Things to remember when compiling/linking C/C++ software
by Angel Leon. March 17, 2015;
Updated August 29, 2019.
Updated last on February 27, 2023
On the compilation phase, you will usually need to specify the different include paths so that the interfaces (.h, .hpp) which define structs, classes, constants, and functions can be found.
llvm include paths are passed with
-I/path/to/includes, you can pass as many
-I as you need.
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.
These compilation-time variables are passed using
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.
When you compile your .c, or .cpp files, you will end up with object files.
These files usually have
.o extensions on Linux, on Windows they might be under
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
They are created like this in Linux/Mac:
ar -cvq libctest.a ctest1.o ctest2.o ctest3.o
libctest.a will contain
They are created like this on Windows:
LIB.EXE /OUT:MYLIB.LIB FILE1.OBJ FILE2.OBJ FILE3.OBJ
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 (symbols you defined in your code), but they will also have symbols defined in shared libraries. Shared libraries are only loaded once in physical memory by the OS, but its symbol's offset are virtually mapped to the memory table of each process, so you'll process will see the same library symbols in different addresses that some other process that uses the library.
Thus, not just making the size of your executable as small as it needs to be, but you won't need to spend more physical memory loading 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.
.so files are created like this:
gcc -Wall -fPIC -c *.c gcc -shared -Wl,-soname,libctest.so.1 -o libctest.so.1.0 *.o
-Wallenables all warnings.
-cmeans compile only, don't run the linker.
-fPICmeans "Position Independent Code", a requirement for shared libraries in Linux.
-sharedmakes the object file created shareable by different executables.
-Wlpasses a comma separated list of arguments to the linker.
-sonamemeans "shared object name" to use.
-o <my.so>means output, in this case the output shared library
.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
.dll files are created like this:
LINK.EXE /DLL /OUT:MYLIB.DLL FILE1.OBJ FILE2.OBJ FILE3OBJ
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
libname.so which should be somewhere inside
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
What if libraries are in different folders?
If you have some other
libbar.so library on another folder outside
LD_LIBRARY_PATH you can explictly pass the full path to that library
/path/to/that/other/library/libbar.so, 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.
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 -L libjlibtorrent.dylib 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
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
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
- 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 prefixed to those methods.
nm is best used along with
Find all Undefined symbols
nm -u libMacOSXUtilsLeopard.jnilib _CFRelease _LSSharedFileListCopySnapshot _LSSharedFileListCreate _LSSharedFileListInsertItemURL _LSSharedFileListItemRemove _LSSharedFileListItemResolve _NSFullUserName _OBJC_CLASS_$_NSArray _OBJC_CLASS_$_NSAutoreleasePool _OBJC_CLASS_$_NSDictionary _OBJC_CLASS_$_NSMutableArray _OBJC_CLASS_$_NSMutableDictionary _OBJC_CLASS_$_NSString _OBJC_CLASS_$_NSURL __Block_copy __NSConcreteGlobalBlock __dyld_register_func_for_add_image __objc_empty_cache __objc_empty_vtable _calloc _class_addMethod _class_getInstanceMethod _class_getInstanceSize _class_getInstanceVariable _class_getIvarLayout
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
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
TxMessage.cpp did include
main.h, the 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
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
.bashrc, it can lead to unintended building consequences in many projects. I've wasted so many hours due to this mistake.