Suppose you have a big project written in c++ which has a thousand of .cpp files and a thousand of .h files.And let's says the project also depends on ten static libraries. Let's says we are on Windows and we build our project in Visual Studio 20xx. When you press Ctrl + F7 Visual Studio to start compiling the whole solution ( suppose we have just one project in the solution )

What's the meaning of compilation ?

• Visual Studio search into file**.vcxproj**and start compiling each file which has the extension .cpp. Order of compilation is undefined.So you must not assume that the file main.cpp is compiled first
• If .cpp files depends on additional .h files in order to find symbols that may or may not be defined in the file .cpp
• If exists one .cpp file in which the compiler could not find one symbol, acompiler time errorraises the message Symbol x could not be found
• For each file with extension .cpp is generated an object file .o and also Visual Studio writes the output in a file named ProjectName.Cpp.Clean.txt which contains all object files that must be processed by the linker.

The Second step of compilation is done by Linker.Linker should merge all the object file and build finally the output ( which may be an executable or a library)

Steps In Linking a project

• Parse all the object files and find the definition which was only declared in headers ( eg: The code of one method of a class as is mentioned in previous answers, or event the initialization of a static variable which is member inside a class)
• If one symbol could not be found in object files he also is searched in Additional Libraries.For adding a new library to a projectConfiguration properties->VC++ Directories->Library Directoriesand here you specified additional folder for searching libraries andConfiguration properties->Linker->Inputfor specifying the name of the library. -If the Linker could not find the symbol which you write in one .cpp he raises alinker time errorwhich may sound like error LNK2001: unresolved external symbol "void __cdecl foo(void)" (?foo@@YAXXZ)

Observation

1. Once the Linker find one symbol he doesn't search in other libraries for it
2. The order of linking librariesdoes matter.
3. If Linker finds an external symbol in one static library he includes the symbol in the output of the project.However, if the library is shared( dynamic ) he doesn't include the code ( symbols ) in output, but Run-Time crashes may occur

How To Solve this kind of error

Compiler Time Error :

• Make sure you write your c++ project syntactical correct.

Linker Time Error

• Define all your symbol which you declare in your header files
• Use #pragma once for allowing compiler not to include one header if it was already included in the current .cpp which are compiled
• Make sure that your external library doesn't contain symbols that may enter into conflict with other symbols you defined in your header files
• When you use the template to make sure you include the definition of each template function in the header file for allowing the compiler to generate appropriate code for any instantiations.

Functions or class-methods are defined in source files with the inline specifier.

An example:-

main.cpp

# include "gum.h"

# include "foo.h"

int main()
{
    gum();
    foo f;
    f.bar();
    return 0;
}

foo.h (1)

# pragma once

struct foo {
    void bar() const;
};

gum.h (1)

# pragma once

extern void gum();

foo.cpp (1)

# include "foo.h"

# include <iostream>

inline /* <- wrong! */ void foo::bar() const {
    std::cout << __PRETTY_FUNCTION__ << std::endl;
}

gum.cpp (1)

# include "gum.h"

# include <iostream>

inline /* <- wrong! */ void gum()
{
    std::cout << __PRETTY_FUNCTION__ << std::endl;
}

If you specify that gum (similarly, foo::bar) is inline at its definition then the compiler will inline gum (if it chooses to), by:-

• not emitting any unique definition of gum, and therefore
• not emitting any symbol by which the linker can refer to the definition of gum, and instead
• replacing all calls to gum with inline copies of the compiled body of gum.

As a result, if you define gum inline in a source file gum.cpp, it is compiled to an object file gum.o in which all calls to gum are inlined and no symbol is defined by which the linker can refer to gum. When you link gum.o into a program together with another object file, e.g. main.o that make references to an external symbol gum, the linker cannot resolve those references. So the linkage fails:

Compile:

g++ -c  main.cpp foo.cpp gum.cpp

Link:

$ g++ -o prog main.o foo.o gum.o
main.o: In function `main':
main.cpp:(.text+0x18): undefined reference to `gum()'
main.cpp:(.text+0x24): undefined reference to `foo::bar() const'
collect2: error: ld returned 1 exit status

