unit which uses Shlwapi.h and the fact it does so is an implementation detail hidden from the outside, it's not shown in the interface. Therefore, in that file, I link using #pragma comment at the point I include the header. On the other hand, if you have something used far more widely - to pick the widest example, kernel32.lib - I would add that the project itself. (Note this is an example, you don't actually need to explicitly link to kernel32, that will be done for you!)
I have a static library, where one of the objects defines a symbol:
nm mylib.a
...
00007340 t _a_local_symbol
...
I need to access the function from my C code. Obviously, I don't have the source code for the library, so I can work only with the archive file that I have at hand.
This is further restricted by iOS linker.
A bit more context. The library is Objective-C++, the function in question is pure C. I don't have original headers, but I've got the function signature restored.
objcopy has a flag to do what you want:
--globalize-symbol <name> Force symbol <name> to be marked as a global
Not sure whether objcopy works on iOS object files though.