undefined reference to defined function

hello

I'm facing a problem with my C++ program as it constantly returns the error

undefined reference to function

I saw that the function takes an argument const int
I was sending just int,so I thought that was the problem.
So I did the following but it still returns the same error.

Code:

---

int b;
const int a = b;
function(a);

---

How can I convert b to const int in order to send it as an argument to the function?

That is probably not the problem.
Where is your function prototype and definition located; w.r.t the line in which the error is shown ?

Quote:

---

Originally Posted by manasij7479

That is probably not the problem.
Where is your function prototype and definition located; w.r.t the line in which the error is shown ?

---

The function prototype is in a .h file ,the definition is in a .o file (linux)
I include the .h file in my .cpp file
(.text+0x839): undefined reference to `close(int)'

Oh, the solution is simple..
You need to give the compiler both the .cpp files together. (" g++ foo.cpp main.cpp -o output")
(... or compile them separately with the -c flag and invoke the linker manually. :D )

Quote:

---

Originally Posted by manasij7479

Oh, the solution is simple..
You need to give the compiler both the .cpp files together. (" g++ foo.cpp main.cpp -o output")
(... or compile them separately with the -c flag and invoke the linker manually. :D )

---

actually I tried that by doing: g++ main.cpp foo.o -o output
but it's still the same...

Quote:

---

Originally Posted by quo

actually I tried that by doing: g++ main.cpp foo.o -o output
but it's still the same...

---

Read my post again.

Quote:

---

Originally Posted by manasij7479

Read my post again.

---

I have.There's no foo.cpp file...I only have the foo.o..
I have compiled separately all my .cpp files and the linking is my problem,
because I want the linker to see the foo.o and link it with the others to one ouput.
When you say manually what do you mean?

Quote:

---

Originally Posted by quo

When you say manually what do you mean?

---

Then compile all the files separately, and give g++ only the object files.
Or
Type "man ld" in the terminal and read the instructions thoroughly.

Quote:

---

Originally Posted by manasij7479

Then compile all the files separately, and give g++ only the object files.
Or
Type "man ld" in the terminal and read the instructions thoroughly.

---

I have all my .o files but when I use g++ -o main A.o B.o C.o
returns the error of undefined reference
I have never used ld before .
Could you please give me an example?

Quote:

---

Originally Posted by quo

I have all my .o files but when I use g++ -o main A.o B.o C.o
returns the error of undefined reference

---

Change the order of the object files.
I though the gcc can looks backwards, but that seems not to be.

Quote:

---

I have never used ld before .
Could you please give me an example?

---

That is what the man page is for.

Quote:

---

Originally Posted by manasij7479

Change the order of the object files.
I though the gcc can looks backwards, but that seems not to be.


That is what the man page is for.

---

I used ld and it returns the same ,even more undefined references ..I also changed the order of the.o but nothing..
Since A.o contains the definitions of functions that every other .o needs and that's why they included all of them the same .h file
is there a way to tell the linker to link against A.o as well?
Maybe linker isn't doing this
I don't know,seems so confusing

maybe the problem I mentioned is the beginning is causing it?

Quote:

---

Originally Posted by quo

I used ld and it returns the same ,even more undefined references ..I also changed the order of the.o but nothing..
Since A.o contains the definitions of functions that every other .o needs and that's why they included all of them the same .h file
is there a way to tell the linker to link against A.o as well?
Maybe linker isn't doing this
I don't know,seems so confusing

---

If A.o contains the object file needed by B.o and C.o, I think A.o must be after B.o and C.o.

Example command of what I am suggesting this "-l" (lower case L)
is for needed library.

Code:

---

g++ B.o C.o A.o -o main -lLibName

---

Link Options - Using the GNU Compiler Collection (GCC)

I think the link below is correct for ld command; I did not find the link I used last time.
http://linux.die.net/man/1/ld

From above I found start-group/end-group this might work for you below.

Code:

---

g++ -Wl,--start-group A.o B.o C.o -Wl,--end-group -o exename

---

Tim S.

Actually in A.o are functions definitions used by every other file in my program.
Their declaration was in A.h
All of my files(.c and .cpp as well as .h) have #include "A.h"
The undefined references I get are ALL about these functions (in A.o)

Quote:

---

Originally Posted by quo

All of my files(.c and .cpp as well as .h) have #include "A.h"

---

Looks like you are mixing c and c++.
A c compiler uses a different name for the same function then a c++ compiler and that might confuse the linker.

What you exactly have to do depends on what A.o contains

Looks like the header "A.h" needs needs to have conditions like

Code:

---

#ifdef __cplusplus
  extern "C" {
#endif

// the functions defined in a c module
void a_c_function(); 

#ifdef __cplusplus
}
#endif

---

This is the way to enable c++-code to call c function.

if A.o contains c++ code then you have to conditionally exclude things like class definitions for the c compiler.


