I'm using gcc on WSL to compile a C++ Linux project. I wanted to use the base64 encode/decode library so I installed it with vcpkg. I'm including the library's functions via #include <cpp-base64/base64.h>. However, this causes a linker error:
undefined reference to `base64_encode[abi:cxx11](unsigned char const*, unsigned int)'
Upon investigating the include directory, I see nothing unusual:
/bin/vcpkg/installed/x64-linux/include/cpp-base64$ ls
base64.cpp base64.h
As a test I decided to copy the base64.cpp into my source code directory directly and it linked correctly. Why is this happening? Shouldn't the linker pick up the base64.cpp implementation from the include directory without any problems?
Related
I am very new to c++ but i know a couple other languages (ie. python, java, a little C). The problem is when i compile my code and try to open a new ImGui window it gives a "undefined reference" error. I think it is a problem with how i the setup the files. both my main.cpp file and the imgui library are in the same directory and when i try to import the file by using "#include "imgui/imgui.h" " it still gives my the undefined reference error. I have tried to pull all the .h and .cpp files out of the imgui folder and into the same directory as my "Main.cpp" file but it still gives me a undefined reference error. I have read the install instructions where you put the .cpp files into the working directory and it still didn't work and gave me the same error. Any help would be appreciated.
--------------------------------------------MyCode--------------------------------------------
#include <iostream>
using namespace std;
#include "imgui/imgui.h"
int main(){
ImGui::Begin("Window");
ImGui::End();
}
---------------------------------------------Output---------------------------------------------
/tmp/ccMXHI3r.o: In function `main':
Main.cpp:(.text+0x16): undefined reference to `ImGui::Begin(char const*, bool*, int)'
collect2: error: ld returned 1 exit status
-------------------------------my working directory-----------------------
imgui // imgui library folder that i downloaded from github
Main.cpp // my cpp file where my code is
The problem seem to be a missing linking step, you need to tell your compilation/linking process to link to imgui library.
When you include a header in C/C++ you are just letting the compiler to know the signatures of functions and other declarations.
Then you need to let your linker know where to look for the code, and that will be telling it to link to the library.
For example with gcc you can make a compilation + linking with the following call:
gcc foo.c -o foo -ldynLib
that example will compile the source code 'foo.c', will generate an object for that and it will link it with the dynamic library 'dynlib' into the final binary 'foo' (the executable).
in your case it could be something like this:
gcc Main.cpp -o program -limgui
Of course flags and steps will depend on your compiler / linker, also you should check the actual name of the library (imgui is just a guess, as I don't know that library)
I'm trying to include some networking code into my C++ application. I downloaded CSimpleSocket and I copied all the .h and .cpp files into the directory where my main file is. Then I tried including one of the headers, but the linker just barfs up a bunch of errors, like:
[Linker error] undefined reference to CPassiveSocket::CPassiveSocket(CSimpleSocket::CSocketType)'
[Linker error] undefined reference to `CSimpleSocket::Initialize()'
[Linker error] undefined reference to `CPassiveSocket::Listen(unsigned char const*, short, int)'
[Linker error] undefined reference to `CPassiveSocket::Accept()'
and others. Everything is in one directory, so I don't think that's the problem. The code I'm using to include is #include "PassiveSocket.h". I'm using Dev-C++, if that makes any difference. I don't understand what I'm doing wrong, so if somebody could help me, that would be great.
Forgive me if this is a really dumb question, but I'm trying to learn C++, and it's not easy. Thanks for your help.
The reason you're getting this error is because your compiler can't find the binary that corresponds to the CSimpleSocket headers. It's as if you wrote
void someFunction(int someArg);
And then never provided the implementation for someFunction.
To use a third party library you need two things:
Header files (.h, .hpp, etc...)
Library files (.a, .lib, etc...)
Once you've got your header files and library files you need to put them in a place your compiler can find them. This place will vary depending on your OS, environment variables and compiler configuration.
Now that they're somewhere the compiler can find them you need to tell the compiler to use them. Header files are used with the #include command and library files are linked by providing arguments to the compiler.
Behind the scenes Dev-C++ uses the MinGW GNU GCC compiler, it invokes a command similar to g++ file1.cpp file2.cpp ... filen.cpp -o filename that tells the program g++ to compile a C++ executable named "filename" using files 1 to n. There are other flags that can be added to g++ such as telling it where to search and what to link.
The name of the CSimpleSocket library when compiled is "clsocket" so we need to find a way to configure Dev-C++ to add -lclsocket to the g++ command. I don't use Dev-C++ so I can't help you here but you're probably looking for "Linking Options" or something similar in your compile configuration. You also need to make sure the .lib and .h files are on the search path which should also be configurable in Dev-C++.
CSimpleSocket also provides an installer that should automatically create the .lib file and place the .lib and .h in places where they can be found, you should consider using that installer.
I think the complexity of this answer highlights the abysmal state of the C++ library integration ecosystem. Unfortunately there is no concept of a "module" in C++ at the time of writing.
When I want to build some OpenCV programme, it shows questions " undefined reference to cvFindHomography' so I check that which header file contains this function, so I include `...
But, it doesn't work.
You already have included the appropriate header file, otherwise you would get a compiler error and not the linker error you reported. In C++ in most cases the header files only expose the declarations of functions you want to use. In your case the definition is found in the library file. You have to tell your linker to link your program against these lib files. See "4) Configure your own projects to use OpenCV" at http://opencv.willowgarage.com/wiki/InstallGuide on how to do this for OpenCV. In addition I recommend that you increase your knowledge about the c++ build system, i.e. what does your compiler, what does your linker etc.
