One Mach-O feature that hits many people by surprise is the distinction between shared libraries and dynamically loadable modules. On ELF systems both are the same; any piece of shared code can be used as a library and for dynamic loading. Use otool -hv some_file to see the filetype of some_file.


Mach-O shared libraries have the file type MH_DYLIB and carry the extension .dylib. They can be linked against with the usual static linker flags, e.g. -lfoo for libfoo.dylib.


Loadable modules are called "bundles" in Mach-O speak. They have the file type MH_BUNDLE. Since no component involved cares about it, they can carry any extension. The extension .bundle is recommended by Apple, but most ported software uses .so for the sake of compatibility.

library

In this tutorial we are going to build a shared library:



The source code

helloworld.c

1 #include <stdio.h>
2 void printHelloWorld(void)
3 {
4     printf("Hello World!\n");
5 }
6 

helloworld.h

1 void printHelloWorld(void);

main.c

1 #include "helloworld.h"
2 int main(int argc, char **argv)
3 {
4     printHelloWorld();
5 }
6 


Compiling the library

 

$ gcc -dynamiclib -o libhello.dylib -dy helloworld.c 

$ ls -al

total 64

drwxr-xr-x   7 xappsoftware  xappsoftware   238 15 Dic 16:48 .

drwx------+ 39 xappsoftware  xappsoftware  1326 14 Dic 19:06 ..

-rw-r--r--@  1 xappsoftware  xappsoftware  6148 14 Dic 19:06 .DS_Store

-rw-r--r--   1 xappsoftware  xappsoftware    78 12 Dic 11:21 helloworld.c

-rw-r--r--   1 xappsoftware  xappsoftware    28 12 Dic 11:21 helloworld.h

-rwxr-xr-x   1 xappsoftware  xappsoftware  8456 15 Dic 16:48 libhello.dylib

-rw-r--r--   1 xappsoftware  xappsoftware    81 12 Dic 11:24 main.c


Verifing the Library

 

$ file libhello.dylib 

libhello.dylib: Mach-O 64-bit dynamically linked shared library x86_64

$ 


Compiling an application

 

$ gcc -L. -lhello -o mainsl main.c 

$ ls -al

total 88

drwxr-xr-x   8 xappsoftware  xappsoftware   272 15 Dic 16:51 .

drwx------+ 39 xappsoftware  xappsoftware  1326 14 Dic 19:06 ..

-rw-r--r--@  1 xappsoftware  xappsoftware  6148 14 Dic 19:06 .DS_Store

-rw-r--r--   1 xappsoftware  xappsoftware    78 12 Dic 11:21 helloworld.c

-rw-r--r--   1 xappsoftware  xappsoftware    28 12 Dic 11:21 helloworld.h

-rwxr-xr-x   1 xappsoftware  xappsoftware  8456 15 Dic 16:48 libhello.dylib

-rw-r--r--   1 xappsoftware  xappsoftware    81 12 Dic 11:24 main.c

-rwxr-xr-x   1 xappsoftware  xappsoftware  8784 15 Dic 16:51 mainsl


Testing the application

 

$ ./mainsl 

Hello World!

$ 


Gg1