Kurt

can you provide the ACTUAL function declaration? can you provide the ACTUAL full error message? it just seems like you're asking for help without providing all of the relevant information. I'm surprised that nobody has said anything before. it's likely an extern "C" sort of thing, but we won't know until you give us more relevant info. also, if you don't have the source, but only the .o of one of the objects, you can maybe use objdump or nm to list the symbols in the .o file. you could then use that list to figure out why you're getting the error.

Quote:

---

Originally Posted by Elkvis

can you provide the ACTUAL function declaration? can you provide the ACTUAL full error message? it just seems like you're asking for help without providing all of the relevant information. I'm surprised that nobody has said anything before. it's likely an extern "C" sort of thing, but we won't know until you give us more relevant info. also, if you don't have the source, but only the .o of one of the objects, you can maybe use objdump or nm to list the symbols in the .o file. you could then use that list to figure out why you're getting the error.

---

Actually I just found out what it is.Yes you're absolutely right it's an extern "C" thing.I will explain exactly what's going on.

I have a A.o file (without the source code) which has been compiled with gcc.This file includes all the functions definitions I get the error message undefined reference to <function>.I get this message for every function of A.o I use anywhere in the code.The A.h contains the functions declarations so every .h,.c,.cpp file includes it as #include "A.h" (I have used #pragma once just in case).

Now,ALL of my other files(no matter if .c or .cpp) have been compiled with g++.So the linker error occurs due to the fact A.o is compiled with gcc.

I guess I have to use extern "C" in every file?Or just in header files(since .cpp and .c include them)?
And if so how exactly should I do that?I've never used it before and I barely know even its syntax..

Quote:

---

Originally Posted by ZuK

Looks like you are mixing c and c++.
A c compiler uses a different name for the same function then a c++ compiler and that might confuse the linker.

What you exactly have to do depends on what A.o contains

Looks like the header "A.h" needs needs to have conditions like

Code:

---

#ifdef __cplusplus
  extern "C" {
#endif

// the functions defined in a c module
void a_c_function(); 

#ifdef __cplusplus
}
#endif

---

This is the way to enable c++-code to call c function.

if A.o contains c++ code then you have to conditionally exclude things like class definitions for the c compiler.


Kurt

---

I just saw your post.
It's the header I have to change?
Or every file of my program which includes the header?

You have to change every header that both the c and the c++ compiler see.
Kurt

Quote:

---

Originally Posted by ZuK

You have to change every header that both the c and the c++ compiler see.
Kurt

---

The A.h is the only header which has functions that have been compiled with gcc (c functions) --A.o
All the others have been compiled with g++ and include the A.h
So only this has to be changed right?

I believe both function prototypes (declarations) and function definitions need to have extern "C" around them if you want to use the code in C++. This means add ZuK's macros-plus-code to both the sources and the headers. Try it and see what happens.

There's a bit of confusion in this thread regarding compiling multiple files etc so I'll try to clear that up. If you have one file, you can obviously just use

Code:

---

$ g++ program.cpp -o program

---

This preprocesses your source, compiles it, and links it. You can stop the compiler after each of these steps, and one common procedure is to have one command preprocess and compile, and another link.

Code:

---

$ g++ -c program.cpp  # -o program.o is optional
$ g++ program.o -o program

---

Of course, this works well with multiple source files too; you can compile each file with -c, and combine them all together with a "link" g++ command.

Code:

---

$ g++ -c file1.cpp
$ g++ -c file2.cpp
$ g++ file1.o file2.o -o program

---

However, you can also combine .cpp and .o files in one command line! g++ doesn't care. It just has to do more work when it sees a .cpp file. Also, there's rarely any need to invoke ld manually (although you can if you want). Just saying

Code:

---

$ g++ file1.o file2.o -o program

---

is sufficient to link object files together, and it has the added benefit of noticing C++ source files or assembly files and just dealing with them.

Finally: you can debug all of this yourself if you have the tools to do so. Specifically, "nm" will list all the symbols in an object file. If the symbol looks just like the function name, it's compiled as a C symbol; if it looks mangled, it's compiled as C++. Really, "mangled" is the technical term. Here's an example.

Code:

---

$ cat code.c
int func() { return 42; }
$ gcc -c code.c
$ nm code.o
0000000000000000 T func
$ cp code.c code.cpp
$ g++ -c code.cpp
$ nm code.o
0000000000000000 T _Z4funcv
$

---

The name of the first symbol is "func", which is clearly not mangled. But "_Z4funcv" is mangled. So you can tell whether C or C++ symbols are being put into the file. extern "C" should put a C symbol into the file (so that you can call it from C); if you're getting undefined references to "_Z4funcv" or whatever then there's no extern "C" in the header file. If you don't have extern "C" in the source file then the symbol will be mangled and you won't be able to call it from C.

There's a useful utility called c++filt that can unmangle symbols for you if they become unreadable: try "nm file.o | c++filt". If you're curious about this stuff, look up "ldd" too, it's pretty useful.

Finally, the order you specify files in for the g++ command line should only matter for libraries; you can put individual .o files in any order you want. I'm pretty sure of this. Otherwise just about every Makefile ever written would get random linker errors.