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Things to remember when compiling and linking C/C++ programs

Things to remember when compiling/linking C/C++ software

by Angel Leon. March 17, 2015; August 29, 2019.

Include Paths

On the compilation phase, you will usually need to specify the different include paths so that the interfaces (.h, .hpp) which define structs, classes, constans, and functions can be found.

With gcc and llvm include paths are passed with -I/path/to/includes, you can pass as many -I as you need.

In Windows, cl.exe takes include paths with the following syntax: /I"c:\path\to\includes\ you can also pass as many as you need.

Some software uses macro definition variables that should be passed during compile time to decide what code to include.

Compilation flags

These compilation-time variables are passed using -D, e.g. -DMYSOFTWARE_COMPILATION_VARIABLE -DDO_SOMETHING=1 -DDISABLE_DEPRECATED_FUNCTIONS=0

These compilation time flags are by convention usually put into a single variable named CXXFLAGS, which is then passed to the compiler as a parameter for convenience when you're building your compilation/make script.

Object files

When you compile your .c, or .cpp files, you will end up with object files. These files usually have .o extensions in Linux, in Windows they might be under .obj extensions.

You can create an .o file for a single or for many source files.

Static Library files

When you have several .o files, you can put them together as a library, a static library. In Linux/Mac these static libraries are simply archive files, or .a files. In windows, static library files exist under the .lib extension.

They are created like this in Linux/Mac:

ar -cvq libctest.a ctest1.o ctest2.o ctest3.o

libctest.a will contain ctest1.o,ctest2.o and ctest2.o

They are created like this in Windows:


When you are creating an executable that needs to make use of a library, if you use these static libraries, the size of your executable will be the sum of all the object files statically linked by the executable. The code is right there along the executable, it's easier to distribute, but again, the size of the executable can be bigger than it needs to... why? because, sometimes, many of the .o files, or even the entire .a file you're linking against might be a standard library that many other programs need.

Shared Libraries (Dynamic Libraries)

So shared or dynamic libraries were invented so that different programs or libraries would make external (shared) references to them, since they're "shared" the symbols defined in them don't need to be part of your executable or library, your executable contain symbols whose entry points or offset addresses might point to somewhere within themselves, but they will also have symbols whose entry points are expected to exist on shared libraries which need only be loaded once in a single portion of the operating shared memory, thus not just making the size of your executable as small as it needs to be, but you won't need to load the library for every process/program that needs its symbols.

On Linux shared files exist under the .so (shared object) file extension, on Mac .dylib (dynamic library), and in Windows they're called .dll (dynamic link libraries)

Another cool thing about dynamic libraries, is that they can be loaded during runtime, not just linked at compile time. An example of runtime dynamic libraries are browser plugins.

In Linux .so files are created like this:

gcc -Wall -fPIC -c *.c
gcc -shared -Wl,-soname, -o   *.o
  • -Wall enables all warnings.
  • -c means compile only, don't run the linker.
  • -fPIC means "Position Independent Code", a requirement for shared libraries in Linux.
  • -shared makes the object file created shareable by different executables.
  • -Wl passes a comma separated list of arguments to the linker.
  • -soname means "shared object name" to use.
  • -o <> means output, in this case the output shared library

In Mac .dylib files are created like this:

clang -dynamiclib -o libtest.dylib file1.o file2.o -L/some/library/path -lname_of_library_without_lib_prefix

In Windows .dll files are created like this:


Linking to existing libraries

When linking your software you may be faced with a situation on which you want to link against several standard shared libraries. If all the libraries you need exist in a single folder, you can set the LD_LIBRARY_PATH to that folder. By common standard all shared libraries are prefixed with the word lib. If a library exists in LD_LIBRARY_PATH and you want to link against it, you don't need to pass the entire path to the library, you simply pass -lname and you will link your executable to the symbols of which should be somewhere inside LD_LIBRARY_PATH.

Tip: You should probably stay away from altering your LD_LIBRARY_PATH, if you do, make sure you keep its original value, and when you're done restore it, as you might screw the build processes of other software in the system which might depend on what's on the LD_LIBRARY_PATH.

What if libraries are in different folders?

If you have some other library on another folder outside LD_LIBRARY_PATH you can explictly pass the full path to that library /path/to/that/other/library/, or you can specify the folder that contains it -L/path/to/that/other/library and then the short hand form -lbar. This latter option makes more sense if the second folder contains several other libraries.

Useful tools

Sometimes you may be dealing with issues like undefined symbol errors, and you may want to inspect what symbols (functions) are defined in your library.

On Mac there's otool, on Linux/Mac there's nm, on Windows there's depends.exe (a GUI tool that can be used to see both dependencies and the symbol's tables. Taking a look at the "Entry Point" column will help you understand clearly the difference between symbols linking to a shared library vs symbols linking statically to the same library)

Useful command options

See shared library dependencies on Mac with otool

otool -L libjlibtorrent.dylib 
	libjlibtorrent.dylib (compatibility version 0.0.0, current version 0.0.0)
	/usr/lib/libc++.1.dylib (compatibility version 1.0.0, current version 120.0.0)
	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1213.0.0)

See shared symbols with nm (Linux/Mac) With nm, you can see the symbol's name list. Familiarize yourself with the meaning of the symbol types:

  • T (text section symbol)
  • U (undefined - useful for those undefined symbol error),
  • I (indirect symbol).

