I am a very new to programming and have a very basic question that may be answered in other threads however I think they are far too advanced for me to understand how. I have actually found many answers so far on this site but this is the first problem that forced me to create an account and ask.
Anyway i am running a very basic example program on linux mint 18.3. Now I have seen this exact code work on a machine with windows 8 I believe so I was wondering if that could be the problem. I have created a class and when i plug in my object then build and run I get:
/usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/crt1.o||In function _start':|
(.text+0x20)||undefined reference tomain'|
This is the entire code:
#include <iostream>
#include "Gladius.h"
using namespace std;
int main()
{
Gladius io;
return 0;
}
Thats it very basic. here is the .h
#ifndef GLADIUS_H
#define GLADIUS_H
class Gladius
{
public:
Gladius();
};
#endif // GLADIUS_H
and the .cpp for the class.
#include "Gladius.h"
#include <iostream>
using namespace std;
Gladius::Gladius()
{
cout << "The Gladius is a short sword" << endl;
}
I know this seems extremely simple but I am just learning to code and i have been looking all over for an explanation why this isn't working yet I see it work on another pc exactly as is. Anyway any explanation would be greatly appreciated.
Here is what i found in command line If this answers your questions about what was in the cmd.
g++ -Wall -fexceptions -g -std=c++11 -Wall -I -c /home/gator/Documents/Spartan1/Gladius.cpp -o obj/Debug/Gladius.o
/usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/crt1.o: In function _start':
(.text+0x20): undefined reference tomain'
collect2: error: ld returned 1 exit status
Know the compiler options(gcc/g++ compiler):
-c : Compile and assemble, but do not link
-o file : Place the output into file
So when you run
g++ filename.cpp -o executable_name
, you generate an application which can be executed.
The problem is you are compiling, assembling as well as linking when you are trying to compile "Gladius.cpp" and compiler is trying to search for main() definition.
So in your case, the compilation steps would be:
First compile "Gladius.cpp" and generate object file "Gladius.o":
g++ -Wall -fexceptions -g -std=c++11 -c Gladius.cpp
Next compile "main.cpp" and generate object file "main.o":
g++ -Wall -fexceptions -g -std=c++11 -c main.cpp
Generate executable by linking "main.o" and "Gladius.o"
g++ -Wall -fexceptions -g -std=c++11 -o main main.o Gladius.o
Now you can run "main":
./main
Your compiler's command line contains -I -c sequence.
This -I option "swallows" your -c option. -I requires an additional argument, which is an include directory name. You failed to supply that argument, which is why -I assumes that -c that follows it is the directory name. So that -I consumes that -c.
The compiler never sees that -c. Without -c it assumes that you want to compile and link your program. Since Gladius.cpp does not have main in it, you get the error at linking stage.
Here 's a simple demo of the same problem: http://coliru.stacked-crooked.com/a/8a37cd3e90a443e2
You need to figure out why you have an orphaned -I in your command line.
If you are compiling this code using a command line like:
g++ -Wall -Wextra -Werror -O gladius.cpp -o output.exe
then make sure that you include all the .cpp files (not .h files) that contain code that your program needs.
g++ -Wall -Wextra -Werror -O gladius.cpp main.cpp -o output.exe
I explain this to beginners all the time as each .cpp being a bag of Lego's in a kit. You need all the bags that came with the box in order to build the kit. If you omitted main.cpp (or the file that contains main) then you will get the linker error that you are currently getting.
What command are you using to compile, link, and then execute? It should look something like
$ g++ main.cpp gladius.cpp -odemo
$ ./demo
check your command line for linking step.. You may forgot file with main as input, or you had forgot output file name after -o (and masked main.o in result)
I had this very kind of problem myself, and though it may not be the conventional, "proper" solution, I simply renamed the ".c" file to ".cpp", and it all worked.
After all, I was compiling both c and c++ together with a c++ compiler (recommended by the library), and the c code already had the proper c++ #extern flags (see here for more on that).