You can only define gum as inline if the compiler can see its definition in every source file in which gum may be called. That means its inline definition needs to exist in a header file that you include in every source file you compile in which gum may be called. Do one of two things:

Either don't inline the definitions

Remove the inline specifier from the source file definition:

foo.cpp (2)

# include "foo.h"

# include <iostream>

void foo::bar() const {
    std::cout << __PRETTY_FUNCTION__ << std::endl;
}

gum.cpp (2)

# include "gum.h"

# include <iostream>

void gum()
{
    std::cout << __PRETTY_FUNCTION__ << std::endl;
}

Rebuild with that:

$ g++ -c  main.cpp foo.cpp gum.cpp
imk@imk-Inspiron-7559:~/develop/so/scrap1$ g++ -o prog main.o foo.o gum.o
imk@imk-Inspiron-7559:~/develop/so/scrap1$ ./prog
void gum()
void foo::bar() const

Success.

Or inline correctly

Inline definitions in header files:

foo.h (2)

# pragma once

# include <iostream>

struct foo {
    void bar() const  { // In-class definition is implicitly inline
        std::cout << __PRETTY_FUNCTION__ << std::endl;
    }
};
// Alternatively...

# if 0

struct foo {
    void bar() const;
};
inline void foo::bar() const  {
    std::cout << __PRETTY_FUNCTION__ << std::endl;
}

# endif

gum.h (2)

# pragma once

# include <iostream>

inline void gum() {
    std::cout << __PRETTY_FUNCTION__ << std::endl;
}

Now we don't need foo.cpp or gum.cpp:

$ g++ -c main.cpp
$ g++ -o prog main.o
$ ./prog
void gum()
void foo::bar() const

When linking against shared libraries, make sure that the used symbols are not hidden.

The default behavior of gcc is that all symbols are visible. However, when the translation units are built with option -fvisibility=hidden, only functions/symbols marked with __attribute__ ((visibility ("default"))) are external in the resulting shared object.

You can check whether the symbols your are looking for are external by invoking:

# -D shows (global) dynamic symbols that can be used from the outside of XXX.so

nm -D XXX.so | grep MY_SYMBOL

the hidden/local symbols are shown by nm with lowercase symbol type, for example t instead of `T for code-section:

nm XXX.so
00000000000005a7 t HIDDEN_SYMBOL
00000000000005f8 T VISIBLE_SYMBOL

You can also use nm with the option -C to demangle the names (if C++ was used).

Similar to Windows-dlls, one would mark public functions with a define, for example DLL_PUBLIC defined as:

# define DLL_PUBLIC __attribute__ ((visibility ("default")))

DLL_PUBLIC int my_public_function(){
  ...
}

Which roughly corresponds to Windows'/MSVC-version:

# ifdef BUILDING_DLL

    #define DLL_PUBLIC __declspec(dllexport) 

# else

    #define DLL_PUBLIC __declspec(dllimport) 

# endif

More information about visibility can be found on the gcc wiki.

When a translation unit is compiled with -fvisibility=hidden the resulting symbols have still external linkage (shown with upper case symbol type by nm) and can be used for external linkage without problem if the object files become part of a static libraries. The linkage becomes local only when the object files are linked into a shared library.

To find which symbols in an object file are hidden run:

>>> objdump -t XXXX.o | grep hidden
0000000000000000 g     F .text  000000000000000b .hidden HIDDEN_SYMBOL1
000000000000000b g     F .text  000000000000000b .hidden HIDDEN_SYMBOL2

Even though this is a pretty old questions with multiple accepted answers, I'd like to share how to resolve anobscure"undefined reference to" error.

