Why does makefile automatically generate object file(*.o) without any specific rule?

Question

I am learning to write makefile recently. All the documents I am reading tell me that I have to write a rule to execute commands. But I found that the following Makefile could generate object files with out any compile command. Why?

SOURCES=$(wildcard  src/*.c)
OBJECTS=$(patsubst %.c, %.o, $(SOURCES))

all: $(OBJECTS)

When I type make in terminal, I get this:

cc -c -o xxx.o xxx.c

How does it happened?

Read on until you reach the chapter about built-in rules. – too honest for this site Jan 18 '16 at 16:19 — too honest for this site, Jan 18 '16 at 16:19

score 10 · Accepted Answer · edited May 23 '17 at 12:13

10

Why does makefile automatically generate object file(*.o) without any specific rule?

Because it has a set of Built-in rules.

Make uses implicit rules like the following:

%.o: %.c  
    $(CC) $(CPPFLAGS) $(CFLAGS) -c

%: %.o    
    $(CC) $(LDFLAGS) n.o $(LOADLIBES) $(LDLIBS)

You can see all of them with make -p. While make -dshows the applied rules.

As Ian Abbott pointed out in the comment:

make -p needs to be run from a directory that has no makefile (and no Makefile, and no GNUmakefile).

See also the relevant GNU make documentation

and this post.

edited May 23 '17 at 12:13

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answered Jan 18 '16 at 16:32

terence hill

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To avoid confusion, the `make -p` needs to be run from a directory that has no makefile (and no Makefile, and no GNUmakefile). – Ian Abbott Jan 18 '16 at 16:35
Thanks for clarification, I'll add it – terence hill Jan 18 '16 at 16:38
Even in a directory with a makefile you can see which rules are built-in (it is in the `-p` output) but you might miss some built-in rules if the makefile overrides them. – Etan Reisner Jan 18 '16 at 16:40

Why does makefile automatically generate object file(*.o) without any specific rule?

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