Ok, so it's been a while, and i'm having problems with #includes
So I'm doing
#include "someheader.h"
but it's giving me
fatal error: someheader.h: No such file or directory
It's a system wide library I guess you could say.
I'm running arch linux and I installed the library from the repo, and I think the .h files are in /usr/include.
I could just copy all the header files into the folder my code is in but that would be a hack.
What is the "right" way to do this?
Edit: I wasn't correct by saying the .h files were in /usr/include, what I meant was that the library folder was in there
So, Emile Cormier's answer worked to a certain extent.
The problem now is that there are some include in the header file and it seems from the methods I'm trying to access that those includes are not happening
it's giving my the error
undefined reference to Namespace::Class::method()
Edit:
Ok so the final answer is:
#include <library_name/someheader.h>
And compile with
g++ code.cpp -llibrary_name
Sometimes, header files for a library are installed in /usr/include/library_name, so you have to include like this:
#include <library_name/someheader.h>
Use your file manager (or console commands) to locate the header file on your system and see if you should prefix the header's filename with a directory name.
The undefined reference error you're getting is a linker error. You're getting this error because you're not linking in libsynaptics along with your program, thus the linker cannot find the "implementation" of the libsynaptics functions you're using.
If you're compiling from the command-line with GCC, you must add the -lsynaptics option to link in the libsynaptics library. If you're using an IDE, you must find the place where you can specify libraries to link to and add synaptics. If you're using a makefile, you have to modify your list of linker flags so that it adds -lsynaptics.
Also the -L <path_to_library> flag for the search path needs to be added, so the linker can find the library, unless it's installed in one of the standard linker search paths.
See this tutorial on linking to libraries with GCC.
You'd use #include <someheader.h> for header files in system locations.
#include "someheader.h" would try to include the file someheader.h in the directory of your .c file.
In addition to including the header file, you also need to link in the library, which is done with the -l argument:
g++ -Wall youprogram.cpp -lname_of_library
Not doing so is the reason for the "undefined reference .. " linker errors.
The quick fix is to do use:
#include <someheader.h>
assuming that someheader.h is in the standard include locations (to find it use the command locate someheader.h in a shell. If it is in /usr/include it is in a standard location. If it is in a subdirectory of /usr/include you only need to add the part of the directory up to /usr/include in the #include directive (e.g. #include <fancy_lib/someheader.h>)
However, this is only half of the story. You also will need to set up your build system in a way that locates the given library and adds its include path (the path under which it's header files are stored) to the compiler command (for gcc that is -I/path/to/header). That way you can also build with different versions by configuring them in your build system. If the library is not header-only you will also have to add it to the linker dependencies. How this is achieved in your build system is best found out by consulting its documentation.
I'm trying to implement the jsoncpp libraries in my C++ code, I wrote a simple piece of code just to try it out, and it's not even compiling.
#include <stdio.h>
#include <stdlib.h>
#include <stddef.h>
#include <string.h>
#ifndef json_included
#define json_included
#include "jsoncpp\include\json\json.h"
#endif
//#include "json\jsonC\json.h"
int main(int argc, char **argv)
{
std::string example = "{\"array\":[\"item1\", \"item2\"], \"not an array\":\"asdf\"}";
Json::Value value;
Json::Reader reader;
bool parsed = reader.parse(example, value, false);
std::cout << parsed;
return 0;
}
The errors i'm getting are:
undefined reference to `Json::Reader::parse(std::string const&, Json::Value&, bool)'
undefined reference to `Json::Reader::Reader()'
undefined reference to `Json::Value::~Value()'
undefined reference to `Json::Value::Value(Json::ValueType)'
I'm a bit new to C++, is there something I'm missing in the include statement? Or does jsonCpp need something extra?
Thank you for your time!
Your code is compiling, but it is not linking. You forgot to provide the JSON shared library files to your linker (or, on newer versions, to add the amalgamated jsoncpp.cpp to your project).
Without knowing more about your development environment, it's hard to give you more specific instructions.
BTW, you're writing C++; use C++ headers like cstdio, not stdio.h, please. You also failed to include C++ string and got lucky that it "worked" through some JSON header including it for you.
"Undefined reference" sounds like a linker problem. Does jsoncpp come with a library that you need to link to, such as a .so, .a, .lib or .dll file?
According to the jsoncpp README, the library must first be built using scons. Presumably this will then output a library file such as a .so, .a, .lib or .dll file. You must then follow your compiler's rule for linking against such a library (e.g. add it to the end of the command line when compiling, or add it to the "additional libraries" field in the project config in your IDE).
In my case (using CodeBlocks IDE on Ubuntu) the problem was that I needed to add the json.cpp file (generated with python amalgamate.py from within the jsoncpp project) to my build targets.
In other words, I added a -c jsoncpp.cpp option to my g++ compile statement.
You need to link to the json libraries, e.g. using -ljson_linux-gcc-4.4.3_libmt
You can find the exact library name by looking in the library directory, e.g. /usr/lib
If you're using Visual Studio, add the .lib file to Project Properties, Linker, Input, Additional Dependencies and specify the path in Project Properties, Linker, General, Additional Library Directories
Two potential issues:
There is a bug in some versions of the jsoncpp library code where amalgated needs to become amalgamation for the linking to work correctly.
As the other answers suggested, #include
After you compile jsoncpp you can find the libraries in the folder libs/ . For convenience you can put it in /usr/lib and then link it at run time by passing -llibjson_linux-gcc-4.4.3_libmt as an argument to g++.
I have renamed libjson_linux-gcc-4.4.3_libmt.so to libjson.so and can link it by specifying -ljson.