Different versions of libraries

I was using an alias to refer to std::filesystem::path: filesystem is in the standard library since C17 but my program needed to**also compile in C14**so I decided to use a variable alias:

# if (defined _GLIBCXX_EXPERIMENTAL_FILESYSTEM) //is the included filesystem library experimental? (C++14 and newer: <experimental/filesystem>)

using path_t = std::experimental::filesystem::path;

# elif (defined _GLIBCXX_FILESYSTEM) //not experimental (C++17 and newer: <filesystem>)

using path_t = std::filesystem::path;

# endif

Let's say I have three files: main.cpp, file.h, file.cpp:

*file.h#include's <experimental::filesystem> and contains the code above
*file.cpp, the implementation of file.h, #include's "file.h"
*main.cpp#include's <filesystem> and "file.h"

Note thedifferent librariesused in main.cpp and file.h. Since main.cpp #include'd "file.h" after <filesystem>, the version of filesystem used there wasthe C++17 one. I used to compile the program with the following commands:

$ g++ -g -std=c++17 -c main.cpp -> compiles main.cpp to main.o
$ g++ -g -std=c++17 -c file.cpp -> compiles file.cpp and file.h to file.o
$ g++ -g -std=c++17 -o executable main.o file.o -lstdc++fs -> links main.o and file.o

This wayany functioncontained in file.o and used in main.o thatrequired path_tgave "undefined reference" errors becausemain.oreferred to**std::filesystem::pathbutfile.otostd::experimental::filesystem::path**.

Resolution

To fix this I just needed tochange experimental::filesystem in file.h to .

Missing "extern" in const variable declarations/definitions (C++ only)

For people coming from C it might be a surprise that in C++ global constvariables have internal (or static) linkage. In C this was not the case, as all global variables are implicitly extern (i.e. when the static keyword is missing).

Example:

// file1.cpp
const int test = 5;    // in C++ same as "static const int test = 5"
int test2 = 5;

// file2.cpp
extern const int test;
extern int test2;

void foo()
{
 int x = test;   // linker error in C++ , no error in C
 int y = test2;  // no problem
}

correct would be to use a header file and include it in file2.cpp and file1.cpp

extern const int test;
extern int test2;

Alternatively one could declare the const variable in file1.cpp with explicit extern

Clean and rebuild

A "clean" of the build can remove the "dead wood" that may be left lying around from previous builds, failed builds, incomplete builds and other build system related build issues.

In general the IDE or build will include some form of "clean" function, but this may not be correctly configured (e.g. in a manual makefile) or may fail (e.g. the intermediate or resultant binaries are read-only).

Once the "clean" has completed, verify that the "clean" has succeeded and all the generated intermediate file (e.g. an automated makefile) have been successfully removed.

This process can be seen as a final resort, but is often a good first step; especially if the code related to the error has recently been added (either locally or from the source repository).

Inconsistent UNICODE definitions

A Windows UNICODE build is built with TCHAR etc. being defined as wchar_t etc. When not building with UNICODE defined as build with TCHAR defined as char etc. These UNICODE and _UNICODE defines affect all the "T" string types; LPTSTR, LPCTSTR and their elk.

Building one library with UNICODE defined and attempting to link it in a project where UNICODE is not defined will result in linker errors since there will be a mismatch in the definition of TCHAR; char vs. wchar_t.

The error usually includes a function a value with a char or wchar_t derived type, these could include std::basic_string<> etc. as well. When browsing through the affected function in the code, there will often be a reference to TCHAR or std::basic_string<TCHAR> etc. This is a tell-tale sign that the code was originally intended for both a UNICODE and a Multi-Byte Character (or "narrow") build.

To correct this, build all the required libraries and projects with a consistent definition of UNICODE (and _UNICODE).

1. This can be done with either;

# define UNICODE

# define _UNICODE

1. Or in the project settings;
   Project Properties > General > Project Defaults > Character Set
2. Or on the command line;

/DUNICODE /D_UNICODE

The alternative is applicable as well, if UNICODE is not intended to be used, make sure the defines are not set, and/or the multi-character setting is used in the projects and consistently applied.

Do not forget to be consistent between the "Release" and "Debug" builds as well.

When your include paths are different

Linker errors can happen when a header file and its associated shared library (.lib file) go out of sync. Let me explain.

How do linkers work? The linker matches a function declaration (declared in the header) with its definition (in the shared library) by comparing their signatures. You can get a linker error if the linker doesn't find a function definition that matches perfectly.

Is it possible to still get a linker error even though the declaration and the definition seem to match? Yes! They might look the same in source code, but it really depends on what the compiler sees. Essentially you could end up with a situation like this:

// header1.h
typedef int Number;
void foo(Number);

// header2.h
typedef float Number;
void foo(Number); // this only looks the same lexically

Note how even though both the function declarations look identical in source code, but they are really different according to the compiler.