Also related:
C++ Error: undefined reference to `main'
Including C Code in C++
Why do you need an explicit `-lm` compiler option
Compilation on Linux - In function '_start': (.text+0x20): undefined reference to 'main'
Related
This may be a bit of a weird request but I'm trying to compile some software with g++ with the -nostdlib flag.
I'm pretty new to c++ and I'm still learning how linking and compiling works but I'm trying to compile this code:
#include "../lib/gamesys/types.h"
int main() {
types::setUid(1001);
}
Using these commands:
g++ -Wall -g -c bin/scripttwo.cpp -o bin/scripttwo.o -std=c++14
and
g++ -o bin/scripttwo bin/scripttwo.o software/*.o hardware/*.o *.o -nostdlib -Llib/lib/ -ltypes -ljsoncpp -lreadline -ltypes
and I'm getting the error:
/usr/bin/ld: software/FS.o: undefined reference to symbol '_Unwind_Resume##GCC_3.0'
/usr/bin/ld: /lib64/libgcc_s.so.1: error adding symbols: DSO missing from command line
I'm assuming it has something to do with the stdlib because once I remove -nostdlib, it compiles and links fine.
How can I solve this (it is pretty important that I don't include stdlibs)
Note: If you want to know why I'm trying to avoid stdlibs, let me know, its a long a convoluted story lol
There are similar questions but their answers did not work for my issue.
I have a c++ program with #include <boost/test/unit_test.hpp> on top (among other includes).
To compile correctly, if I understood, I should do the command:
g++ -g -L/path_to_boost_lib -lboost_lib myprog.cpp -o myprog.exe
If i do a locate, I get /usr/lib/x86_64-linux-gnu/libboost_unit_test_framework.so.
Hence I edited my call to g++ by doing:
g++ -g -L/usr/lib/x86_64-linux-gnu -lboost_unit_test_framework myprog.cpp -o myprog.exe
But I still get errors of the type undefined reference to boost::unit_test.
I also tried the option -I/usr/include/ which contains the boost folder, without success.
It's because of the order. The GCC linker goes through the artifacts left-to-right, and every unknown symbol it encounters in an object file must be resolved by an artifact occurring afterwards.
The right command is thus:
g++ -g myprog.cpp -L/usr/lib/x86_64-linux-gnu -lboost_unit_test_framework -o myprog.exe
See this answer for a more thorough explanation.
I suggest using a build tool like CMake that takes care of such low-level details for you.
So, I was following a simple C++ with SDL tutorial for linux but i encounter some errors on my way.
First of all I'm using Geany and i downloaded the corresponding SDL2 libs, here is the thing:
in my project folder there is a main.cxx file, which i open with geany as i mentioned before:
I included this libraries:
#include <iostream>
#include <SDL2/SDL.h>
#include <SDL2/SDL_image.h>
#include <SDL2/SDL_mixer.h>
First i encountered a pelculiar error, compilation performs sucessfully but when it comes to build i got this error:
main.cxx: undefined reference to `SDL_Init'
After searching a bit i found out that i had to add the tag -lSDL to my geany build options so they would end up being somethinf like this:
Compile:
g++ -Wall -c -lSDL "%f"
Build:
g++ -Wall -o -lSDL "%e" "%f"
But there is a problem, now when I execute the build command i get a:
G ++: error: main: There is no such file or directory
Why am i getting this error, am I including a wrong library or g++ has problems with .cxx files?
I already tried converting between .cxx and .cpp.
Thanks in advance.
g++ -Wall -c -lSDL2 "%f"
There is absolutely no need to specify libraries during compilation phase. Remove -lSDL.
g++ -Wall -o -lSDL2 "%e" "%f"
It invokes compiler, implies linking (no -c or other operation-specific flags), and sets output file name to -lSDL2. That is, linker will output resulting binary in a file named -lSDL2 in current working directory. Then, when it comes what files to link, it goes main, which supposed to be -o main, but since you've broken flags order it is now just ordinary file name that linker will try to link into resulting binary. It so happens that this file doesn't exist.
