Im trying to compile program on Ubuntu and dont understand some things

Question

Im a Windows dev who has no expirience on building C/C++ programs on Linux, but now I need to. Right way would be to go and learn Make and g++ compiler, but before I commit to that I want to figure out some basic stuff.

So I have .c program which is compiled with this makefile:

CUDA_VER=11.5
ifeq ($(CUDA_VER),)
  $(error "CUDA_VER is not set")
endif

APP:= deepstream-test3-app

TARGET_DEVICE = $(shell gcc -dumpmachine | cut -f1 -d -)

NVDS_VERSION:=6.0

LIB_INSTALL_DIR?=/opt/nvidia/deepstream/deepstream-$(NVDS_VERSION)/lib/
APP_INSTALL_DIR?=/opt/nvidia/deepstream/deepstream-$(NVDS_VERSION)/bin/

ifeq ($(TARGET_DEVICE),aarch64)
  CFLAGS:= -DPLATFORM_TEGRA
endif

SRCS:= $(wildcard *.c)
$(info info is $(SRCS))

INCS:= $(wildcard *.h)

PKGS:= gstreamer-1.0

OBJS:= $(SRCS:.c=.o)

CFLAGS+= -I../../../includes \
        -I /usr/local/cuda-$(CUDA_VER)/include

CFLAGS+= $(shell pkg-config --cflags $(PKGS))

LIBS:= $(shell pkg-config --libs $(PKGS))

LIBS+= -L/usr/local/cuda-$(CUDA_VER)/lib64/ -lcudart -lnvdsgst_helper -lm \
        -L$(LIB_INSTALL_DIR) -lnvdsgst_meta -lnvds_meta \
        -lcuda -Wl,-rpath,$(LIB_INSTALL_DIR)

$(info info is $(CFLAGS))

all: $(APP)

%.o: %.c $(INCS) Makefile
    gcc -c -o $@ $(CFLAGS) $<

$(APP): $(OBJS) Makefile
    gcc -o $(APP) $(OBJS) $(LIBS)

install: $(APP)
    cp -rv $(APP) $(APP_INSTALL_DIR)

clean:
    rm -rf $(OBJS) $(APP)

First thing I tried is to change this Makefile to compile it as C++ program. I changed .c file into .cpp, in makefile I change gcc to g++ everywhere and .c to .cpp everywhere. It gave me error that it couldnt find "main" entry point.

I gave up on that pretty fast and decided just to use lines output of original makefile, ending up with this:

g++ -c -o deepstream_test3_app.o -I../../../includes -I /usr/local/cuda-11.5/include -pthread -I/usr/include/gstreamer-1.0 -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include ./deepstream_test3_app.cpp
g++ -o deepstream-test3-app deepstream_test3_app.o -lgstreamer-1.0 -lgobject-2.0 -lglib-2.0 -L/usr/local/cuda-11.5/lib64/ -lcudart -lnvdsgst_helper -lm -L/opt/nvidia/deepstream/deepstream-6.0/lib/ -lnvdsgst_meta -lnvds_meta -lcuda -Wl,-rpath,/opt/nvidia/deepstream/deepstream-6.0/lib/

First question, can I combine this 2 launches of g++ into one?

Second, when I make changes to "./deepstream_test3_app.cpp" they are not noticed by compiler. I added

#include <iostream>
...
std::cout << "hello!" << std::endl;

and they are ignored. Its like g++ gets as input some other copy/older version of the file and I dont understand how to go about it.

Hope for any help, sorry if it's all sounds stupid.

Answer 1

Ignoring for the moment the issues surrounding compiling C code with a C++ compiler,

g++ -c -o deepstream_test3_app.o -I../../../includes -I /usr/local/cuda-11.5/include -pthread -I/usr/include/gstreamer-1.0 -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include ./deepstream_test3_app.cpp
g++ -o deepstream-test3-app deepstream_test3_app.o -lgstreamer-1.0 -lgobject-2.0 -lglib-2.0 -L/usr/local/cuda-11.5/lib64/ -lcudart -lnvdsgst_helper -lm -L/opt/nvidia/deepstream/deepstream-6.0/lib/ -lnvdsgst_meta -lnvds_meta -lcuda -Wl,-rpath,/opt/nvidia/deepstream/deepstream-6.0/lib/

First question, can I combine this 2 launches of g++ into one?

Yes. It is a common practice in makefiles to separate the compilation and linking steps, but that is not mandatory. When there are multiple sources, the separation makes it possible to limit recompilations to only the source files that have changed, but it doesn't make much difference, makefile or not, when there is only one source file.

The one-command version would be mostly a concatenation of the two commands you gave. One would omit the -c option, which instructs g++ to compile but not link, and one would omit the -o deepstream_test3_app.o, which specifies the name of the object file that we are no longer going to create. One would also omit the appearance of deepstream_test3_app.o drawn from the link (second) command, as we are going straight from source file to program. The rest of the options can be reordered to some extent, but all the -l options need to remain in the same order relative to each other and to any object files among the inputs. Here is how I would write it:

g++ -c -o deepstream_test3_app -I../../../includes -I /usr/local/cuda-11.5/include -pthread -I/usr/include/gstreamer-1.0 -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include  -Wl,-rpath,/opt/nvidia/deepstream/deepstream-6.0/lib/ ./deepstream_test3_app.cpp -lgstreamer-1.0 -lgobject-2.0 -lglib-2.0 -L/usr/local/cuda-11.5/lib64/ -lcudart -lnvdsgst_helper -lm -L/opt/nvidia/deepstream/deepstream-6.0/lib/ -lnvdsgst_meta -lnvds_meta -lcuda

Second, when I make changes to "./deepstream_test3_app.cpp" they are not noticed by compiler.

The compiler compiles the source file(s) you tell it to.

Its like g++ gets as input some other copy/older version of the file

It is possible that you are indeed telling it to compile a different version than the one you modified. It is also possible that compilation fails, so you don't get a new executable. And it is possible that when you try to run the result, you are not running the program you think you are running. We don't have enough information to know.

With regard to the last, however, do be aware that on Linux, unlike on Windows, the working directory is not automatically in the executable search path. If you want to run the compiled result from the above command, you would want to specify the path to it, which you could most easily do by prepending ./ to its simple name: ./deepstream-test3-app.