I'm trying to create a program that uses some of the code from WebKit/GTK+. Specifically, I want to load a string, use WebKit's parser to construct a DOM tree and then iterate over that tree.
I'm trying to use a class called HTMLDocument. WebKit/GTK+ doesn't expose this as part of its API and I'm running into some trouble linking against it.
I'm able to build WebKit/GTK+ normally, which gives me a file called: libwebkit-1.0.so. My program is:
#include <iostream>
#include <WebCore/config.h>
#include <WebCore/html/HTMLDocument.h>
using namespace WebCore;
int main() {
String title = "test";
RefPtr<HTMLDocument> d = HTMLDocument::create(0);
d->open();
d->write("<!doctype html><html><head><title>" + title + "</title></head><body></body></html>");
}
This compiles fine (I'm using the same include directives used by webkit to build), but results in linking errors.
...test_doc.cpp:18: undefined reference to `WebCore::String::String(char const*)'
...test_doc.cpp:21: undefined reference to WebCore::Document::open(WebCore::Document*)'
...(similar for every function I use)
If I run:
nm -C .libs/libwebkit-1.0.so | grep 'WebCore::Document::open'
I see:
003b1830 T WebCore::Document::open(WebCore::Document*)
which seems to indicate that the function is available. I have a reasonable amount of C++ experience, but not much experience with linking files under Linux.
I'm not expecting this exact problem to be solved, but I'm hoping someone can correct me if I have conceptual problems. My main question is why I see "undefined reference" errors when I'm linking with an .so file that lists that function as being defined. Is another file or build step needed?
Thank you very much.
Using:
Ubuntu 9.10
g++ 4.4.1
g++ is invoked with:
g++ --debug -DHAVE_CONFIG_H -I. `pkg-config --cflags libsoup-2.4` \
-DBUILDING_CAIRO__=1 -DBUILDING_GTK__=1 -DWTF_CHANGES -DWTF_USE_ICU_UNICODE=1 \
-DNDEBUG -I./WebCore -I./WebCore/accessibility -I./WebCore/bindings/js \
-I./WebCore/bridge -I./WebCore/bridge/c -I./WebCore/css -I./WebCore/dom \
...many more webkit include directories...
-DDATA_DIR=\"/usr/local/share\" \
test_doc.cpp -o test_doc.out \
./webkit-1.1.15.3/.libs/libwebkit-1.0.so
(I get the same result with -L/path/to/lib -lwebkit-1.0)
I think you might be running into an ordering problem: man g++ specifies that the order of the -l option is significant, and from memory the linker will only look for symbols in objects which have preceeded the current file on the command line.
I suspect what is happening is that the linker is trying to link test_doc before it's seen libwebkit-1.0.so, so it hasn't seen any of those symbols yet and bails.
You should use the -L/path/to/web and -lwebkit-1.0.
Also, I would compile your .cpp file in to a .o and then build your executable separately to make sure things are isolated.
Anyway, you may need to set your $LD_LIBRARY_PATH environment variable to include the path where that .so is stored. If you link to a shared library, you will need that library at run-time. Therefore, you do not want to have your webkit SO stored in its build directory (build/.libs). You want to install it. If you are not root, then you should ./configure with a --prefix=/some/path to install it to some local directory. Alternatively, you can link against the static library. One way to do this is to use the -bstatic (or similar) flag before your -lwebkit-1.0.
This is a good resource for Linux library creation and use.
I think you're issue is that the symbols you need are not exported. You can do objdump --dynamic-syms libwebkit-1.0.so to see which symbols are available. In the WebKit GTK build files there is use of the -fvisibility=hidden flag to restrict the symbols. Check your generated GNUMakefile and you'll see SYMBOL_VISIBILITY = -fvisibility=hidden. You should be able to modify the build files to get what you need.
Related
I want to compile a C++ program with a twitter library, on Linux.
I'm current using twitcurl as the twitter API library and installed g++ and all the necessary files and packages that are listed on the official website: http://code.google.com/p/twitcurl/wiki/WikiHowToUseTwitcurlLibrary
However, when I compile my program using this command g++ twitterClient.cpp -ltwitcurl, I get this error: cannot find -ltwitcurl
I also used CodeBlocks IDE to compile it but got this error: undefined reference to twitCurl::~twitCurl()
`
My code only contains a few lines:
#include <iostream>
#include "Twitter/Twitter.hpp"
using namespace std ;
int main ()
{
Twitter t ;
return 0 ;
}
I've already spent a lot of time on this but am unable to solve the problem. What should I do in order to compile the program on the command-line and CodeBlocks?
