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Header Files (.h) - Static Libraries (.lib, .a) - Dynamic／Shared Libraries (.dll, .so)

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read: gcc - g++ (Preprocessor - Compiler - Assembler - Linker - Execution)

Thing

Description

header file
  • a collection of C/C++ code

library
  • a collection of object files (i.e. compiled C/C++ code) that can be linked into your programs via the linker
  • you can list the contents of a library via “nm filename”

Static Library vs. Shared Library

External Libraries

Extension

Description

static library
  • .a (archive file) in Unixes
  • .lib (library) in Windows

When your program is linked against a static library, the machine code of external functions used in your program is copied into the executable. A static library can be created via the archive program “ar.exe”

shared library
  • .so (shared objects) in Unixes
  • .dll (dynamic link library) in Windows

When your program is linked against a shared library, only a small table is created in the executable. Before the executable starts running, the operating system loads the machine code needed for the external functions - a process known as dynamic linking. Dynamic linking makes executable files smaller and saves disk space, because one copy of a library can be shared between multiple programs. Furthermore, most operating systems allows one copy of a shared library in memory to be used by all running programs, thus, saving memory. The shared library codes can be upgraded without the need to recompile your program.

Searching for Header Files and Object-Files/Libraries (-I, -L and -l)

when building a C/C++ program:

  • preprocessor needs to know where the header files are to preprocess the source codes
  • linker needs to know where the libraries are to resolve the source code’s external references to other object files

The preprocessor and linker will not find the headers & libraries unless you set the appropriate options (e.g. -I, -L, -l) specifying where things are

Header Files (-I)

For each of the headers (e.g. #include “glew.h”) used in the C/C++ program, the preprocessor searches through the include-paths for files that match the headers.

The include-paths are specified:

  • -I option - (e.g. g++ main.cpp -I/path/to/directory/containing/header-file)
  • environment variable CPATH

Since the header’s filename is known (e.g., iostream.h, stdio.h), the compiler only needs the directories

Object-Files/Libraries (-L, -l)

The linker searches the library-paths for libraries needed to link the program into an executable.

The library-path is specified:

  • -L option - (e.g. g++ main.cpp -L/path/to/directory/containing/object-files)
  • environment variable LIBRARY_PATH

In addition, you also have to specify the library name:

  • In Unixs, the library libxxx.a is specified via -lxxx option (lowercase letter ‘l’, without the prefix “lib” and “.a” extension)
  • In Windows, provide the full name such as -lxxx.lib.

The linker needs to know both the directories as well as the library names. Hence, two options (-L, -l) need to be specified

GCC Environment Variables

GCC uses the following environment variables:

  • PATH: for searching the executables and run-time shared libraries (.dll, .so).
  • CPATH: for searching the include-paths for headers. It is searched after paths specified in -I<dir> options
    • C_INCLUDE_PATH & CPLUS_INCLUDE_PATH can be used to specify C and C++ headers if the particular language was indicated in pre-processing.
  • LIBRARY_PATH: for searching library-paths for link libraries. It is searched after paths specified in -L<dir> options.

Default Include-Paths, Library-Paths and Libraries

list default include-paths

cpp -v
......
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/gcc/x86_64-pc-cygwin/6.4.0/include
 /usr/include
 /usr/lib/gcc/x86_64-pc-cygwin/6.4.0/../../../../lib/../include/w32api

list default library-paths

ld --verbose | grep SEARCH_DIR | tr -s ' ;' \\012

gcc passes a few extra -L paths to the linker, which you can list with the following command

gcc -print-search-dirs | sed '/^lib/b 1;d;:1;s,/[^/.][^/]*/\.\./,/,;t 1;s,:[^=]*=,:;,;s,;,;  ,g' | tr \; \\012