Unresolved external symbol error lnk2001 unresolved external symbol

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 to something 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("%sn",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 added eg1.o to 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':

Possible causes

There are many ways to get this error. All of them involve a reference to a function or variable that the linker couldn’t resolve, or find a definition for. The compiler can identify when a symbol isn’t declared, but it can’t tell when the symbol isn’t defined. It’s because the definition may be in a different source file or library. If a symbol is referred to but never defined, the linker generates an unresolved external symbol error.

Here are some common problems that cause LNK2019:

The source file that contains the definition of the symbol isn’t compiled

In Visual Studio, make sure the source file that defines the symbol gets compiled as part of your project. Check the intermediate build output directory for a matching .obj file. If the source file isn’t compiled, right-click on the file in Solution Explorer, and then choose Properties to check the properties of the file. The Configuration Properties > General page should show an Item Type of C/C++ Compiler. On the command line, make sure the source file that contains the definition is compiled.

The object file or library that contains the definition of the symbol isn’t linked

In Visual Studio, make sure the object file or library that contains the symbol definition is linked as part of your project. On the command line, make sure the list of files to link includes the object file or library.

The declaration of the symbol isn’t spelled the same as the definition of the symbol

Verify you use the correct spelling and capitalization in both the declaration and the definition, and wherever the symbol is used or called.

A function is used but the type or number of the parameters don’t match the function definition

The function declaration must match the definition. Make sure the function call matches the declaration, and that the declaration matches the definition. Code that invokes function templates must also have matching function template declarations that include the same template parameters as the definition. For an example of a template declaration mismatch, see sample LNK2019e.cpp in the Examples section.

A function or variable is declared but not defined

LNK2019 can occur when a declaration exists in a header file, but no matching definition is implemented. For member functions or static data members, the implementation must include the class scope selector. For an example, see Missing Function Body or Variable.

The calling convention is different between the function declaration and the function definition

Some calling conventions (__cdecl, __stdcall, __fastcall, and __vectorcall) are encoded as part of the decorated name. Make sure the calling convention is the same.

A symbol is defined in a C file, but declared without using `extern "C"` in a C++ file

A file that’s compiled as C creates decorated names for symbols that are different from the decorated names for the same symbols declared in a C++ file, unless you use an extern "C" modifier. Make sure the declaration matches the compilation linkage for each symbol. Similarly, if you define a symbol in a C++ file that will be used by a C program, use extern "C" in the definition.

A symbol is defined as static and then later referenced outside the file

In C++, unlike C, global constants have static linkage. To get around this limitation, you can include the const initializations in a header file and include that header in your .cpp files, or you can make the variable non-constant and use a constant reference to access it.

A static member of a class isn’t defined

A static class member must have a unique definition, or it will violate the one-definition rule. A static class member that can’t be defined inline must be defined in one source file by using its fully qualified name. If it isn’t defined at all, the linker generates LNK2019.

A build dependency is only defined as a project dependency in the solution

In earlier versions of Visual Studio, this level of dependency was sufficient. However, starting with Visual Studio 2010, Visual Studio requires a project-to-project reference. If your project doesn’t have a project-to-project reference, you may receive this linker error. Add a project-to-project reference to fix it.

An entry point isn’t defined

The application code must define an appropriate entry point: main or wmain for console applications, and WinMain or wWinMain for Windows applications. For more information, see main function and command-line arguments or WinMain function. To use a custom entry point, specify the /ENTRY (Entry-Point Symbol) linker option.

You build a console application by using settings for a Windows application

If the error message is similar to unresolved external symbol WinMain referenced in function function_name, link by using /SUBSYSTEM:CONSOLE instead of /SUBSYSTEM:WINDOWS. For more information about this setting, and for instructions on how to set this property in Visual Studio, see /SUBSYSTEM (Specify Subsystem).

You attempt to link 64-bit libraries to 32-bit code, or 32-bit libraries to 64-bit code

Libraries and object files linked to your code must be compiled for the same architecture as your code. Make sure the libraries your project references are compiled for the same architecture as your project. Make sure the /LIBPATH or Additional Library Directories property points to libraries built for the correct architecture.

You use different compiler options for function inlining in different source files

Using inlined functions defined in .cpp files and mixing function inlining compiler options in different source files can cause LNK2019. For more information, see Function Inlining Problems.

