Simply including the OpenCV header results in linking error. Why is that?
// test.cpp
#include <opencv2/opencv.hpp>
int foo();
int bar();
int main() {
}
If I compile the file with g++ test.cpp, the following linking error occurs:
/tmp/ccugmQl4.o: In function `cv::String::~String()':
test.cpp:(.text._ZN2cv6StringD2Ev[_ZN2cv6StringD5Ev]+0x14): undefined reference to `cv::String::deallocate()'
/tmp/ccugmQl4.o: In function `cv::String::operator=(cv::String const&)':
test.cpp:(.text._ZN2cv6StringaSERKS0_[_ZN2cv6StringaSERKS0_]+0x28): undefined reference to `cv::String::deallocate()'
collect2: error: ld returned 1 exit status
If I compile with g++ test.cpp -lopencv_core, it works all right.
My question is:
It seems to me that there's no need to resolve undefined symbols if I do not use it, like the functions foo and bar. There's no definition for them but the compile-link process works alright.
I don't use any OpenCV functions either. Why is there linking error only for OpenCV functions?
And what kinds of stuff defined in headers can cause such a linking error?
If you tweak your example a little bit
// test.cpp
int foo();
int bar() {
foo();
}
int main() {
}
You would notice that it'd stop working because linker won't be able to understand what is foo();
The same thing happens when you include opencv header - there are references to functions which are declared but since you never link opencv itself - linker can't figure what those functions are and where to get them.
Related
The problem I'm facing is difficult to describe and explain, but let's try...
Enrivornment: Ubuntu, C++ and g++
So I have an hierarchy of c++ projects and namespaces:
main: My main program - that calls....
objectaccess: Access objects (read, write, update, delete) - that calls...
commonaccess: Encapsulate access to sqlite3 functions (sqlite3_open, sqlite3_exec, etc.) - that calls...
sqlite3.so: The Sqlite3 library.
So, an example call code would be:
#include "objectaccess.hpp"
#include "commonaccess.hpp"
int main()
{
int id = 10;
myobjecttype mo = objectaccess::get(id);
}
At objectaccess I have:
#include "commonaccess.hpp"
namespace objectaccess {
myobjecttype get(int id)
{
myobjecttype mo = commonaccess::getFromTableX(id);
return mo;
}
}
At commonaccess I have:
#include <sqlite3.h>
namespace commonaccess {
myobjecttype getFromTableX(int id)
{
sqlite3_init(whatever...);
sqlite3_exe(whatever...);
myobjecttype retobject;
retobject.whatever = data1;
return retobject;
}
}
The code runs fine and is tested, except for one problem. All three namespaces are in different projects delivering a dynamic library (commonaccess.so and objectaccess.so) except for the main program that returns a binary executable.
My problem is:
At main, if I make a call to any of commonaccess functions, I get the following errors on linking:
g++ -L"/home/workspace/objectaccess/Debug"
-L"/home/workspace/commonaccess/Debug"
-L/usr/lib/i386-linux-gnu -Xlinker -rpath="./" -o "test" ./src/test.o
-lsqlite3 -lobjectaccess -lcommonaccess
/home/workspace/commonaccess/Debug/libcommonaccess.so: undefined reference to `sqlite3_close'
/home/workspace/commonaccess/Debug/libcommonaccess.so: undefined reference to `sqlite3_exec'
/home/workspace/commonaccess/Debug/libcommonaccess.so: undefined reference to `sqlite3_free'
/home/workspace/commonaccess/Debug/libcommonaccess.so: undefined reference to `sqlite3_errmsg'
/home/workspace/commonaccess/Debug/libcommonaccess.so: undefined reference to `sqlite3_open'
/home/workspace/commonaccess/Debug/libcommonaccess.so: undefined reference to `sqlite3_last_insert_rowid'
collect2: error: ld returned 1 exit status
This is simply solved adding at main() a call to any function of commonaccess, like:
#include "objectaccess.hpp"
#include "commonaccess.hpp"
int main()
{
commonaccess::dummycall();
int id = 10;
myobjecttype mo = objectaccess::get(id);
}
Actually calling a dummy function is not desired, so:
a) Why the linker is not being able to solve these references without directly calling any of the commonaccess functions?
b) Why only adding a call to the lower hierarchy namespace in the main program "teaches" the linker about real reference to sqlite3 functions?
Switch the order of libraries being linked. GNU linker can't reorder them, nor keeps references to calls not used so far, so -lsqlite3 is currently useless. Put it after the library that actually uses sqlite3 calls, something like g++ -L"/home/workspace/objectaccess/Debug" -L"/home/workspace/commonaccess/Debug" -L/usr/lib/i386-linux-gnu -Xlinker -rpath="./" -o "test" ./src/test.o -lobjectaccess -lcommonaccess -lsqlite3
I moved from Windows to Ubuntu and I wanted to try some C++ programming on Ubuntu. So here is very simple code and very stupid error which I can't resolve:
horse.h
#ifndef _horse_
#define _horse_
class Horse{
int speed;
public:
void saySomething();
};
#endif
horse.cpp
#include "horse.h"
#include <iostream>
using namespace std;
void Horse::saySomething(){
cout << "iiiihaaaaaaa brrrrr."<<endl;
}
and Main.cpp
#include "horse.h"
int main(){
Horse h;
h.saySomething();
}
After I compile (compilation is successful) and run this I get this error message:
/tmp/ccxuDyrd.o: In function `main':
Main.cpp:(.text+0x11): undefined reference to `Horse::saySomething()'
collect2: ld returned 1 exit status
Please help me somehow.
