How can I make GNU Make use a different compiler without manually editing the makefile?
You should be able to do something like this:
make CC=my_compiler
This is assuming whoever wrote the Makefile used the variable CC.
You can set the environment variables CC and CXX, which are used for compiling C and C++ files respectively. By default they use the values cc and g++
If the makefile is written like most makefiles, then it uses $(CC) when it wishes to invoke the C compiler. That's what the built-in rules do, anyway. If you specify a different value for that variable, then Make will use that instead. You can provide a new value on the command line:
make CC=/usr/bin/special-cc
You can also specify that when you run configure:
./configure CC=/usr/bin/special-cc
The configuration script will incorporate the new CC value into the makefile that it generates, so you don't need to manually edit it, and you can just run make by itself thereafter (instead of giving the custom CC value on the command line every time).
Many makefiles use 'CC' to define the compiler. If yours does, you can override that variable with
make CC='/usr/bin/gcc'
Use variables for the compiler program name.
Either pass the new definition to the make utility or set them in the environment before building.
See Using Variables in Make
Makefile:
#!MAKE
suf=$(suffix $(src))
ifeq ($(suf), .c)
cc=gcc
else
ifeq ($(suf), .cpp)
cc=g++
endif
endif
all:
$(cc) $(src) -o $(src:$(suf)=.exe)
clean:
rm *.exe
.PHONY: all clean
Terminal:
$ make src=main.c
Related
I want to type the following:
make fileName.cpp
To compile, and then to execute:
./fileName
If I have a make file like this:
commandToCompileFileName:
g++ -o fileName fileName.cpp
Then I can do:
make commandToCompileFileName
And then:
./fileName
I want to be able to do this with different files, without having to write a different prompt for each of them in makefile. So something would be in place of
commandToCompileFileName
in the makefile that would just compile whatever I type in after make, and the executable would just be named the same minus the .cpp.
This page in the very first paragraph describes precisely what I want and probably answers my question, yet I couldn't figure it out after playing around with '$#' and '$<'.
You actually do not need a Makefile to do this: make ships with a whole bunch of default rules, one of which creates programs from .cpp files.
In other words, just type make fileName, and be happy :)
(if you want to custom compilation flags, see the CXXFLAGS and LDFLAGS variables)
I use the following:
run_% : %
#echo "---- running $< ----"
$<
.PHONY : run_%
So that when you do, for example, make run_test it builds test target and runs it.
What I'm trying to do is create a c++ file from an object file but I cannot figure out a way to do so.
INCLUDEDIR = ../headers
CXXFLAGS = -std=c++11 -I $(INCLUDEDIR) -Wall -Wfatal-errors -O2
all:primeFactors.o
primeFactors.o: primeFactors.cpp $(INCLUDEDIR)/primeFactors.h
g++ $(CXXFLAGS) -c $< -o $#
When I try to build this I get
make: *** No rule to make target 'primeFactors.cpp', needed by
'primeFactors.o'. Stop.
which I understand but when I take out the primeFactor.cpp argument I then get told there is nothing to be done with the make file. So is there a way to do this?
In general; no, you cannot do that. An object file (.o file) is the result of the code passing through the compiler front-end (to parse the language) the optimizer (to make it run fast) and the compiler back-end (to produce code that will run on your CPU).
This process is not reversible. You cannot go from compiled code back to source.
can you create a c++ file from an .o object file with makefile?
A makefile will allow you to do that only if you have an underlying tool to do it. make, which uses makefiles to do its job, does not have any built-in mechanisms to pull that off.
Your makefile has a rule for building primeFactors.o.
primeFactors.o: primeFactors.cpp $(INCLUDEDIR)/primeFactors.h
It says that primeFactors.cpp and $(INCLUDEDIR)/primeFactors.h are needed to build primeFactors.o. If you don't have those files, or no rules to build them, there is no way for make to build primeFactors.o.
I have a c++ codebase. I have a CMakeLists. When I run make, it creates the binary myexec.
I would like to be able to run, say, make -- v2, and that it creates the exact same binary, but called myexec_v2. It would be great if it did not have to recompile everything to create this binary, but only the modified files.
How can I do that?
myexec myexec_v2: $(prerequisites)
#echo compile $#
observe:
$ make myexec
compile myexec
$ make myexec_v2
compile myexec_v2
if nothing else is needed to compile myexec_v2 then you are done.
see here for more information: https://www.gnu.org/software/make/manual/html_node/Multiple-Targets.html
here is a slightly more advanced version
myexec myexec_v2: myexec%: $(prerequisites)
#echo compile $# $*
this is called a static pattern rule. the myexec% in the middle is called the target pattern. in this example we only use it to get the so called stem. the stem would be _v2 and the variable $* expands to the stem.
for more information: https://www.gnu.org/software/make/manual/html_node/Static-Usage.html
observe:
$ make myexec
compile myexec
$ make myexec_v2
compile myexec_v2 _v2
now you can use either $# or $* in the recipe to modify the behaviour to compile on or the other.
but if you want to do if branches in the recipe read this first: Basic if else statement in Makefile
I should preface this by saying I am very new to Makefiles.
