Question

Here is a tutorial for make file in Linux: http://mrbook.org/tutorials/make/

Here is a make file example:

all: hello

hello: main.o factorial.o hello.o
    g++ main.o factorial.o hello.o -o hello

main.o: main.cpp
    g++ -c main.cpp

factorial.o: factorial.cpp
    g++ -c factorial.cpp

hello.o: hello.cpp
    g++ -c hello.cpp

clean:
    rm -rf *o hello

For me this line is confusing main.o: main.cpp
What does it mean? As I understand, it means target main.o has a main.cpp dependency. But there is no target with main.cpp
It means a target (main.o) has a dependency (main.cpp) which that dependency (main.cpp) itself is not a target. So what is it (main.cpp)?

Answer 1

main.cpp is a file.

From the extension we expect it to be a c++ source file, and the associated rule suggest that this is the case.

The rule will be exectuted any time that main.cpp is newer than main.o (which is also a file).