You might ask how one ends up in a situation like that?Include pathsof course! If when compiling the shared library, the include path leads to header1.h and you end up using header2.h in your own program, you'll be left scratching your header wondering what happened (pun intended).

An example of how this can happen in the real world is explained below.

Further elaboration with an example

I have two projects: graphics.lib and main.exe. Both projects depend on common_math.h. Suppose the library exports the following function:

// graphics.lib    

# include "common_math.h"

void draw(vec3 p) { ... } // vec3 comes from common_math.h

And then you go ahead and include the library in your own project.

// main.exe

# include "other/common_math.h"

# include "graphics.h"

int main() {
    draw(...);
}

Boom! You get a linker error and you have no idea why it's failing. The reason is that the common library uses different versions of the same include common_math.h (I have made it obvious here in the example by including a different path, but it might not always be so obvious. Maybe the include path is different in the compiler settings).

Note in this example, the linker would tell you it couldn't find draw(), when in reality you know it obviously is being exported by the library. You could spend hours scratching your head wondering what went wrong. The thing is, the linker sees a different signature because the parameter types are slightly different. In the example, vec3 is a different type in both projects as far as the compiler is concerned. This could happen because they come from two slightly different include files (maybe the include files come from two different versions of the library).

Debugging the linker

DUMPBIN is your friend, if you are using Visual Studio. I'm sure other compilers have other similar tools.

The process goes like this:

1. Note the weird mangled name given in the linker error. (eg. draw@graphics@XYZ).
2. Dump the exported symbols from the library into a text file.
3. Search for the exported symbol of interest, and notice that the mangled name is different.
4. Pay attention to why the mangled names ended up different. You would be able to see that the parameter types are different, even though they look the same in the source code.
5. Reason why they are different. In the example given above, they are different because of different include files.

[1] By project I mean a set of source files that are linked together to produce either a library or an executable.

EDIT 1: Rewrote first section to be easier to understand. Please comment below to let me know if something else needs to be fixed. Thanks!

Befriending templates...

Given the code snippet of a template type with a friend operator (or function);

template <typename T>
class Foo {
    friend std::ostream& operator<< (std::ostream& os, const Foo<T>& a);
};

The operator<< is being declared as a non-template function. For every type T used with Foo, there needs to be a non-templated operator<<. For example, if there is a type Foo<int> declared, then there must be an operator implementation as follows;

std::ostream& operator<< (std::ostream& os, const Foo<int>& a) {/*...*/}

Since it is not implemented, the linker fails to find it and results in the error.

To correct this, you can declare a template operator before the Foo type and then declare as a friend, the appropriate instantiation. The syntax is a little awkward, but is looks as follows;

// forward declare the Foo
template <typename>
class Foo;

// forward declare the operator <<
template <typename T>
std::ostream& operator<<(std::ostream&, const Foo<T>&);

template <typename T>
class Foo {
    friend std::ostream& operator<< <>(std::ostream& os, const Foo<T>& a);
    // note the required <>        ^^^^
    // ...
};

template <typename T>
std::ostream& operator<<(std::ostream&, const Foo<T>&)
{
  // ... implement the operator
}

The above code limits the friendship of the operator to the corresponding instantiation of Foo, i.e. the operator<< <int> instantiation is limited to access the private members of the instantiation of Foo<int>.

Alternatives include;

• Allowing the friendship to extend to all instantiations of the templates, as follows;

template <typename T>
class Foo {
    template <typename T1>
    friend std::ostream& operator<<(std::ostream& os, const Foo<T1>& a);
    // ...
};

• Or, the implementation for the operator<< can be done inline inside the class definition;

template <typename T>
class Foo {
    friend std::ostream& operator<<(std::ostream& os, const Foo& a)
    { /*...*/ }
    // ...
};

• Note*, when the declaration of the operator (or function) only appears in the class, the name is not available for "normal" lookup, only for argument dependent lookup, from cppreference;

A name first declared in a friend declaration within class or class template X becomes a member of the innermost enclosing namespace of X, but is not accessible for lookup (except argument-dependent lookup that considers X) unless a matching declaration at the namespace scope is provided...

