Using make for cross platform compilation

Question

I am currently developing a C project under Linux and Win32. The \'deliverable\' is a shared library, and all the development is done under Linux with the GNU tool chain. I am u

Answer 1

I use UNAME := $(shell uname) within my Makefile to detect the platform (Linux or MS-Windows).

I provide below a complete example based on make and gcc to build a shared library: *.so or *.dll depending on the platform.

The example is basic/simple/stupid to be more understandable :-)

To use make and gcc on MS-Windows, Cygwin or MinGW can be installed.

The example uses five files:

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

The Makefiles

app/Makefile

app.exe: main.o
        gcc -o $@ $^ -L../lib -lhello
        # '-o $@'    => output file => $@ = the target file (app.exe)
        # '   $^'    => no options => Link all depended files 
        #            => $^ = main.o and other if any
        # '-L../lib' => look for libraries in directory ../lib
        # '-lhello   => use shared library hello (libhello.so or hello.dll)

%.o: %.c
        gcc -o $@ -c $< -I ../lib
        # '-o $@'     => output file => $@ = the target file (main.o)
        # '-c $<'     => COMPILE the first depended file (main.cpp)
        # '-I ../lib' => look for headers (*.h) in directory ../lib

clean:
        rm -f *.o *.so *.dll *.exe

lib/Makefile

UNAME := $(shell uname)

ifeq ($(UNAME), Linux)
TARGET = libhello.so
else
TARGET = hello.dll
endif

$(TARGET): hello.o
        gcc  -o $@  $^  -shared
        # '-o $@'    => output file => $@ = libhello.so or hello.dll
        # '   $^'    => no options => Link all depended files => $^ = hello.o
        # '-shared'  => generate shared library

%.o: %.c
        gcc  -o $@  -c $<  -fPIC
        # '-o $@' => output file => $@ = the target file (main.o)
        # '-c $<' => compile the first depended file (main.cpp)
        # '-fPIC' => Position-Independent Code (required for shared lib)

clean:
        rm -f *.o *.so *.dll *.exe

The source code

app/main.c

#include "hello.h" //hello()
#include  //puts()

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

lib/hello.h

#ifndef __HELLO_H__
#define __HELLO_H__

const char* hello();

#endif

lib/hello.c

#include "hello.h"

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

The build

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

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

The make command is the same on both platforms. The given output is for MS-Windows (unnecessary lines removed).

> cd lib
> make clean
> make
gcc  -o hello.o  -c hello.c  -fPIC
gcc  -o hello.dll  hello.o  -shared
> cd ../app
> make clean
> make
gcc -o main.o -c main.c -I ../lib
gcc -o app.exe main.o -L../lib -lhello

The run

The application requires to know where is the shared library.

On MS-Windows, the simple/basic/stupid way is to copy the library where the application is:

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

On Linux, use the LD_LIBRARY_PATH environment variable:

> export LD_LIBRARY_PATH=lib

The run command line and output are the same on both platforms:

> app/app.exe
hello