C shared library problems

Question

I am trying to create a shared library called -lrfc7539 with the structure below:

rfc7539.o: rfc7539.c chacha20poly1305.o
    $(CC) $(CFLAGS) -c -o $@ $<

chacha20poly1305.o: chacha20poly1305.c chacha20.o poly1305.o
    $(CC) $(CFLAGS) -c -o $@ $<

chacha20.o: chacha_merged.c
    $(CC) -fPIC $(CFLAGS) -c -o $@ $<

poly1305.o: poly1305-donna.c
    $(CC) -fPIC $(CFLAGS) -DPOLY1305_16BIT -c -o $@ $<

rfc7539_test: rfc7539.o chacha20poly1305.o poly1305.o chacha20.o

.PHONY: clean

clean: 
    @rm -f *.o
    @rm -f rfc7539_test

I then do this command gcc -shared -o lrfc7539.so *.o to create .so file Is there a better practice for my makefile to be able to do this automatically ?

Answer 1

You need to create a target in your makefile that runs the gcc command you gave. If this library is the main output, make an all target pointing to in.

all: lrfc7539.so

lrfc7539.so: rfc7539.o chacha20poly1305.o chacha20.o poly1305.o 
    gcc -shared -o $@ *.o

Answer 2

You can just make a rule with that target:

librfc7539.so: rfc7539.o chacha20poly1305.o poly1305.o chacha20.o
    $(LINK.c) $(OUTPUT_OPTION) -shared $^  $(LOADLIBES) $(LDLIBS)

I copied the command from make --print-data-base (you could update your other commands likewise). You may need to add -fPIC to LDFLAGS , too.

I called your library librfc7539.so so that you can link to it using -lrfc7539 - I think that's what you're wanting.

---

I believe it's best practice to explicitly specify the object files you intend to link, but some like to assume that every source file must be compiled and linked:

sources := $(wildcard *.c)
librfc7539.so: $(sources:.c=.o)

This wouldn't work for you, though, unless you renamed the source files that are compiled to differently-named object files.

---

I note that it's strange that your object files depend on other object files. That shouldn't be the case, although they may need dependencies on some header files.

Here's a complete Makefile (assuming GNU Make):

CFLAGS += -Wall -Wextra
CFLAGS += -fPIC

LDFLAGS += -fPIC

%.so: LDFLAGS += -shared

all: rfc7539_test librfc7539.so

librfc7539.so: rfc7539.o chacha20poly1305.o poly1305.o chacha20.o
    $(LINK.c) $(OUTPUT_OPTION) $^ $(LOADLIBES) $(LDLIBS)

rfc7539_test: rfc7539.o chacha20poly1305.o poly1305.o chacha20.o
    $(LINK.c) $(OUTPUT_OPTION) $^ $(LOADLIBES) $(LDLIBS)

# Default %.o:%.c rule works, except for these files with misnamed sources:

chacha20.o: chacha_merged.c
    $(COMPILE.c) $(OUTPUT_OPTION) $<

poly1305.o: poly1305-donna.c
    $(COMPILE.c) $(OUTPUT_OPTION) $<

# Specific flags for this source file
poly1305.o: CFLAGS += -DPOLY1305_16BIT

.PHONY: clean

clean: 
    @$(RM) *.o
    @$(RM) rfc7539_test

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