$ g++ twitterClient.cpp -ltwitcurl
cannot find -ltwitcurl
This means your compiler doesn't find the libtwitcurl.so.1. in its library directories.
First, make sure you correctly build the twitcurl library and obtained the libtwitcurl.so.1. file with something like this :
svn co http://twitcurl.googlecode.com/svn/trunk/libtwitcurl
cd libtwitcurl/
make
Secondly, make sure you put the file (or a symlink) in one of your compiler's library path :
cp libtwitcurl.so.1.0 /usr/lib/
You can check g++ library paths using the following command :
g++ --print-search-dirs | grep libraries
(/usr/lib/ is usually at the end.)
If you don't want/can't put the file in your compiler's library path, you can also tell it where to find libtwitcurl.so.1. by adding -L/path/to/twitcurl/ in the g++ options, but it is not needed if the file is already in one of the compiler's library path.
You need to specify path to twitter lib:
g++ twitterClient.cpp -L/path/to/lib/dir -ltwitcurl
Alright, I realize that this might seem like a duplicate, but I've tried most variations and attempts via' searched posts, and I'm still coming up with nothing. Here's the issue: I am working with an open-source C library called libxls that is used for reading .xls files (aptly named project.. :) ). The code I'm using is straight off their website, essentially just a beginning toy example to make sure I can access the libraries:Link to their website with the source
#include <stdio.h>
#include "libxls/xls.h"
int main(){
xlsWorkBook* pWB;
pWB = xls_open("Book1.xls", "iso-8859-15//TRANSLIT");
return 0;
}
That's it. Now, the syntax of that is fine. I know for a fact through the nm command that xls_open is indeed available as a function in the .a library, so that's not a problem. in my directory I have the following files (pardon the redundant naming, I was just trying to force it to work real quick):
Book1.xls
libxlsreader.a
libxlsReader.c
libxlsreader.so
Although I don't THINK I need the .so file here since I've tried dynamically linking to where that lives, figured it wasn't a bad plan to try. Alright, so, on the include line, I keep getting the common:
libxlsreader.c:3:37: fatal erro: libxls/xls.h: No such file or directory
Ok fine, so I probably linked in the library wrong, take a look at my compile line arguments:
gcc -o libxlsWrapper libxlsReader.c -L /usr/local/lib/ -lxlsreader -lpthread
huh, well.. that certainly LOOKS right, /usr/local/lib/ is where the library created itself with all of it's .so files, and the .a one in my PWD. Now, I'm a java developer by trade, so I might be missing something blaringly obvious, but for the life of me I can't determine what it is. it certainly seems ok to me.
I'm using GCC (ubuntu/Linaro 4.7.3-lubunutul) 4.7.3 on Linux Mint 15 KDE 32-bit.
If you'd like to reproduce the library for your own testing or problem solving, it can be obtained from
I don't recommend recreating it on windows, I tried for a couple hours yesterday and gave up, so just do the regular ./configure -> make -> make install and that should produce the appropriate libraries for you.
Thanks!
-Will
edit #1: here are some of the other linking attempts I've tried, all with the identical result.
gcc -o libxlsWrapper libxlsReader.c -L. -lxlsreader -I.
gcc -o libxlsWrapper libxlsReader.c -L. -lxlsreader -lpthread
gcc -o libxlsWrapper libxlsReader.c -L. -lxlsreader.so
I also tried a bunch with g++ instead of gcc (throwing darts, I know), same result.
Please do a find your_folder -type f -iname xls.h and then use a gcc -I /the_path to include the path for xls.h. Because the problem you have is that gcc could not locate the header file xls.h.
I am getting undefined reference to 'mysql_suchandsuch##' messages when trying to link my program with the MySQL libraries supplied with the 5.5 server. When MySQL was installed, I used the default path, which for me on Windows is C:\Program Files\MySQL\MySQL Server 5.5\. Originally, I had thought that the spaces are causing my grief, but I think I've correctly worked out how to point to the library path without spaces (still with no luck). If there's another probable cause, please let me know.
I have reviewed a series of questions on this site trying to resolve my issue...