If the symbol is local (non-external) the symbol type is presented in lowercase letters, for example a lowercase u represents an undefined reference to a private external in another module in the same library.

nm's documentation says that if you're working on Mac and you see that the symbol is preceeded by + or - it means it's an ObjectiveC method, if you're familiar with ObjectiveC you will know that + is for class methods and - is for instance methods, but in practice it seems to be a bit more explicit and you will often see objc or OBJC prefixed to those methods.

nm is best used along with grep ;)

Find all Undefined symbols

nm -u libMacOSXUtilsLeopard.jnilib

My C++ code compiles but it won't link

Linking is simply "linking" a bunch of .o files to make an executable.

Each one of these .o's may be compiled on their own out of their .cpp files, but when one references symbols that are supposed to exist in other .o's and they're not to be found then you get linking errors.

Perhaps through forward declarations you managed your compilation phase to pass, but then you get a bunch of symbol not found errors. Make sure to read them slowly, see where these symbols are being referenced, you will see that these issues occur due to namespace visibility in most cases.

Perhaps you copied the signature of a method that exists in a private space elsewhere into some other namespace where your code wasn't compiling, all you did was make it compilable, but the actual symbol might not be visible outside the scope where it's truly defined and implemented.

Function symbols can be private if they're declared inside anonymous namespaces, or if they're declared as static functions.

An example:

Undefined symbols for architecture x86_64:
  "FlushStateToDisk(CValidationState&, FlushStateMode)", referenced from:
      Network::TxMessage::handle(CNode*, CDataStream&, long long, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >&, bool, bool) in libbitcoin_server.a(libbitcoin_server_a-TxMessage.o)

Here, when I read the code of Network::TxMessage::handle(...) there was a call to FlushStateToDisk, which was declared in main.h, and coded in main.cpp. My TxMessage.cpp did include main.h, compilation was fine, I had a TxMessage.o file and a main.o, but the linker was complaining.

The issue was that FlushStateToDisk was declared as a static, therefore only visible inside main.o, once I removed the static from the declaration and implementation the error went away and my executable was linked. Similar things happen when functions are declared in anonymous spaces in other files, even if you forward declare them on your local .h

In other cases your code compiles and you get this error linking because your library can't be added using -lfoo, and adding its containing folder to -L doesn't cut it, in this case you just add the full path to the library in your compilation command: gcc /path/to/the/missing/library.o ... my_source.cpp -o my_executable


DO NOT EXPORT CFLAGS, CPPFLAGS and the like on your .bash_profile/.bashrc, it can lead to unintended building consequences in many projects. I've wasted so many hours due to this mistake.


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@aldenml aldenml commented Mar 17, 2015

A warning regarding the macro definition via flags (not compilation-time variables). If you pass -DMY_MACRO=0, still you have the MY_MACRO defined.

The shared library it's a little more complex than what's explained here, since you are open to shared memory and interprocess communications and if you are developing a driver (a kind of shared library) you need to be aware of in which OS space you are loaded.

I don't think it is correct to say "linked during runtime", instead it's more appropriate to use "loaded during runtime". The linking process is a static one. One particular point of Windows is that usually, the libraries come with a .lib stub, if you don't have this stub, you need to GetProcAddress and cast to the desired signature (just to deal with the stack).

I did't know about the ObjectiveC nm detail. Always learning :)


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@Petross404 Petross404 commented Dec 24, 2017

Nice article, but how can I find in which library of my Linux, my undefined function exists?

I might use ceil and not know which library I must link with - llibraryname


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@gubatron gubatron commented Apr 30, 2018

If it's undefined that means it does not exist in the library, so you'll have to grep your source code to know exactly where it exists and figure out what might be the cause why it's not being included in the corresponding .o (object) file.


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@iweigele iweigele commented May 1, 2018

@Petross404 In the case of ceil if you try "man ceil" there ought to be this line in the SYNOPSIS section:

Link with -lm.

so you know to have to pass -lm to the linker. You'll find it in

>objdump -t /usr/lib/ | grep ceil
0000aa40 l F .text 0000002b __ceil
000232b0 l F .text 0000002b __ceill
0001ab00 l F .text 0000002b __ceilf
0000aa40 w F .text 0000002b ceil
000232b0 w F .text 0000002b ceill
0001ab00 w F .text 0000002b ceilf

("nm -g" and "readelf -s" will also get you the same information in slightly different formats).

If the man page doesn't tell you then you can use guatron's grep idea on the libraries:
>grep -l "\<ceil\>" /usr/lib/*.so

(-l is lower case ell and specifies name only, the "\<ceil\>" is a regex the '\<' and '\>' are just to specify the start and end of the word, so with them "ceil" and "ceil." would match but "ceiling" would not. They do not have to be paired: "\<ceil" matches "ceilf" and "ceil\>" matches "__ceil". It excludes a bunch of matches from the pthreads library that I didn't want in my output)


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@Celthi Celthi commented Sep 28, 2020

Awesome article!


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@emgullufsen emgullufsen commented Apr 29, 2021

very useful reference - thanks! -eric


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@gubatron gubatron commented Apr 30, 2021

@emgullufsen Thank you. I often end up here every few months when I need to maintain some C++ builds.

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