how to write a simple makefile for c

Question

I need to write a simple make file for my.c, and so after

make

then my program can be run by

./my

Answer 1

Just a minor correction: put the -lm to the linking step, and there after all object files.

all: my
my: cJ.o my.o
    gcc cJ.o my.o -o my -lcrypt -lm
cJ.o: cJ/cJ.c
    gcc -c cJ/cJ.c
my.o: my.c
    gcc -c my.c

And then, you could work more with automatic variables:

all: my
my: cJ.o my.o
    gcc $^ -o $@ -lcrypt -lm
cJ.o: cJ/cJ.c
    gcc -c $^
my.o: my.c
    gcc -c $^

where $@ is the target of the current rule and $^ are the prerequisites.

See also http://www.gnu.org/software/make/manual/make.html.