C/C++#
Useful GCC flags#
-L<path>Add
<path>to linker search path. Should come before-l.-l<lib>Link against library
<lib>. Prependsliband.soor.ato library name. Must come after object file.-fPICCompile to Position Independent Code. Required for building dynamically linked libraries.
extern keyword#
The extern keyword is used to explicitly declare rather than define functions and variables over a global scope [1].
In C++, it can be used with "C" to declare pure-C functions/variables which are however defined in C++. This can be useful for example to expose C++ class methods to C [2] [3].
Building dynamically linked libaries#
Object files to be linked into shared library must be compiled with -fPIC option [4] [5], e.g.:
g++ -fPIC file1.cpp -o file1.o
Whilst should only affect some speciifc architecures, in pratice -fPIC option seems to be required for shared-library builds on most architectures. Does not seem to be detrimental to non-library builds however [6].
Link object files into shared library with -shared option:
g++ -shared file1.o -o libfile1.so
Once the library has been built, the OS must be told where to find it at runtime — this may not be the same place as at link-time. Exact link-/run-time serach rules can vary across Linux distributions, but generally can be set temporarily by setting LD_LIBRARY_PATH environment variable, or permanently by copying to a default location (e.g. /usr/local/lib) [7].
N.b. Simply copying library to e.g. /usr/local/lib after building may not be enough however as some distributions (e.g. Ubuntu) cache dynamic libraries, which therefore must be updated by calling ldconfig [8].