对于 C 项目，如何有效地修补外部库并将其编译到 Makefile 中？

Question

I'm working on a C project which needs an external open source library. In particular, it needs a version I patched myself in order to add some needed features.

At the moment I'm using a Makefile which expects a statically compiled version of the patched library inside the ./lib folder (let's call it libpatched.a ), and the corresponding header files in ./include/libpatched . The following are the main parts of the aforementioned Makefile:

EXECNAME=MyExecutable

CC=gcc

SRC_DIR=src
OBJ_DIR=obj

SRC=$(wildcard $(SRC_DIR)/*.c)

OBJ=$(SRC:$(SRC_DIR)/%.c=$(OBJ_DIR)/%.o)

CFLAGS += -Wall -O2 -Iinclude -Iinclude/libpatched
LDFLAGS += -Llib
LDLIBS += -lpatched

.PHONY: all clean

all: $(EXECNAME)

$(EXECNAME): $(OBJ_CC)
    $(CC) $(LDFLAGS) $^ $(LDLIBS) -o $@

$(OBJ_DIR)/%.o: $(SRC_DIR)/%.c
    $(CC) $(CFLAGS) -c $< -o $@

clean:
    $(RM) $(OBJ_DIR)/*.o

This Makefile is working correctly; however, I was looking for a more flexible solution, which does not need any statically compiled library before make is called. What I would like to accomplish is having a Makefile which does something like:

1. Download a specific version of the original library (in order to never have any compatibility problem)

2. Apply a patch using patch and a diff file ( .patch )

3. Compile the patched library (either statically or dynamically) for the current platform, using cmake , as required by the original library

4. Compile my project, using libpatched

Are these steps valid in your opinion, or is there a much better way to handle this need for a patched library?

If yes, as I'm not an expert at all in creating Makefiles, is there an easy way to reach this goal by simply leveraging on a properly written Makefile?

Which could be the best way to do so?

Thank you very much in advance.

Answer 1

I've done exactly this before, when building cross compiler etc with my patches for my operating system kernel. You can use the wget or curl commands in the Makefile. For example something like

# foo.tar.gz needs to be downloaded
foo.tar.gz:
    wget https://download.source.from/here/foo.tar.gz -O foo.tar.gz

# the makefile requires the downloaded file.
foo_src/CMakeLists.txt: foo.tar.gz
    mkdir -p foo_src
    cd foo_src && tar xfz ../foo.tar.gz

# patch the library if flag not present
foo_patched.flag:
    cd foo_src && patch -p1 ../foo.patch
    touch foo_patched.flag

# this depends on patching
libpatched.a: foo_src/CMakeLists.txt foo_patched.flag 
    cd foo_src && cmake
    cp foo_src/libfoo.a libpatched.a

The Makefile format is very simple - unlike CMake: - the rules just say, "to generate the file on the left. please build the prerequisites on the right side first. Then execute these commands to actually generate the file on the left hand side"