What are the differences between so and dylib on osx

Question

dylib is the dynamic library extension on OSX  but it s never been clear to me when I can t   shouldn t use a traditional unix  so shared object   Some of the questions I have    At a conceptual level  what are the main differences between  so and  dylib  When can should I use one over the other   Compilation tricks  amp  tips  For example  the replacement for gcc -shared -fPIC  since that doesn t work on osx

User · Answer

Just an observation I just made while building naive code on OSX with cmake:

cmake ... -DBUILD_SHARED_LIBS=OFF ...

creates .so files

while

cmake ... -DBUILD_SHARED_LIBS=ON ...

creates .dynlib files.

Perhaps this helps anyone.

User · Answer

The Mach-O object file format used by Mac OS X for executables and libraries distinguishes between shared libraries and dynamically loaded modules  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   They can be created by passing the -dynamiclib flag to the compiler   -fPIC is the default and needn t be specified    Loadable modules are called  bundles  in Mach-O speak  They have the file type MH BUNDLE  They can carry any extension  the extension  bundle is recommended by Apple  but most ported software uses  so for the sake of compatibility   Typically  you ll use bundles for plug-ins that extend an application  in such situations  the bundle will link against the application binary to gain access to the application   s exported API   They can be created by passing the -bundle flag to the compiler   Both dylibs and bundles can be dynamically loaded using the dl APIs  e g  dlopen  dlclose    It is not possible to link against bundles as if they were shared libraries  However  it is possible that a bundle is linked against real shared libraries  those will be loaded automatically when the bundle is loaded   Historically  the differences were more significant   In Mac OS X 10 0  there was no way to dynamically load libraries   A set of dyld APIs  e g  NSCreateObjectFileImageFromFile  NSLinkModule  were introduced with 10 1 to load and unload bundles  but they didn t work for dylibs   A dlopen compatibility library that worked with bundles was added in 10 3  in 10 4  dlopen was rewritten to be a native part of dyld and added support for loading  but not unloading  dylibs  Finally  10 5 added support for using dlclose with dylibs and deprecated the dyld APIs   On ELF systems like Linux  both use the same file format  any piece of shared code can be used as a library and for dynamic loading   Finally  be aware that in Mac OS X   bundle  can also refer to directories with a standardized structure that holds executable code and the resources used by that code   There is some conceptual overlap  particularly with  loadable bundles  like plugins  which generally contain executable code in the form of a Mach-O bundle   but they shouldn t be confused with Mach-O bundles discussed above   Additional references     Fink Porting Guide  the basis for this answer  though pretty out of date  as it was written for Mac OS X 10 3   ld 1  and dlopen 3  Dynamic Library Programming Topics Mach-O Programming Topics

User · Answer

The file  so is not a UNIX file extension for shared library   It just happens to be a common one   Check line 3b at ArnaudRecipes sharedlib page  Basically  dylib is the mac file extension used to indicate a shared lib

User · Answer

The difference between  dylib and  so on mac os x is how they are compiled  For  so files you use -shared and for  dylib you use -dynamiclib  Both  so and  dylib are interchangeable as dynamic library files and either have a type as DYLIB or BUNDLE  Heres the readout for different files showing this   libtriangle dylib  Mach header       magic cputype cpusubtype  caps    filetype ncmds sizeofcmds      flags MH MAGIC 64  X86 64        ALL  0x00       DYLIB    17       1368   NOUNDEFS DYLDLINK TWOLEVEL NO REEXPORTED DYLIBS    libtriangle so  Mach header       magic cputype cpusubtype  caps    filetype ncmds sizeofcmds      flags MH MAGIC 64  X86 64        ALL  0x00       DYLIB    17       1256   NOUNDEFS DYLDLINK TWOLEVEL NO REEXPORTED DYLIBS  triangle so  Mach header       magic cputype cpusubtype  caps    filetype ncmds sizeofcmds      flags MH MAGIC 64  X86 64        ALL  0x00      BUNDLE    16       1696   NOUNDEFS DYLDLINK TWOLEVEL   The reason the two are equivalent on Mac OS X is for backwards compatibility with other UNIX OS programs that compile to the  so file type   Compilation notes  whether you compile a  so file or a  dylib file you need to insert the correct path into the dynamic library during the linking step  You do this by adding -install name and the file path to the linking command  If you dont do this you will run into the problem seen in this post  Mac Dynamic Library Craziness  May be Fortran Only

[c++] What are the differences between .so and .dylib on osx?

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