Question

When I use make, it says

gcc -I./header -c -g main.c
gcc: error: main.c: No such file or directory

My makefile is here:

vpath %.c ./src
vpath %.h ./header

CC = gcc
FLAG = -I./header -c -g
objects = main.o a.o b.o

app: ${objects}
    ${CC} -o app ${objects}

main.o: main.c command.h
    ${CC} ${FLAG} main.c

a.o: a.c command.h
    ${CC} ${FLAG} a.c

b.o: b.c command.h
    ${CC} ${FLAG} b.c

clean: 
    -rm *.o app

.PHONY: clean

and files are stored like this:

.
|-- header
|   `-- command.h
|-- Makefile
`-- src
    |-- a.c
    |-- b.c
    `-- main.c

What’s wrong with the Makefile?

Answer 1

You should use automatic variables instead of hard coded values for file names in recipes:

main.o: main.c command.h
    ${CC} ${FLAG} $<

...

This allows Make to insert the right file names found in vpath:

When a prerequisite is found in another directory through directory search, this cannot change the recipe of the rule; they will execute as written. Therefore, you must write the recipe with care so that it will look for the prerequisite in the directory where make finds it.

This is done with the automatic variables such as $^ (see Automatic Variables). For instance, the value of $^ is a list of all the prerequisites of the rule, including the names of the directories in which they were found, and the value of $@ is the target.