Try
g++ -c main.cpp horse.cpp (to compile)
g++ -o a.out main.o horse.o (to link)
It seems you only compiled your code but did not link the resulting object files. You probably invoked the compiler like this:
g++ main.cpp
You should instead compile every *.cpp file separately and then link each resulting *.o file. And you should do this with a Makefile.
Actually, the basic idea is the same on Windows with MSVC. The compiler produces object files, the linker links them together.
I installed polarssl:
make
sudo make install
tried to compile very simple file, named test.c:
#include <stdio.h>
#include "polarssl/md5.h"
int main(int argc, char * argv[])
{
int i;
for (i=1;i<1;i++)
{
char res[16];
if (md5_file("file.txt",res) == 0)
{
int count;
for (count=0;count<16;count++)
printf("%02x",res[count]);
printf("n");
}
}
return 0;
}
Compiled it like this:
gcc -lpolarssl test.c -I /usr/local/include/polarssl/
but it shows me:
/tmp/cczptlsk.o: In function `main':
test.c:(.text+0x36): undefined reference to `md5_file'
collect2: ld returned 1 exit status
whats the problem, how to fix it? I know for 100% that polarssl files are in /usr/local/include/polarssl/
The compiler will attempt to complete linkage in the order the objects or files are presented. In this case, since you had put -lpolarssl first, there were no unresolved symbols needed from that library, so nothing got linked in.
Putting -lpolarssl last lets the compiler resolve unresolved symbols from your source file from that library.
Includes are fine.
But linking is wrong. Try to put the -lpolarssl last in the linker command.
Then add a -L if libpolarssl.a is not found by the linker to point it to the right location.
I have made the following C++ program which is made up of 3 files:
The thing.h file
#ifndef THING_H
#define THING_H
class thing{
double something;
public:
thing(double);
~thing();
double getthing();
void setthing(double);
void print();
};
#endif
The thing.cpp file
#include <iostream>
#include "thing.h"
thing::thing(double d){
something=d;
}
thing::~thing(){
std::cout << "Destructed" << std::endl;
}
double thing::getthing(){
return something;
}
void thing::setthing(double d){
something = d;
}
void thing::print(){
std::cout <<"The someting is " << something << std::endl;
}
The main file
#include <iostream>
#include "thing.h"
int main(){
thing t1(5.5);
t1.print();
t1.setthing(7.);
double d=t1.getthing();
std::cout << d << std::endl;
system("pause");
return 0;
}
I had made this program previously all in one file and it ran perfectly but when I try split it into seperate files to create a header I get a linker error, here is the errors I get when I try run it from the main file:
[Linker error] undefined reference to `thing::thing(double)'
[Linker error] undefined reference to `thing::print()'
[Linker error] undefined reference to `thing::setthing(double)'
[Linker error] undefined reference to `thing::getthing()'
[Linker error] undefined reference to `thing::~thing()'
[Linker error] undefined reference to `thing::~thing()'
ld returned 1 exit status
From the above errors it seems asthough the main file doesnt recognise the functions inside the header, how do I fix this please?
In slightly less pedantic terms:
Your header file thing.h declares "what class thing should look like", but not its implementation, which is in the source file thing.cpp. By including the header in your main file (we'll call it main.cpp), the compiler is informed of the description of class thing when compiling the file, but not how class thing actually works. When the linker tries to create the entire program, it then complains that the implementation (thing::print() and friends) cannot be found.
The solution is to link all the files together when creating the actual program binary. When using the g++ frontend, you can do this by specifying all the source files together on the command line. For example:
g++ -o main thing.cpp main.cpp
will create the main program called "main".
It seems you are not linking thing.cpp into your "project".
If you are compiling using gcc:
g++ thing.cpp -o thing.o
g++ main.cpp -o main.o
g++ main.o thing.o -o my-best-application-ever
How to add the file to your project depends on the compiler/IDE/build-system you are using.
#sheu is right.. But you don't have to do anything if you just include thing.cpp in your main.cpp
Since you're already including thing.h in thing.cpp everything will work just fine if you include thing.cpp
Compiler knows about declarations of functions, but nothing about definitions. You need to say them where they are. The easiest way to do that is to create 'project' and add all files to it. Then compiler knows where to search all files.
Put some code in thing.cpp which let you know that it is being compiled i.e.
error ...
apparently it is not being compiled and linked...
When I run and build a simple program, it fails.
Here is the error message:
g++ -Wall -o "main" "main.cpp" (in directory: /home/markuz/Desktop)
/tmp/ccHV9wPu.o: In function main':
main.cpp:(.text+0x11): undefined reference toTest::display()'
collect2: ld returned 1 exit status
Compilation failed.
Here are the files. The compile and build command is the default of geany 1.22
//main.cpp
#include "imba.h"
int main(){
Test t;
t.display();
return 0;
}
//imba.h
class Test{
public:
void display();
};
//imba.cpp
#include <iostream>
#include "imba.h"
void Test::display(){
std::cout << "oi";
}
Any ideas about this?
Thanks.
You need to also add the imba.cpp file in the compilation step. Although you have included the header in your main file, you have not compiled the source for it and so the linker cannot find the object file for imba.cpp - that is what the error is complaining about