There is further reading on template friends at cppreference and the C++ FAQ.

Code listing showing the techniques above.

• As a side note to the failing code sample; g++ warns about this as follows*

warning: friend declaration 'std::ostream& operator<<(...)' declares a non-template function [-Wnon-template-friend]

note: (if this is not what you intended, make sure the function template has already been declared and add <> after the function name here)

A wrapper around GNU ld that doesn't support linker scripts

Some .so files are actually GNU ld linker scripts, e.g. libtbb.so file is an ASCII text file with this contents:

INPUT (libtbb.so.2)

Some more complex builds may not support this. For example, if you include -v in the compiler options, you can see that the mainwin gcc wrapper mwdip discards linker script command files in the verbose output list of libraries to link in. A simple work around is to replace the linker script input command file with a copy of the file instead (or a symlink), e.g.

cp libtbb.so.2 libtbb.so

Or you could replace the -l argument with the full path of the .so, e.g. instead of -ltbb do /home/foo/tbb-4.3/linux/lib/intel64/gcc4.4/libtbb.so.2

Your linkage consumes libraries before the object files that refer to them

• You are trying to compile and link your program with the GCC toolchain.
• Your linkage specifies all of the necessary libraries and library search paths
• If libfoo depends on libbar, then your linkage correctly puts libfoo before libbar.
• Your linkage fails with undefined reference tosomething errors.
• But all the undefined somethings are declared in the header files you have #included and are in fact defined in the libraries that you are linking.

Examples are in C. They could equally well be C++

A minimal example involving a static library you built yourself

my_lib.c

# include "my_lib.h"

# include <stdio.h>

void hw(void)
{
    puts("Hello World");
}

my_lib.h

# ifndef MY_LIB_H

# define MT_LIB_H

extern void hw(void);

# endif

eg1.c

# include <my_lib.h>

int main()
{
    hw();
    return 0;
}

You build your static library:

$ gcc -c -o my_lib.o my_lib.c
$ ar rcs libmy_lib.a my_lib.o

You compile your program:

$ gcc -I. -c -o eg1.o eg1.c

You try to link it with libmy_lib.a and fail:

$ gcc -o eg1 -L. -lmy_lib eg1.o 
eg1.o: In function `main':
eg1.c:(.text+0x5): undefined reference to `hw'
collect2: error: ld returned 1 exit status

The same result if you compile and link in one step, like:

$ gcc -o eg1 -I. -L. -lmy_lib eg1.c
/tmp/ccQk1tvs.o: In function `main':
eg1.c:(.text+0x5): undefined reference to `hw'
collect2: error: ld returned 1 exit status

A minimal example involving a shared system library, the compression library libz

eg2.c

# include <zlib.h>

# include <stdio.h>

int main()
{
    printf("%s\n",zlibVersion());
    return 0;
}

Compile your program:

$ gcc -c -o eg2.o eg2.c

Try to link your program with libz and fail:

$ gcc -o eg2 -lz eg2.o 
eg2.o: In function `main':
eg2.c:(.text+0x5): undefined reference to `zlibVersion'
collect2: error: ld returned 1 exit status

Same if you compile and link in one go:

$ gcc -o eg2 -I. -lz eg2.c
/tmp/ccxCiGn7.o: In function `main':
eg2.c:(.text+0x5): undefined reference to `zlibVersion'
collect2: error: ld returned 1 exit status

And a variation on example 2 involving pkg-config:

$ gcc -o eg2 $(pkg-config --libs zlib) eg2.o 
eg2.o: In function `main':
eg2.c:(.text+0x5): undefined reference to `zlibVersion'

What are you doing wrong?

In the sequence of object files and libraries you want to link to make your program, you are placing the libraries before the object files that refer to them. You need to place the libraries after the object files that refer to them.

Link example 1 correctly:

$ gcc -o eg1 eg1.o -L. -lmy_lib

Success:

$ ./eg1 
Hello World

Link example 2 correctly:

$ gcc -o eg2 eg2.o -lz

Success:

$ ./eg2 
1.2.8

Link the example 2 pkg-config variation correctly:

$ gcc -o eg2 eg2.o $(pkg-config --libs zlib) 
$ ./eg2
1.2.8

The explanation

• Reading is optional from here on*.