You use automatic variables outside their scope

Automatic (function scope) variables can only be used in the scope of that function. These variables can’t be declared extern and used in other source files. For an example, see Automatic (Function Scope) Variables.

You call intrinsic functions or pass argument types to intrinsic functions that aren’t supported on your target architecture

For example, if you use an AVX2 intrinsic, but don’t specify the /ARCH:AVX2 compiler option, the compiler assumes that the intrinsic is an external function. Instead of generating an inline instruction, the compiler generates a call to an external symbol with the same name as the intrinsic. When the linker tries to find the definition of this missing function, it generates LNK2019. Make sure you only use intrinsics and types supported by your target architecture.

You mix code that uses native `wchar_t` with code that doesn’t

C++ language conformance work that was done in Visual Studio 2005 made wchar_t a native type by default. If not all files have been compiled by using the same /Zc:wchar_t settings, type references may not resolve to compatible types. Make sure wchar_t types in all library and object files are compatible. Either update from a wchar_t typedef, or use consistent /Zc:wchar_t settings when you compile.

You get errors for `printf` and `scanf` functions when you link a legacy static library

A static library that was built using a version of Visual Studio before Visual Studio 2015 may cause LNK2019 errors when linked with the UCRT. The UCRT header files <stdio.h>, <conio.h>, and <wchar.h>now define many *printf* and *scanf* variations as inline functions. The inlined functions are implemented by a smaller set of common functions. Individual exports for the inlined functions aren’t available in the standard UCRT libraries, which only export the common functions. There are a couple of ways to resolve this issue. The method we recommend is to rebuild the legacy library with your current version of Visual Studio. Make sure the library code uses the standard headers for the definitions of the *printf* and *scanf* functions that caused the errors. Another option for a legacy library that you can’t rebuild is to add legacy_stdio_definitions.lib to the list of libraries you link. This library file provides symbols for the *printf* and *scanf* functions that are inlined in the UCRT headers. For more information, see the Libraries section in Overview of potential upgrade issues.

Third-party library issues and vcpkg

If you see this error when you’re trying to configure a third-party library as part of your build, consider using vcpkg. vcpkg is a C++ package manager that uses your existing Visual Studio tools to install and build the library. vcpkg supports a large and growing list of third-party libraries. It sets all the configuration properties and dependencies required for successful builds as part of your project.

Diagnosis tools

Sometimes it’s difficult to tell why the linker can’t find a particular symbol definition. Often the problem is that you haven’t included the code that contains the definition in your build. Or, build options have created different decorated names for external symbols. There are several tools and options that can help you diagnose LNK2019 errors.

The /VERBOSE linker option can help you determine which files the linker references. This option can help you verify whether the file that contains the definition of the symbol is included in your build.
The /EXPORTS and /SYMBOLS options of the DUMPBIN utility can help you discover which symbols are defined in your .dll and object or library files. Make sure the exported decorated names match the decorated names the linker searches for.
The UNDNAME utility can show you the equivalent undecorated external symbol for a decorated name.

Examples

Here are several examples of code that causes LNK2019 errors, together with information about how to fix the errors.

A symbol is declared but not defined

In this example, an external variable is declared but not defined:

// LNK2019.cpp
// Compile by using: cl /EHsc /W4 LNK2019.cpp
// LNK2019 expected
extern char B[100];   // B isn't available to the linker
int main() {
   B[0] = ' ';   // LNK2019
}

Here’s another example where a variable and function are declared as extern but no definition is provided:

// LNK2019c.cpp
// Compile by using: cl /EHsc LNK2019c.cpp
// LNK2019 expected
extern int i;
extern void g();
void f() {
   i++;
   g();
}
int main() {}

Unless i and g are defined in one of the files included in the build, the linker generates LNK2019. You can fix the errors by including the source code file that contains the definitions as part of the compilation. Alternatively, you can pass .obj files or .lib files that contain the definitions to the linker.

A static data member is declared but not defined

LNK2019 can also occur when a static data member is declared but not defined. The following sample generates LNK2019, and shows how to fix it.

// LNK2019b.cpp
// Compile by using: cl /EHsc LNK2019b.cpp
// LNK2019 expected
struct C {
   static int s;
};

// Uncomment the following line to fix the error.
// int C::s;

int main() {
   C c;
   C::s = 1;
}

Declaration parameters don’t match the definition

Code that invokes function templates must have matching function template declarations. Declarations must include the same template parameters as the definition. The following sample generates LNK2019 on a user-defined operator, and shows how to fix it.

