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About makefile : OS-detecting-makefile

Question Detail

I routinely work on several different computers and several different operating systems, which are Mac OS X, Linux, or Solaris. For the project I’m working on, I pull my code from a remote git repository.

I like to be able to work on my projects regardless of which terminal I’m at. So far, I’ve found ways to get around the OS changes by changing the makefile every time I switch computers. However, this is tedious and causes a bunch of headaches.

How can I modify my makefile so that it detects which OS I’m using and modifies syntax accordingly?

Here is the makefile:

cc = gcc -g
CC = g++ -g
yacc=$(YACC)
lex=$(FLEX)

all: assembler

assembler: y.tab.o lex.yy.o
        $(CC) -o assembler y.tab.o lex.yy.o -ll -l y

assembler.o: assembler.c
        $(cc) -o assembler.o assembler.c

y.tab.o: assem.y
        $(yacc) -d assem.y
        $(CC) -c y.tab.c

lex.yy.o: assem.l
        $(lex) assem.l
        $(cc) -c lex.yy.c

clean:
        rm -f lex.yy.c y.tab.c y.tab.h assembler *.o *.tmp *.debug *.acts

Question Answer

There are many good answers here already, but I wanted to share a more complete example that both:

  • doesn’t assume uname exists on Windows
  • also detects the processor

The CCFLAGS defined here aren’t necessarily recommended or ideal; they’re just what the project to which I was adding OS/CPU auto-detection happened to be using.

ifeq ($(OS),Windows_NT)
    CCFLAGS += -D WIN32
    ifeq ($(PROCESSOR_ARCHITEW6432),AMD64)
        CCFLAGS += -D AMD64
    else
        ifeq ($(PROCESSOR_ARCHITECTURE),AMD64)
            CCFLAGS += -D AMD64
        endif
        ifeq ($(PROCESSOR_ARCHITECTURE),x86)
            CCFLAGS += -D IA32
        endif
    endif
else
    UNAME_S := $(shell uname -s)
    ifeq ($(UNAME_S),Linux)
        CCFLAGS += -D LINUX
    endif
    ifeq ($(UNAME_S),Darwin)
        CCFLAGS += -D OSX
    endif
    UNAME_P := $(shell uname -p)
    ifeq ($(UNAME_P),x86_64)
        CCFLAGS += -D AMD64
    endif
    ifneq ($(filter %86,$(UNAME_P)),)
        CCFLAGS += -D IA32
    endif
    ifneq ($(filter arm%,$(UNAME_P)),)
        CCFLAGS += -D ARM
    endif
endif

The uname command (http://developer.apple.com/documentation/Darwin/Reference/ManPages/man1/uname.1.html) with no parameters should tell you the operating system name. I’d use that, then make conditionals based on the return value.

Example

UNAME := $(shell uname)

ifeq ($(UNAME), Linux)
# do something Linux-y
endif
ifeq ($(UNAME), Solaris)
# do something Solaris-y
endif

Detect the operating system using two simple tricks:

  • First the environment variable OS
  • Then the uname command
ifeq ($(OS),Windows_NT)     # is Windows_NT on XP, 2000, 7, Vista, 10...
    detected_OS := Windows
else
    detected_OS := $(shell uname)  # same as "uname -s"
endif

Or a more safe way, if not on Windows and uname unavailable:

ifeq ($(OS),Windows_NT) 
    detected_OS := Windows
else
    detected_OS := $(shell sh -c 'uname 2>/dev/null || echo Unknown')
endif

Ken Jackson proposes an interesting alternative if you want to distinguish Cygwin/MinGW/MSYS/Windows. See his answer that looks like that:

ifeq '$(findstring ;,$(PATH))' ';'
    detected_OS := Windows
else
    detected_OS := $(shell uname 2>/dev/null || echo Unknown)
    detected_OS := $(patsubst CYGWIN%,Cygwin,$(detected_OS))
    detected_OS := $(patsubst MSYS%,MSYS,$(detected_OS))
    detected_OS := $(patsubst MINGW%,MSYS,$(detected_OS))
endif

Then you can select the relevant stuff depending on detected_OS:

ifeq ($(detected_OS),Windows)
    CFLAGS += -D WIN32
endif
ifeq ($(detected_OS),Darwin)        # Mac OS X
    CFLAGS += -D OSX
endif
ifeq ($(detected_OS),Linux)
    CFLAGS   +=   -D LINUX
endif
ifeq ($(detected_OS),GNU)           # Debian GNU Hurd
    CFLAGS   +=   -D GNU_HURD
endif
ifeq ($(detected_OS),GNU/kFreeBSD)  # Debian kFreeBSD
    CFLAGS   +=   -D GNU_kFreeBSD
endif
ifeq ($(detected_OS),FreeBSD)
    CFLAGS   +=   -D FreeBSD
endif
ifeq ($(detected_OS),NetBSD)
    CFLAGS   +=   -D NetBSD
endif
ifeq ($(detected_OS),DragonFly)
    CFLAGS   +=   -D DragonFly
endif
ifeq ($(detected_OS),Haiku)
    CFLAGS   +=   -D Haiku
endif

Notes:

  • Command uname is same as uname -s because option -s (--kernel-name) is the default. See why uname -s is better than uname -o.

