Static vs. Dynamic Linking

Since iOS 8, dynamic linking is also available on Apple’s mobile platform. Before the release, we had only static libraries and frameworks as options to include third-party dependencies in applications. The new opportunities offer some advantages, but have also minor drawbacks inherent. This talk should give some overview of what’s possible, help on problem discovery and points out where to pay attention.

Transcript

1. STATIC VS. Dynamic LINKING
   DOTSWIFT 2015, MARIUS RACKWITZ,
   @MRACKWITZ
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2. LIBRARIES
   VS.
   FRAMEWORKS
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3. ❓
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4. PERSPECTIVE
   CLARIFICATION
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5. ARCHITECTURAL PERSPECTIVE:
   Hollywood
   PRINCIPLE
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6. DON'T CALL us,
   WE'LL CALL you!
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8. TOOLING PERSPECTIVE:
   DIFFERENT
   BUILD PRODUCTS
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9. A STATIC LIBRARY (AKA STATIC ARCHIVE)
   … is a collection of .o files with a table of contents that lists the global
   symbols in the .o files.
   — the man page of ld
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11. BUILDING A STATIC LIBRARY
    $ xcodebuild -target BananaLib … MACH_O_TYPE=staticlib
    # Pseudo (⾠) Code of what happens under the hood:
    PRODUCT_PATHS=''
    for ARCH in $ARCHS do
    OBJECT_FILES=''
    for FILE in $SRC_FILES do
    FILE_BASENAME=$(basename $FILE)
    OBJECT_FILE="$OBJECT_FILE_DIR_normal/$ARCH/$FILE_BASENAME.o"
    clang … -c $FILE -o $OBJECT_FILE
    OBJECT_FILES="$OUTPUT_FILES $OBJECT_FILE"
    end
    PRODUCT_PATH="$OBJECT_FILE_DIR_normal/$ARCH/$PRODUCT_NAME.a"
    ranlib $PRODUCT_PATH - $OBJECT_FILES
    PRODUCT_PATHS="$PRODUCT_PATHS $PRODUCT_PATH"
    end
    lipo -create $PRODUCT_PATHS -output $CONFIGURATION_BUILD_DIR/$PRODUCT_NAME.a
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12. STATIC LIBS ON LINK TIME
    ld will only pull .o files out of a static library if needed to resolve some
    symbol reference.
    — the man page of ld
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15. FRAMEWORK
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16. STATIC FRAMEWORK
    $ xcodebuild -target BananaFramework … MACH_O_TYPE=staticlib
    … # Build the static library
    mkdir $PRODUCT_NAME.framework
    mv $PRODUCT_NAME.a $PRODUCT_NAME.framework/$PRODUCT_NAME
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17. ⾠
    CLASSES OF STATIC
    FRAMEWORKS ARE PART OF
    THE MAIN BUNDLE
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18. A DYNAMIC LIBRARY (AKA DYLIB OR FRAMEWORK)
    … is a final linked image.
    — the man page of ld
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20. DYNAMIC LIBRARY
    $ xcodebuild -target BananaLib … MACH_O_TYPE=mh_dylib
    PRODUCT_PATHS=''
    for ARCH in $ARCHS do
    … # Build $OBJECT_FILES
    PRODUCT_PATH="$OBJECT_FILE_DIR_normal/$ARCH/$PRODUCT_NAME.dylib"
    ld $OBJECT_FILES -o $PRODUCT_PATH -framework Foundation …
    PRODUCT_PATHS="$PRODUCT_PATHS $PRODUCT_PATH"
    end
    lipo -create $PRODUCT_PATHS -output $CONFIGURATION_BUILD_DIR/$PRODUCT_NAME.dylib
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21. DYNAMIC FRAMEWORK
    $ xcodebuild -target BananaFramework … MACH_O_TYPE=mh_dylib
    … # Build the dylib
    mkdir $PRODUCT_NAME.framework
    mv $PRODUCT_NAME.dylib $PRODUCT_NAME.framework/$PRODUCT_NAME
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22. DYLIBS ON LINK TIME
    Exported symbols from the dynamic library are used to resolve
    references.
    — the man page of ld
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31. DYLIBS ON LINK TIME
    The generated final linked image will have encoded that it depends on
    that dynamic library.
    — the man page of ld
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32. MACH-O FILE FORMAT BASIC STRUCTURE
    SOURCE: OS X ABI MACH-O FILE FORMAT REFERENCE
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33. $ otool -l BananaKit.framework/BananaKit
    BananaKit.framework/BananaKit:
    …
    Load command 3
    cmd LC_ID_DYLIB
    cmdsize 96
    name @rpath/BananaKit.framework/BananaKit (offset 24)
    time stamp 1 Thu Jan 1 01:00:01 1970
    current version 1.0.0
    compatibility version 1.0.0
    …
    Load command 9
    cmd LC_LOAD_DYLIB
    cmdsize 56
    name @rpath/libswiftCore.dylib (offset 24)
    time stamp 2 Wed Dec 31 16:00:02 1969
    current version 0.0.0
    compatibility version 0.0.0
    …
    Load command 22
    cmd LC_RPATH
    cmdsize 40
    path @executable_path/Frameworks (offset 12)
    Load command 23
    cmd LC_RPATH
    cmdsize 40
    path @loader_path/Frameworks (offset 12)
    …
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34. MACH-O LOAD COMMANDS
    ▸ LC_LOAD_DYLIB
    (OTHER_LD_FLAGS -l/-framework)
    ▸ LC_ID_DYLIB
    LD_DYLIB_INSTALL_NAME
    ▸ LC_RPATH
    LD_RUNPATH_SEARCH_PATHS
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35. @rpath?
    ▸ Should be used as prefix in LC_ID_DYLIB /
    LC_LOAD_DYLIB
    ▸ Searched by dyld at runtime.
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36. WRAP UP
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37. ADVANTAGES OF DYNAMIC FRAMEWORKS
    ▸ Easier to distribute & integrate if compiled
    ▸ Encoded Dependencies
    ▸ Two-Level Namespacing
    ▸ Reduces file size if used for app & extensions
    ▸ Separate resources in distinct bundles
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38. ADVANTAGES OF DYNAMIC FRAMEWORKS
    ▸ Easier to distribute & integrate if compiled
    ▸ Encoded Dependencies
    ▸ Two-Level Namespacing
    ▸ Reduces file size if used for app & extensions
    ▸ Separate resources in distinct bundles
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39. DISADVANTAGES OF DYNAMIC LINKING
    ▸ Limits dead-code stripping
    ▸ Increases load times
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40. RESTRICTIONS
    ▸ Dynamic Frameworks are needed for @IBDesignable.
    ▸ Dynamic Linking is needed for Swift.
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41. WHEN ALTERING BUILD SETTINGS ACROSS TARGETS
    CLEAN WON'T DO THE JOB!
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42. GOLDEN RULE #1
    WHEN ALTERING BUILD SETTINGS ACROSS TARGETS
    NUKE DERIVEDDATA !
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43. GOLDEN RULE #1
    WHEN ALTERING BUILD SETTINGS ACROSS TARGETS
    Always NUKE DERIVEDDATA !
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44. THANKS FOR YOUR
    ATTENTION!
    @MRACKWITZ
    MR@REALM.IO
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45. for question in questions {
    question.ask()!
    }
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