Getting Make to automatically detect changes in protocol buffers - makefile

Question

I want Make to automatically compile protos when I update them, here is what I've got so far:
TARGET=main
BIN_DIR=bin
SRC_DIR=src
OBJ_DIR=obj
PROTO_DIR=protos/
PROTO_COMPILE_DIR=src/$(PROTO_DIR)
CC = g++
CFLAGS = -Wall -std=c++17 -ggdb -pipe -I.
LINKER = g++
LFLAGS = $(CFLAGS) -lprotobuf
SOURCES = $(wildcard src/*.cc) \
$(wildcard src/protos/*.cc) \
$(wildcard src/db_handler/*.cc)
OBJECTS := $(SOURCES:$(SRC_DIR)/%.cc=$(OBJ_DIR)/%.o)
$(BIN_DIR)/$(TARGET): proto $(OBJECTS)
#mkdir -p $(BIN_DIR)/
$(LINKER) $(OBJECTS) $(LFLAGS) -o $#
$(OBJECTS): $(OBJ_DIR)/%.o : $(SRC_DIR)/%.cc
#mkdir -p obj/ obj/protos obj/db_handler
$(CC) $(CFLAGS) -c $< -o $#
.PHONY: proto
proto:
#printf "Compiling protos...\n"
#cd $(PROTO_DIR) && protoc * --grpc_out=../$(PROTO_COMPILE_DIR)\
--cpp_out=../$(PROTO_COMPILE_DIR)\
--plugin=protoc-gen-grpc="/usr/local/bin/grpc_cpp_plugin"\
&& cd ../
It successfully compiles protos; but, it does so every time, even if there are no changes in files. How can I prevent this and compile protos only if protos change?
Edit: Added project structure
├── LICENSE
├── makefile
├── protos
│   ├── client.proto
│   └── person.proto
├── README.md
└── src
├── db_handler
│   ├── db_handler.cc
│   └── db_handler.h
├── main.cc
└── protos
├── client.grpc.pb.cc
├── client.grpc.pb.h
├── client.pb.cc
├── client.pb.h
├── person.grpc.pb.cc
├── person.grpc.pb.h
├── person.pb.cc
└── person.pb.h

Answer 1

You want to add the source files of the protocol buffers to the right (the prerequisites) of the rule that cares about them. This is how Make understands and tracks their time stamps.
By adding the files to the prerequisites, Make will understand that this is the rule which cares about those source files.
PROTO_SOURCES := $(wildcard $(PROTO_DIR)/*.proto)
PROTOS := $(patsubst $(PROTO_DIR)/%.proto,$(PROTO_COMPILE_DIR)/%.cc,$(PROTO_SOURCES))
$(PROTOS): $(PROTO_SOURCES)
#printf "Compiling protos...\n"
#cd $(PROTO_DIR) && protoc * --grpc_out=../$(PROTO_COMPILE_DIR)\
--cpp_out=../$(PROTO_COMPILE_DIR)\
--plugin=protoc-gen-grpc="/usr/local/bin/grpc_cpp_plugin"
However, this $(PROTOS) : $(PROTO_SOURCES) is not good if you use parallel builds; because, Make will try to run the command once for each output file. So, make will run N instances of the command at the same time, which means they may clobber each other.
In order to know the fully correct solution you need to provide more information (for those not familiar with protoc). Is it a requirement that you invoke protoc once with all inputs? Or is it valid to run protoc individually on each input .proto file to get its output? Then you can write a pattern rule that generates one file at a time.
Note 1: As to your other attempt, if you use .PHONY to mark the rule then Make will rebuild the rule every time weather it needs to or not.
Note 2: You don't need the cd .. at the end of the second instruction, since it is run in a sub shell.

Answer 2

Thanks to #FiddlingBits I managed to figure out how to do it correctly
TARGET=main
BIN_DIR=bin
SRC_DIR=src
OBJ_DIR=obj
PROTO_DIR=protos/
PROTO_COMPILE_DIR=src/$(PROTO_DIR)
rm = rm -f
CC = g++
CFLAGS = -Wall -std=c++17 -ggdb -pipe -I.
LINKER = g++
LFLAGS = $(CFLAGS) -lprotobuf
SOURCES = $(wildcard src/*.cc) \
$(wildcard src/protos/*.cc) \
$(wildcard src/db_handler/*.cc)
OBJECTS := $(SOURCES:$(SRC_DIR)/%.cc=$(OBJ_DIR)/%.o)
PROTOS := $($(PROTO_DIR)/%.proto=$(PROTO_COMPILE_DIR)/%.cc)
$(BIN_DIR)/$(TARGET): $(PROTOS) $(OBJECTS)
echo $(PROTOS)
#mkdir -p $(BIN_DIR)/
$(LINKER) $(OBJECTS) $(LFLAGS) -o $#
$(OBJECTS): $(OBJ_DIR)/%.o : $(SRC_DIR)/%.cc
#mkdir -p obj/ obj/protos obj/db_handler
$(CC) $(CFLAGS) -c $< -o $#
$(PROTOS):
#printf "Compiling protos...\n"
#cd $(PROTO_DIR) && protoc * --grpc_out=../$(PROTO_COMPILE_DIR)\
--cpp_out=../$(PROTO_COMPILE_DIR)\
--plugin=protoc-gen-grpc="/usr/local/bin/grpc_cpp_plugin"\
&& cd ../
.PHONY: clean
clean:
#$(rm) -r $(OBJ_DIR)/*
#$(rm) -r $(BIN_DIR)/*
#printf "Cleanup complete!\n"