  • The use of OS (instead of uname) simplifies the identification algorithm. You can still use solely uname, but you have to deal with if/else blocks to check all MinGW, Cygwin, etc. variations.

  • The environment variable OS is always set to "Windows_NT" on different Windows versions (see %OS% environment variable on Wikipedia).

  • An alternative of OS is the environment variable MSVC (it checks the presence of MS Visual Studio, see example using Visual C++).


Below I provide a complete example using make and gcc to build a shared library: *.so or *.dll depending on the platform. The example is as simplest as possible to be more understandable.

To install make and gcc on Windows see Cygwin or MinGW.

My example is based on five files

 ├── lib
 │   └── Makefile
 │   └── hello.h
 │   └── hello.c
 └── app
     └── Makefile
     └── main.c

Reminder: Makefile is indented using tabulation. Caution when copy-pasting below sample files.

The two Makefile files

1. lib/Makefile

ifeq ($(OS),Windows_NT)
    uname_S := Windows
else
    uname_S := $(shell uname -s)
endif

ifeq ($(uname_S), Windows)
    target = hello.dll
endif
ifeq ($(uname_S), Linux)
    target = libhello.so
endif
#ifeq ($(uname_S), .....) #See https://stackoverflow.com/a/27776822/938111
#    target = .....
#endif

%.o: %.c
    gcc  -c $<  -fPIC  -o [email protected]
    # -c $<  => $< is first file after ':' => Compile hello.c
    # -fPIC  => Position-Independent Code (required for shared lib)
    # -o [email protected]  => [email protected] is the target => Output file (-o) is hello.o

$(target): hello.o
    gcc  $^  -shared  -o [email protected]
    # $^      => $^ expand to all prerequisites (after ':') => hello.o
    # -shared => Generate shared library
    # -o [email protected]   => Output file (-o) is [email protected] (libhello.so or hello.dll)

2. app/Makefile

ifeq ($(OS),Windows_NT)
    uname_S := Windows
else
    uname_S := $(shell uname -s)
endif

ifeq ($(uname_S), Windows)
    target = app.exe
endif
ifeq ($(uname_S), Linux)
    target = app
endif
#ifeq ($(uname_S), .....) #See https://stackoverflow.com/a/27776822/938111
#    target = .....
#endif

%.o: %.c
    gcc  -c $< -I ../lib  -o [email protected]
    # -c $<     => compile (-c) $< (first file after :) = main.c
    # -I ../lib => search headers (*.h) in directory ../lib
    # -o [email protected]     => output file (-o) is [email protected] (target) = main.o

$(target): main.o
    gcc  $^  -L../lib  -lhello  -o [email protected]
    # $^       => $^ (all files after the :) = main.o (here only one file)
    # -L../lib => look for libraries in directory ../lib
    # -lhello  => use shared library hello (libhello.so or hello.dll)
    # -o [email protected]    => output file (-o) is [email protected] (target) = "app.exe" or "app"

To learn more, read Automatic Variables documentation as pointed out by cfi.

The source code

lib/hello.h

#ifndef HELLO_H_
#define HELLO_H_

const char* hello();

#endif

lib/hello.c

#include "hello.h"

const char* hello()
{
    return "hello";
}

app/main.c

#include "hello.h" //hello()
#include <stdio.h> //puts()

int main()
{
    const char* str = hello();
    puts(str);
}

The build

Fix the copy-paste of Makefile (replace leading spaces by one tabulation).