I created the following Makefile:
all: tiling_graph.o
g++ -o tiling_graph tiling_graph.o -L/usr/local/lib -ligraph
I am trying to make sure that -ligraph is included. However, when I type "make", I get the following output: "c++ -c -o tiling_graph.o tiling_graph.cpp"
Why is it not using the Makefile that I created in the current directory? I have tried using "make -f Makefile" and "make --file=Makefile" but none of these are working.
Also, right after I first made the Makefile, it worked correctly. The output after typing make was
"g++ -o tiling_graph tiling_graph.o -L/usr/local/lib -ligraph"
I executed ./tiling_graph and it was successful.
Then I edited tiling_graph.cpp, ran make again, and the output was "c++ -c -o tiling_graph.o tiling_graph.cpp" and has been ever since.
I would really appreciate any help. Thanks!
A simple way to think about a make rule:
target: dependency list
commands to make the target
is that it is a recipe for making the file called target from the list of files in the dependency list. Since make can see the file system, it can tell whether any file in the dependency list is newer than the file named target, which is its signal for recreating target. After all, if none of the dependencies have changed, the target must be up-to-date.
Note that make knows quite a lot about how to build files. In particular, it has a lot of built-in "pattern" rules, so it knows, for example, how to make an object file (prog.o) from a C++ source file (prog.cpp) or from a C source file (prog.c) or many other things. So you only need to actually write a makefile when you have other requirements, like a library (and even then you could just add that to an environment variable, but the makefile is better).
Now, you don't actually want to build a file called all. You want to build a file called tiling_graph. So the correct make rule would be:
tiling_graph: tiling_graph.o
g++ -o tiling_graph tiling_graph.o -L/usr/local/lib -ligraph
Since make already knows how to create tiling_graph.o, it can actually figure out how to make tiling_graph from tiling_graph.cpp.
So where does this all come from? The usual way to use all is:
all: program1 program2 program3
which tells make that the all target requires program1, program2 and program3. So if you need to build all three of those programs, the all rule would let you just do one make command. Since there is no file named all, that's a "phony" target and (with gnu make, at least) it should be declared as a phony target:
all: tiling_graph
.PHONY: all
But you really don't need that if you just want to build one program.
When you just type make (as opposed to make target), make chooses the first target in the makefile. So if you put the thing you usually want to build first, you'll save some typing.
I'm using make to build a C++ project. During the course of the project, I wanted to make some changes to the Makefile. Unfortunately, ever since I executed make once, it keeps using that particular version of the Makefile and just doesn't do anything with the changes at all.
I have run make clean, I have renamed the makefile, I've searched for other Makefiles which might be used instead, all to no avail. There is no mention of any caching mechanism in the man pages for make, nor anywhere on Google.
Does anyone have any idea why make isn't using the new version and what I can do about it? I'm compiling on a Ubuntu 12.04.2 LTS (x86_64) box, with (GNU) make version 3.81.
Update:
Some additional information. It seems make is using the current version of the makefile after all. If I change something in the main target, it's working just fine. But if I change something in the obj/%.o target, it just keeps running the same command, no matter what changes I make to that target.
Full Makefile an be found here: http://pastebin.com/WK43NRcL
CC_FILES = $(shell find -name "*.cc" -exec echo "{}" +;)
That find command is incorrect, shouldn't it be looking in the src directory? And why use echo to print the name when that's what find does anyway?
That means your list of CC_FILES and so also list of OBJ_FILES is empty.
I think you want:
CC_FILES := $(shell find src -name "*.cc")
Note that this uses := not = because otherwise the shell function gets run every time you reference the CC_FILES variable. Using := means it is run and evaluated only once.
However, since it seems all your .cc files are in the same directory you don't need a recursive find, just do:
CC_FILES := $(wildcard src/*.cc)
As you've realised, your patsubst is broken, you can just do:
OBJ_FILES := $(patsubst src/%.cc,obj/%.o,$(CC_FILES))
(Again, use := here)
Also:
obj/%.o: obj src/%.cc
$(CXX) $(CFLAGS) -c -o $# $<
I think you need to read what the $< variable expands to, because the rule above isn't going to do what you expect.
This makefile is full of errors, you need to use echo in pattern rules to print out the values of variables, so you can verify they have the values you expect. (As another option for debugging, set SHELL=bash -x so every shell command is echoed)
make does not somehow magically keep track of your old makefile; it will use whatever file is first in the list of files it looks for, in the current directory.
To find out which Makefile is actually used, see this question: Getting the name of the makefile from the makefile
Since you're using GNU make, check its excellent manual on what filenames it looks for, and in which order.