CODEWITHSUNDEEP
gccmakefilelinker
Kevin Young
Kevin Young

Reputation: 47

Link static library using gcc with gnu make

I'm following Zed Shaw's tutorial "Learn C the Hard Way" and trying to teach myself c programming language.

On my ubuntu desktop, I encountered the linking problem he mentioned in the note of this post.

That is, when linking a static library with gcc, using a command like this:

gcc -Wall -g -DNDEBUG -lmylib ex29.c -o ex29

The linker fails to find the functions in the lib. To link correctly, I have to change the order of source file and lib to this:

gcc -Wall -g -DNDEBUG ex29.c -lmylib -o ex29

And I'm trying to use the makefile offered by Zed to automate unit test. The makefile looks like this:

TEST_SRC=$(wildcard tests/*_tests.c)
TESTS=$(patsubst %.c,%,$(TEST_SRC))

TARGET=build/libYOUR_LIBRARY.a

tests: CFLAGS += $(TARGET)
tests: $(TESTS)
   sh ./tests/runtests.sh

The rest part of the makefile that isn't listed here can build the $(TARGET) lib flawlessly.

The problem is Zed append the lib to the $(CFLAGS) and use the implicit rule to compile the test files which leads to a command like this:

gcc -g -O2 -Wall -Wextra -Isrc -rdynamic -DNDEBUG tests/hashmap_tests.c build/mylib.a -o tests/list_tests

The command fails because of the link problem mentioned before as expected.

The solution I came up with was to write the compilation command explicitly like this so I can change the order:

$(TESTS): $(TARGET)
    $(CC) $(CFLAGS) $^ $(TARGET) -o $@

This works fine if there is only one main source file. Unfortunately, I have several out there under the ./tests directory, and a command like this is a total disaster.

My question is, how should I change my makefile to make it work or is there any other way I can do the same work as elegant as expected?

Upvotes: 1

Views: 3023

Answers (1)

MadScientist
MadScientist

Reputation: 100781

CFLAGS holds compiler flags, like -g -O2. You should not add linker flags to it. CPPFLAGS holds preprocessor flags like -Isrc -DNDEBUG. LDFLAGS holds linker flags, which would include things like -L (capital L) if you need it to find libraries, and -rdynamic. And the LDLIBS variable holds libraries, so you should do this:

CPPFLAGS = -Isrc -DNDEBUG
CFLAGS = -g -O2 -Wall -Wextra
LDFLAGS = -rdynamic
LDLIBS = -lmylib

Now you can use the built-in rules for GNU make to build your program. You can see a list of the build-in rules by running make -p -f/dev/null.

Of course the above are just the default variables make defines and uses with its default rules. You don't have to use them, but in general it's better to follow conventions rather than flaunt them.

Upvotes: 2

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