> sed  's/^  */\t/'  -i  */Makefile

The make command is the same on both platforms. The given output is on Unix-like OSes:

> make -C lib
make: Entering directory '/tmp/lib'
gcc  -c hello.c  -fPIC  -o hello.o
# -c hello.c  => hello.c is first file after ':' => Compile hello.c
# -fPIC       => Position-Independent Code (required for shared lib)
# -o hello.o  => hello.o is the target => Output file (-o) is hello.o
gcc  hello.o  -shared  -o libhello.so
# hello.o        => hello.o is the first after ':' => Link hello.o
# -shared        => Generate shared library
# -o libhello.so => Output file (-o) is libhello.so (libhello.so or hello.dll)
make: Leaving directory '/tmp/lib'

> make -C app
make: Entering directory '/tmp/app'
gcc  -c main.c -I ../lib  -o main.o
# -c main.c => compile (-c) main.c (first file after :) = main.cpp
# -I ../lib => search headers (*.h) in directory ../lib
# -o main.o => output file (-o) is main.o (target) = main.o
gcc  main.o  -L../lib  -lhello  -o app
# main.o   => main.o (all files after the :) = main.o (here only one file)
# -L../lib => look for libraries in directory ../lib
# -lhello  => use shared library hello (libhello.so or hello.dll)
# -o app   => output file (-o) is app.exe (target) = "app.exe" or "app"
make: Leaving directory '/tmp/app'

The run

The application requires to know where is the shared library.

On Windows, a simple solution is to copy the library where the application is:

> cp -v lib/hello.dll app
`lib/hello.dll' -> `app/hello.dll'

On Unix-like OSes, you can use the LD_LIBRARY_PATH environment variable:

> export LD_LIBRARY_PATH=lib

Run the command on Windows:

> app/app.exe
hello

Run the command on Unix-like OSes:

> app/app
hello

I was recently experimenting in order to answer this question I was asking myself. Here are my conclusions:

Since in Windows, you can’t be sure that the uname command is available, you can use gcc -dumpmachine. This will display the compiler target.

There may be also a problem when using uname if you want to do some cross-compilation.

Here’s a example list of possible output of gcc -dumpmachine:

  • mingw32
  • i686-pc-cygwin
  • x86_64-redhat-linux

You can check the result in the makefile like this:

SYS := $(shell gcc -dumpmachine)
ifneq (, $(findstring linux, $(SYS)))
 # Do Linux things
else ifneq(, $(findstring mingw, $(SYS)))
 # Do MinGW things
else ifneq(, $(findstring cygwin, $(SYS)))
 # Do Cygwin things
else
 # Do things for others
endif

It worked well for me, but I’m not sure it’s a reliable way of getting the system type. At least it’s reliable about MinGW and that’s all I need since it does not require to have the uname command or MSYS package in Windows.

To sum up, uname gives you the system on which you’re compiling, and gcc -dumpmachine gives you the system for which you are compiling.

The git makefile contains numerous examples of how to manage without autoconf/automake, yet still work on a multitude of unixy platforms.

Update: I now consider this answer to be obsolete. I posted a new perfect solution further down.

If your makefile may be running on non-Cygwin Windows, uname may not be available. That’s awkward, but this is a potential solution. You have to check for Cygwin first to rule it out, because it has WINDOWS in its PATH environment variable too.

ifneq (,$(findstring /cygdrive/,$(PATH)))
    UNAME := Cygwin
else
ifneq (,$(findstring WINDOWS,$(PATH)))
    UNAME := Windows
else
    UNAME := $(shell uname -s)
endif
endif

That’s the job that GNU’s automake/autoconf are designed to solve. You might want to investigate them.

Alternatively you can set environment variables on your different platforms and make you Makefile conditional against them.

I ran into this problem today and I needed it on Solaris so here is a POSIX standard way to do (something very close to) this.

#Detect OS
UNAME = `uname`

# Build based on OS name
DetectOS:
    [email protected] $(UNAME)


# OS is Linux, use GCC
Linux: program.c
    @SHELL_VARIABLE="-D_LINUX_STUFF_HERE_"
    rm -f program
    gcc $(SHELL_VARIABLE) -o program program.c

# OS is Solaris, use c99
SunOS: program.c
    @SHELL_VARIABLE="-D_SOLARIS_STUFF_HERE_"
    rm -f program
    c99 $(SHELL_VARIABLE) -o program program.c

I finally found the perfect solution that solves this problem for me.

ifeq '$(findstring ;,$(PATH))' ';'
    UNAME := Windows
else
    UNAME := $(shell uname 2>/dev/null || echo Unknown)
    UNAME := $(patsubst CYGWIN%,Cygwin,$(UNAME))
    UNAME := $(patsubst MSYS%,MSYS,$(UNAME))
    UNAME := $(patsubst MINGW%,MSYS,$(UNAME))
endif

The UNAME variable is set to Linux, Cygwin, MSYS, Windows, FreeBSD, NetBSD (or presumably Solaris, Darwin, OpenBSD, AIX, HP-UX), or Unknown. It can then be compared throughout the remainder of the Makefile to separate any OS-sensitive variables and commands.