Long story short, make correct linking line - g++ -o "%e" %f -lSDL2 (libraries comes last, library order is also important).
Ok, so this might not be a great question, but I'm a bit stuck.
I have 3 programs:
pegio.c/pegio.h - Does hardware initialization and functions for my device
PegIOHandler.cpp/PegIOHandler.h - Handles I/O for device
PegRTU.cpp - Main function
My project implements the opendnp3 (which allows one to transmit data using the DNP3 protocol) library.
Now, to compile it I first compile the pegio.c file
gcc -c pegio.c -o pegio.o
Easy, up to here I understand.
Secondly I compile the IOHandler and reference pegio.c as it uses it.
g++ -c PegIOHandler.cpp pegio.c -o PegIOHandler.o -std=c++0x
Now it gives me the following output.
g++: warning: pegio.o: linker input file unused because linking not done
But, it still creates the object file. HOWEVER, PegIOHandler implements opendnp3 header files, which is included in PegIOHandler.cpp and PegIOHandler.h. When I attempt to compile these files without using the '-c', it tells me that there are undefined references to the opendnp3 files. I know I have to link them to my project, but how do I do that?
Then compililing my third and final file:
g++ PegRTU.cpp pegio.o PegIOHandler.o -o pegrtu -std=c++0x
This now tells me that there are undefined references to the opendnp3 files.
So: How do I link the opendnp3 library to my project / code files???
Thanks!
Ouch. So many misunderstandings. You don't need the previously compiled object files to compile further source files to object code. However, you do need to link them together at the end of the compilation process, including any libraries required. All in all:
gcc -c pegio.c -o pegio.o
g++ -c PegIOHandler.cpp -o PegIOHandler.o -std=c++0x
g++ -c PegRTU.cpp -o PegRTU.o -std=c++0x
g++ -o executable_name *.o -lopendnp3 # or whatever linker flag is needed for OpenDNP3
And pretty please, read this.
I came across this while trying to get an answer. But it seems like the poster had multiple files and they were not getting linked, and hence the error.
But, why do I get this error when using a single file?
g++ myClass.cpp
/usr/lib/gcc/i686-redhat-linux/4.6.3/../../../crt1.o: In function `_start':
(.text+0x18): undefined reference to `main'
collect2: ld returned 1 exit status
And why is main necessary here at compile time (from where does it find a mention of main in my code) ? main is the starting point of code execution, but why does the compiler assume i need a main here. I can have it defined in some other file and use gcc -o to make an executable?
Or maybe I am missing something else in the code which causes the error?
#include<iostream>
class myClass
{
public:
myClass()
{
std::cout<<"Constructor";
}
~myClass()
{
std::cout<<"Destructor";
}
};
main is not necessary to compile a source file. It is necessary to link a program into an executable [1], because the program has to start somewhere.
You need to tell the compiler that "this is not the whole of my program, just compile, but don't link", using the '-c' option, so
g++ -c myClass.cpp
which will produce a myClass.o file that you can then use later, e.g.
g++ -o myprog myClass.o myOtherClass.o something_that_has_main.o -lsomelib
(Obviously, substitute names with whatever you have in your project)
[1] Assuming you use the regular linker scrips that come with the compiler. There are "ways around that too", but I think that's beyond this answer.
You are trying to compile an executable, so a main function is expected. You should compile an object file by using the -c flag:
g++ -c myClass.cpp
While you are at it, I suggest adding warning flags -Wall -Wextra at the very least.
You are building your source as an application. Add -c option to produce only object file:
g++ -c myClass.cpp
Compile only?! use -c option
g++ -c file.cpp
otherwise the project needs a main.
You need to use the -c flag to compile and only tell the compiler to generate an object file. You're telling the compiler to make an executable.