CODEWITHSUNDEEP
makefilegnu-make
chris01
chris01

Reputation: 12311

make: 2 dependencies expected but only 1 exists

Lets assume I have a little project with the following make-file. Please attention to the generation of the o-file (2 dependencies - to the .cpp and the .hpp).

LIB_OPTS = -lpthread
CPP_OPTS = -std=c++11 -Wall

all: main
   ${CXX} ${CPP_OPTS} ${LIB_OPTS} *.o -o main

main: a.o b.o c.o main.o

%.o: %.cpp %.hpp
    ${CXX} ${CPP_OPTS} ${LIB_OPTS} -g -c $< -o $@

clean:
   rm *.o

a, b and c do have cpp and hpp-files.

main does only have a .cpp.

Here is the output.

g++ -std=c++11 -Wall -lpthread -g -c a.cpp -o a.o
g++ -std=c++11 -Wall -lpthread -g -c b.cpp -o b.o
g++ -std=c++11 -Wall -lpthread -g -c c.cpp -o c.o
g++    -c -o main.o main.cpp
g++ -std=c++11 -Wall -lpthread *.o -o main

What about the generation of main.o???

Where does that come from? Thats nothing to do with my rule.

If I have a main.hpp it would catch my rule. Sure I can create a separate rule for main.o but I expected %.o to make it.

Thanks for help! Chris

Upvotes: 0

Views: 32

Answers (1)

MadScientist
MadScientist

Reputation: 100781

GNU make has a number of built-in rules. Among them is a rule that knows how to create a .o file from a .cpp file. Since none of your rules are able to create main.o, make looks through its own built-in rules and finds one that it can use.

You can see a complete list of built-in rules using a command such as:

make -pf/dev/null

You can run make so that it doesn't use any of its own built-in rules with make -r.

Upvotes: 2

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