aprilandjan's blog

understand cpp build

08月 25日, 2019
3 minute read

create executable

  1. generate an execuatble(a.exe file in windows):

     # build executable
 g++ main.cpp

  2. generate and set output executable file name to hello.exe:

     # build executable and set output name
 g++ main.cpp -o hello

  3. The above commands do compile into object file and link with other object files and system libraries. Can also seperate into:

  # just compile, the output is defaultly `main.o`
  g++ -c main.cpp

  # if g++ target are object files, then it is `automatically linked`
  g++ main.o

understand library

A library is a set of pre-compiled object files, which can be linked to our program via linker.

There are two type:

  • static library. it’s extension is .a(archive) in unix or .lib(library) in windows, and when linked, the machine code is directly copied into executable.
  • shared library. it’s extension is .so(shared object) in unix or .dll(dynamic link library) in windows. When used, when linked, only small table was created in executable, and system will try to load external machine code, which is so-called dynamic linking. It’s just a sharable code library in operating system.

headers and libraries

When build program:

  • compiler will check header files to compile source codes. Each #include directive in codes will be checked. Very similar to import in JavaScript or some other.
  • linker will check libraries to resolve references of object files or libraries. library path(-Ldir, -L followed with library path) and library name(-lxxx, -l followed with library name) are both needed.

make and makefile

make is a utility provided by GNU to build source files. In windows, make is not directly suportted, we can install mingw and add its bin folder to system PATH to make these mingw unitilies available.

make is similar to npm scripts. Create a file named as makefile, and define scripts as following:

# default task. it neeed file `hello.exe` to finish
all: hello.exe

# task of create `executable` from `object files`. it need file `hello.o` as prerequisites. if the file exists, script below will be executed.
hello.exe: hello.o
	g++ -o hello.exe hello.o

# task of create `object files` from `source files`. it need file `main.cpp` as prerequisites. if the file exists, script below will be executed.
hello.o: main.cpp
	g++ -c main.cpp -o hello.o

# task of cleaning jobs. if run in windows powershell, it needs to be 'del' instead of `rm`
clean:
	rm hello.o hello.exe

when using make in terminal, it will defaultly try command all, then check if the prerequisite files exist. if not, it will find command to create it; otherwise, the defined script below will be executed. A litte similar to pre scripts in npm.

If files are all existed, none script will be excuted; if source file changed, the script will do both compile and link jobs.

Tasks that do not require files are phony targets, such as clean in above example. It is just a label representing the command, so it will be executed everytime when called.

References