By default, a linkage command generated by GCC, on your distro, consumes the files in the linkage from left to right in commandline sequence. When it finds that a file refers to something and does not contain a definition for it, to will search for a definition in files further to the right. If it eventually finds a definition, the reference is resolved. If any references remain unresolved at the end, the linkage fails: the linker does not search backwards.

First,example 1, with static library my_lib.a

A static library is an indexed archive of object files. When the linker finds -lmy_lib in the linkage sequence and figures out that this refers to the static library ./libmy_lib.a, it wants to know whether your program needs any of the object files in libmy_lib.a.

There is only object file in libmy_lib.a, namely my_lib.o, and there's only one thing defined in my_lib.o, namely the function hw.

The linker will decide that your program needs my_lib.o if and only if it already knows that your program refers to hw, in one or more of the object files it has already added to the program, and that none of the object files it has already added contains a definition for hw.

If that is true, then the linker will extract a copy of my_lib.o from the library and add it to your program. Then, your program contains a definition for hw, so its references to hw are resolved.

When you try to link the program like:

$ gcc -o eg1 -L. -lmy_lib eg1.o

the linker has not addedeg1.oto the program when it sees -lmy_lib. Because at that point, it has not seen eg1.o. Your program does not yet make any references to hw: it does not yet make any references at all, because all the references it makes are in eg1.o.

So the linker does not add my_lib.o to the program and has no further use for libmy_lib.a.

Next, it finds eg1.o, and adds it to be program. An object file in the linkage sequence is always added to the program. Now, the program makes a reference to hw, and does not contain a definition of hw; but there is nothing left in the linkage sequence that could provide the missing definition. The reference to hw ends up unresolved, and the linkage fails.

Second,example 2, with shared library libz

A shared library isn't an archive of object files or anything like it. It's much more like a program that doesn't have a main function and instead exposes multiple other symbols that it defines, so that other programs can use them at runtime.

Many Linux distros today configure their GCC toolchain so that its language drivers (gcc,g++,gfortran etc) instruct the system linker (ld) to link shared libraries on an as-needed basis. You have got one of those distros.

This means that when the linker finds -lz in the linkage sequence, and figures out that this refers to the shared library (say) /usr/lib/x86_64-linux-gnu/libz.so, it wants to know whether any references that it has added to your program that aren't yet defined have definitions that are exported by libz

If that is true, then the linker will not copy any chunks out of libz and add them to your program; instead, it will just doctor the code of your program so that:-

• At runtime, the system program loader will load a copy of libz into the same process as your program whenever it loads a copy of your program, to run it.
• At runtime, whenever your program refers to something that is defined in libz, that reference uses the definition exported by the copy of libz in the same process.

Your program wants to refer to just one thing that has a definition exported by libz, namely the function zlibVersion, which is referred to just once, in eg2.c. If the linker adds that reference to your program, and then finds the definition exported by libz, the reference is resolved

But when you try to link the program like:

gcc -o eg2 -lz eg2.o

the order of events is wrong in just the same way as with example 1. At the point when the linker finds -lz, there are no references to anything in the program: they are all in eg2.o, which has not yet been seen. So the linker decides it has no use for libz. When it reaches eg2.o, adds it to the program, and then has undefined reference to zlibVersion, the linkage sequence is finished; that reference is unresolved, and the linkage fails.

Lastly, the pkg-config variation of example 2 has a now obvious explanation. After shell-expansion:

gcc -o eg2 $(pkg-config --libs zlib) eg2.o

becomes:

gcc -o eg2 -lz eg2.o

which is just example 2 again.

I can reproduce the problem in example 1, but not in example 2

The linkage:

gcc -o eg2 -lz eg2.o

works just fine for you!

(Or: That linkage worked fine for you on, say, Fedora 23, but fails on Ubuntu 16.04)

That's because the distro on which the linkage works is one of the ones that does not configure its GCC toolchain to link shared libraries as-needed.

Back in the day, it was normal for unix-like systems to link static and shared libraries by different rules. Static libraries in a linkage sequence were linked on the as-needed basis explained in example 1, but shared libraries were linked unconditionally.