Question 1
Question 2
Question 3
Question 4
Question 5
Question 6
Question 7
Using mingw/g++, I have tried to link using the following options, based on my own research as well as suggestions here:
-L"C:\Program Files\MySQL\MySQL Server 5.5\lib\" -llibmysql.lib
-L"C:\Program Files\MySQL\MySQL Server 5.5\lib\" -lmysqlclient.lib
-L"C:\Progra~1\MySQL\MySQLS~1.5\lib\" -llibmysql.lib
-LC:\Progra~1\MySQL\MySQLS~1.5\lib\ -lmysqlclient.lib
-L"C:\Progra~1\MySQL\MySQLS~1.5\lib\" -lmysql
In all cases, I have put the -L/-l options at the right-most part of the statement, as I understand this can matter.
I have confirmed the libraries do exist. In the /lib dir, I have libmysql.lib, mysqlclient.lib, and libmysql.dll. I have not tried to link with the .dll, as no tutorial/forum I've reviewed suggested that.
I am not using MAKEFILES.
Does anyone have specific experience with g++/MySQL?
The following commands work fine for me using a GCC 4.6.1 from November 2011:
g++ my.cpp -I D:\Opt\MySQL5.5\include ^
D:\Opt\MySQL5.5\lib\libmysql.dll -o myWithDll.exe
g++ my.cpp -I D:\Opt\MySQL5.5\include ^
-L D:\Opt\MySQL5.5\lib -lmysql -o myWithLib.exe
So both linking against the LIB and the DLL do work.
You may get a warning (see Gaffi's comment). This is because the linker does fuzzy linking for you without you having it specified; normally, it would have failed to link. It is being nice, though, and making it work for you, at the same time warning you about things happening without your having requested them. The way to suppress the warning is to make fuzzy linking explicit:
g++ -Wl,--enable-stdcall-fixup my.cpp -I D:\Opt\MySQL5.5\include ^
D:\Opt\MySQL5.5\lib\libmysql.dll -o myWithDll.exe
g++ -Wl,--enable-stdcall-fixup my.cpp -I D:\Opt\MySQL5.5\include ^
-L D:\Opt\MySQL5.5\lib -lmysql -o myWithLib.exe
This is a Cygwin/RedHat/MinGW extension to the linker; the docs are here:
--enable-stdcall-fixup
--disable-stdcall-fixup
If the link[er] finds a symbol that it cannot resolve, it will attempt to
do “fuzzy linking” by looking for another defined symbol that differs
only in the format of the symbol name (cdecl vs stdcall) and will
resolve that symbol by linking to the match. For example, the
undefined symbol _foo might be linked to the function _foo#12, or the
undefined symbol _bar#16 might be linked to the function _bar. When
the linker does this, it prints a warning, since it normally should
have failed to link, but sometimes import libraries generated from
third-party dlls may need this feature to be usable. If you specify
--enable-stdcall-fixup, this feature is fully enabled and warnings
are not printed. If you specify --disable-stdcall-fixup, this feature is
disabled and such mismatches are considered to be errors. [This option
is specific to the i386 PE targeted port of the linker]
did you try -lmysql as the linker automatically prepends lib and appends .lib
My first post on this site with huge hope::
I am trying to understand static linking,dynamic linking,shared libraries,static libraries etc, with gcc. Everytime I try to delve into this topic, I have something which I don't quite understand.
Some hands-on work:
bash$ cat main.c
#include "printhello.h"
#include "printbye.h"
void main()
{
PrintHello();
PrintBye();
}
bash$ cat printhello.h
void PrintHello();
bash$ cat printbye.h
void PrintBye();
bash$ cat printbye.c
#include <stdio.h>
void PrintBye()
{
printf("Bye bye\n");
}
bash$ cat printhello.c
#include <stdio.h>
void PrintHello()
{
printf("Hello World\n");
}
gcc -Wall -fPIC -c *.c -I.
gcc -shared -Wl,-soname,libcgreet.so.1 -o libcgreet.so.1.0 *.o
ln -sf libcgreet.so.1.0 libcgreet.so
ln -sf libcgreet.so.1.0 libcgreet.so.1
So I have created a shared library.
Now I want to link this shared library with my main program to create an executable.
gcc -Wall -L. main.c -lcgreet -o greet
It very well works and if I set the LD_LIBRARY_PATH before running greet( or link it with rpath option) I can make it work.
My question is however different:
Since I am anyway using shared library, is it not possible to force symbol resolution at runtime (not sure about the terminology but perhaps called dynamic linking as per the book "Linkers and Loaders"). I understand that we may not want to do it because this makes the program run slow and has overhead everytime we want to run the program, but I am trying to understand this to clear my concepts.