The key is that Windows uses semicolons to separate paths in the PATH variable whereas everyone else uses colons. (It’s possible to make a Linux directory with a ‘;’ in the name and add it to PATH, which would break this, but who would do such a thing?) This seems to be the least risky method to detect native Windows because it doesn’t need a shell call. The Cygwin and MSYS PATH use colons so uname is called for them.

Note that the OS environment variable can be used to detect Windows, but not to distinguish between Cygwin and native Windows. Testing for the echoing of quotes works, but it requires a shell call.

Unfortunately, Cygwin adds some version information to the output of uname, so I added the ‘patsubst’ calls to change it to just ‘Cygwin’. Also, uname for MSYS actually has three possible outputs starting with MSYS or MINGW, but I use also patsubst to transform all to just ‘MSYS’.

If it’s important to distinguish between native Windows systems with and without some uname.exe on the path, this line can be used instead of the simple assignment:

UNAME := $(shell uname 2>NUL || echo Windows)

Of course in all cases GNU make is required, or another make which supports the functions used.

Here’s a simple solution that checks if you are in a Windows or posix-like (Linux/Unix/Cygwin/Mac) environment:

ifeq ($(shell echo "check_quotes"),"check_quotes")
   WINDOWS := yes
else
   WINDOWS := no
endif

It takes advantage of the fact that echo exists on both posix-like and Windows environments, and that in Windows the shell does not filter the quotes.

Note that Makefiles are extremely sensitive to spacing. Here’s an example of a Makefile that runs an extra command on OS X and which works on OS X and Linux. Overall, though, autoconf/automake is the way to go for anything at all non-trivial.

UNAME := $(shell uname -s)
CPP = g++
CPPFLAGS = -pthread -ansi -Wall -Werror -pedantic -O0 -g3 -I /nexopia/include
LDFLAGS = -pthread -L/nexopia/lib -lboost_system

HEADERS = data_structures.h http_client.h load.h lock.h search.h server.h thread.h utility.h
OBJECTS = http_client.o load.o lock.o search.o server.o thread.o utility.o vor.o

all: vor

clean:
    rm -f $(OBJECTS) vor

vor: $(OBJECTS)
    $(CPP) $(LDFLAGS) -o vor $(OBJECTS)
ifeq ($(UNAME),Darwin)
    # Set the Boost library location
    install_name_tool -change libboost_system.dylib /nexopia/lib/libboost_system.dylib vor
endif

%.o: %.cpp $(HEADERS) Makefile
    $(CPP) $(CPPFLAGS) -c $

Another way to do this is by using a “configure” script. If you are already using one with your makefile, you can use a combination of uname and sed to get things to work out. First, in your script, do:

UNAME=uname

Then, in order to put this in your Makefile, start out with Makefile.in which should have something like

[email protected]@[email protected]@

in it.

Use the following sed command in your configure script after the UNAME=uname bit.

sed -e "s|@@[email protected]@|$UNAME|" < Makefile.in > Makefile

Now your makefile should have UNAME defined as desired. If/elif/else statements are all that’s left!

I had a case where I had to detect the difference between two versions of Fedora, to tweak the command-line options for inkscape:
– in Fedora 31, the default inkscape is 1.0beta which uses --export-file
– in Fedora < 31, the default inkscape is 0.92 which uses --export-pdf

My Makefile contains the following

# set VERSION_ID from /etc/os-release

$(eval $(shell grep VERSION_ID /etc/os-release))

# select the inkscape export syntax

ifeq ($(VERSION_ID),31)
EXPORT = export-file
else
EXPORT = export-pdf
endif

# rule to convert inkscape SVG (drawing) to PDF

%.pdf : %.svg
    inkscape --export-area-drawing $< --$(EXPORT)[email protected]

This works because /etc/os-release contains a line

VERSION_ID=<value>

so the shell command in the Makefile returns the string VERSION_ID=<value>, then the eval command acts on this to set the Makefile variable VERSION_ID.
This can obviously be tweaked for other OS’s depending how the metadata is stored. Note that in Fedora there is not a default environment variable that gives the OS version, otherwise I would have used that!

An alternate way that I have not seen anyone talking about is using the built-in variable SHELL. The program used as the shell is taken from the variable SHELL. On MS-Windows systems, it is most likely to be an executable file with .exe extension (like sh.exe).

In that case, the following conditional test:

ifeq ($(suffix $(SHELL)),.exe)
    # Windows system
else
    # Non-Windows system
endif

Would have the same result as using the environment variable OS:

ifeq ($(OS),Windows_NT)
    # Windows system
else
    # Non-Windows system
endif

However, it seems the latter is the most popular solution, so I would recommend you stick with it.

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