// LNK2019e.cpp
// compile by using: cl /EHsc LNK2019e.cpp
// LNK2019 expected
#include <iostream>
using namespace std;

template<class T> class
Test {
   // The operator<< declaration doesn't match the definition below:
   friend ostream& operator<<(ostream&, Test&);
   // To fix, replace the line above with the following:
   // template<typename T> friend ostream& operator<<(ostream&, Test<T>&);
};

template<typename T>
ostream& operator<<(ostream& os, Test<T>& tt) {
   return os;
}

int main() {
   Test<int> t;
   cout << "Test: " << t << endl;   // LNK2019 unresolved external
}

Inconsistent wchar_t type definitions

This sample creates a DLL that has an export that uses WCHAR, which resolves to wchar_t.

// LNK2019g.cpp
// compile with: cl /EHsc /LD LNK2019g.cpp
#include "windows.h"
// WCHAR resolves to wchar_t
__declspec(dllexport) void func(WCHAR*) {}

The next sample uses the DLL in the previous sample, and generates LNK2019 because the types unsigned short* and WCHAR* aren’t the same.

// LNK2019h.cpp
// compile by using: cl /EHsc LNK2019h LNK2019g.lib
// LNK2019 expected
__declspec(dllimport) void func(unsigned short*);

int main() {
   func(0);
}

To fix this error, change unsigned short to wchar_t or WCHAR, or compile LNK2019g.cpp by using /Zc:wchar_t-.

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

A minimal example involving a static library you built yourself

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

What are you doing wrong?

The explanation

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

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

See also

Possible causes

The source file that contains the definition of the symbol isn’t compiled

The object file or library that contains the definition of the symbol isn’t linked

The declaration of the symbol isn’t spelled the same as the definition of the symbol

A function is used but the type or number of the parameters don’t match the function definition

A function or variable is declared but not defined

The calling convention is different between the function declaration and the function definition

A symbol is defined in a C file, but declared without using `extern "C"` in a C++ file

A symbol is defined as static and then later referenced outside the file

A static member of a class isn’t defined

A build dependency is only defined as a project dependency in the solution

An entry point isn’t defined

You build a console application by using settings for a Windows application

You attempt to link 64-bit libraries to 32-bit code, or 32-bit libraries to 64-bit code

You use different compiler options for function inlining in different source files

You use automatic variables outside their scope

You call intrinsic functions or pass argument types to intrinsic functions that aren’t supported on your target architecture

You mix code that uses native `wchar_t` with code that doesn’t

You get errors for `printf` and `scanf` functions when you link a legacy static library

Third-party library issues and vcpkg

Diagnosis tools

Examples

A symbol is declared but not defined

A static data member is declared but not defined

Declaration parameters don’t match the definition

Inconsistent wchar_t type definitions

See also

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

A minimal example involving a static library you built yourself

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

What are you doing wrong?

The explanation

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

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

See also

Possible causes

The source file that contains the definition of the symbol isn’t compiled

The object file or library that contains the definition of the symbol isn’t linked

The declaration of the symbol isn’t spelled the same as the definition of the symbol

A function is used but the type or number of the parameters don’t match the function definition

A function or variable is declared but not defined

The calling convention is different between the function declaration and the function definition

A symbol is defined in a C file, but declared without using extern "C" in a C++ file

A symbol is defined as static and then later referenced outside the file

A static member of a class isn’t defined

A build dependency is only defined as a project dependency in the solution

An entry point isn’t defined

You build a console application by using settings for a Windows application

You attempt to link 64-bit libraries to 32-bit code, or 32-bit libraries to 64-bit code

You use different compiler options for function inlining in different source files

You use automatic variables outside their scope

You call intrinsic functions or pass argument types to intrinsic functions that aren’t supported on your target architecture

You mix code that uses native wchar_t with code that doesn’t

You get errors for *printf* and *scanf* functions when you link a legacy static library

Third-party library issues and vcpkg

Diagnosis tools

Examples

A symbol is declared but not defined

A static data member is declared but not defined

Declaration parameters don’t match the definition

Inconsistent wchar_t type definitions

See also

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

A symbol is defined in a C file, but declared without using `extern "C"` in a C++ file

You mix code that uses native `wchar_t` with code that doesn’t

You get errors for `printf` and `scanf` functions when you link a legacy static library