Does gcc linker provide any option to delay symbol resolution at runtime? (to do it with the library we are actually going to run the program with)(as library available at compile time may be different than the one available at runtime if any changes in the library)
I want to be able to do sth like:
bash$ gcc main.c -I.
(what option needed here?)
so that I don't have to give the library name, and just tell it that I want to do symbol resolution at runtime, so headers are good enough for now, actual library names are not needed.
Thanks,
Learner For Ever.
Any linker (gcc, ld or any other) only resolves links at compile-time. That is because the ELF standard (as most others) do not define 'run-time' linkage as you describe. They either link statically (i.e. lib.a) or at start-up time (lib.so, which must be present when the ELF is loaded). However, if you use a dynamic link, the linker will only put in the ELF the name of the file and the symbols it must find, it does not link the file directly. So, if you want to upgrade the lib to a newer version later, you can do so, as long as system can find the same filename (the path can actually be different) and the same symbol names.
The other option, to get symbols at run-time, is to use dlopen, which has nothing to do with gcc or ld. dlopen simply put, opens a dynamic link library, just like fopen might, and returns you a handle, which then you pass to dlsym with the name of the symbol you want, which might be a function name for example. dlsym will then pass you a pointer to that symbol, which you can then use to call the function or use as a variable. This is how plugins are implemented.
I think you are looking for ld option '--unresolved-symbols=ignore-all', yes it can actually do it (ignore prev answer). Imagine scenario where a shared library loaded late (when program is already running), it can use all symbols that are already resolved/loaded by the main process, no need to bother to do it again . btw it does not nervelessly makes it slow , at least on Linux
I am writing a fairly large C++ shared-object library, and have run into a small issue that makes debugging a pain:
If I define a function/method in a header file, and forget to create a stub for it (during development), since I am building as a shared object library rather than an executable, no errors appear at compile-time telling me I have forgotten to implement that function. The only way I find out something is wrong is at runtime, when eventually an application linking against this library falls over with an 'undefined symbol' error.
I am looking for an easy way to check if I have all the symbols I need at compile time, perhaps something I can add to my Makefile.
One solution I did come up with is to run the compiled library through nm -C -U to get a demangled list of all undefined references. The problem is this also comes up with the list of all references that are in other libraries, such as GLibC, which of course will be linked against along with this library when the final application is put together. It would be possible to use the output of nm to grep through all my header files and see if any of the names corresponding.. but this seems insane. Surely this is not an uncommon issue and there is a better way of solving it?
Check out the linker option -z defs / --no-undefined. When creating a shared object, it will cause the link to fail if there are unresolved symbols.
If you are using gcc to invoke the linker, you'll use the compiler -Wl option to pass the option to the linker:
gcc -shared ... -Wl,-z,defs
As an example, consider the following file:
#include <stdio.h>
void forgot_to_define(FILE *fp);
void doit(const char *filename)
{
FILE *fp = fopen(filename, "r");
if (fp != NULL)
{
forgot_to_define(fp);
fclose(fp);
}
}
Now, if you build that into a shared object, it will succeed:
> gcc -shared -fPIC -o libsilly.so silly.c && echo succeeded || echo failed
succeeded
But if you add -z defs, the link will fail and tell you about your missing symbol:
> gcc -shared -fPIC -o libsilly.so silly.c -Wl,-z,defs && echo succeeded || echo failed
/tmp/cccIwwbn.o: In function `doit':
silly.c:(.text+0x2c): undefined reference to `forgot_to_define'
collect2: ld returned 1 exit status
failed
On Linux (which you appear to be using) ldd -r a.out should give you exactly the answer you are looking for.
UPDATE: a trivial way to create a.out against which to check:
echo "int main() { return 0; }" | g++ -xc++ - ./libMySharedLib.so
ldd -r ./a.out
What about a testsuite ? You create mock executables that link to the symbols you need. If the linking fails, it means that your library interface is incomplete.
I had the same problem once. I was developing a component model in C++, and, of course, components should load at runtime dynamically. Three solutions come to mind, that were the ones I applied:
Take some time to define a build system that is able to compile statically. You'll lose some time engineering it, but it will save you much time catching these annoying runtime errors.
Group your functions in well-known and well-understood sections, so that you can group of functions/stubs to be sure that each corresponding function has its stub. If you take the time on documenting it well, you can write perhaps a script that checks the definitions (via, for example, its doxygen comments) and check the corresponding .cpp file for it.
Do several test executables that load the same set of libraries and specify the RTLD_NOW flag to dlopen (if you're under *NIX). They will signal the missing symbols.
Hope that helps.