This behaviour is economical at linktime because the linker doesn't have to ponder whether a shared library is needed by the program: if it's a shared library, link it. And most libraries in most linkages are shared libraries. But there are disadvantages too:-

• It is uneconomical at runtime, because it can cause shared libraries to be loaded along with a program even if doesn't need them.
• The different linkage rules for static and shared libraries can be confusing to inexpert programmers, who may not know whether -lfoo in their linkage is going to resolve to /some/where/libfoo.a or to /some/where/libfoo.so, and might not understand the difference between shared and static libraries anyway.

This trade-off has led to the schismatic situation today. Some distros have changed their GCC linkage rules for shared libraries so that the as-needed principle applies for all libraries. Some distros have stuck with the old way.

Why do I still get this problem even if I compile-and-link at the same time?

If I just do:

$ gcc -o eg1 -I. -L. -lmy_lib eg1.c

surely gcc has to compile eg1.c first, and then link the resulting object file with libmy_lib.a. So how can it not know that object file is needed when it's doing the linking?

Because compiling and linking with a single command does not change the order of the linkage sequence.

When you run the command above, gcc figures out that you want compilation + linkage. So behind the scenes, it generates a compilation command, and runs it, then generates a linkage command, and runs it, as if you had run the two commands:

$ gcc -I. -c -o eg1.o eg1.c
$ gcc -o eg1 -L. -lmy_lib eg1.o

So the linkage fails just as it does if you do run those two commands. The only difference you notice in the failure is that gcc has generated a temporary object file in the compile + link case, because you're not telling it to use eg1.o. We see:

/tmp/ccQk1tvs.o: In function `main'

instead of:

eg1.o: In function `main':

See also

The order in which interdependent linked libraries are specified is wrong

Putting interdependent libraries in the wrong order is just one way in which you can get files that need definitions of things coming later in the linkage than the files that provide the definitions. Putting libraries before the object files that refer to them is another way of making the same mistake.

Since people seem to be directed to this question when it comes to linker errors I am going to add this here.

One possible reason for linker errors with GCC 5.2.0 is that a new libstdc++ library ABI is now chosen by default.
If you get linker errors about undefined references to symbols that involve types in the std::__cxx11 namespace or the tag [abi:cxx11] then it probably indicates that you are trying to link together object files that were compiled with different values for the _GLIBCXX_USE_CXX11_ABI macro. This commonly happens when linking to a third-party library that was compiled with an older version of GCC. If the third-party library cannot be rebuilt with the new ABI then you will need to recompile your code with the old ABI.

So if you suddenly get linker errors when switching to a GCC after 5.1.0 this would be a thing to check out.

Linked .lib file is associated to a .dll

I had the same issue. Say i have projects MyProject and TestProject. I had effectively linked the lib file for MyProject to the TestProject. However, this lib file was produced as the DLL for the MyProject was built. Also, I did not contain source code for all methods in the MyProject, but only access to the DLL's entry points.

To solve the issue, i built the MyProject as a LIB, and linked TestProject to this .lib file (i copy paste the generated .lib file into the TestProject folder). I can then build again MyProject as a DLL. It is compiling since the lib to which TestProject is linked does contain code for all methods in classes in MyProject.

A bug in the compiler/IDE

I recently had this problem, and it turned out it was a bug in Visual Studio Express 2013. I had to remove a source file from the project and re-add it to overcome the bug.

Steps to try if you believe it could be a bug in compiler/IDE:

• Clean the project (some IDEs have an option to do this, you can also manually do it by deleting the object files)
• Try start a new project, copying all source code from the original one.

Use the linker to help diagnose the error

Most modern linkers include a verbose option that prints out to varying degrees;

• Link invocation (command line),
• Data on what libraries are included in the link stage,
• The location of the libraries,
• Search paths used.

For gcc and clang; you would typically add -v -Wl,--verbose or -v -Wl,-v to the command line. More details can be found here;

• Linux ld man page.
• LLVM linker page.
• "An introduction to GCC" chapter 9.

For MSVC, /VERBOSE (in particular /VERBOSE:LIB) is added to the link command line.

• The MSDN page on the /VERBOSE linker option.