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STATIC VS. Dynamic LINKING
DOTSWIFT 2015, MARIUS RACKWITZ,
@MRACKWITZ
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LIBRARIES
VS.
FRAMEWORKS
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❓
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PERSPECTIVE
CLARIFICATION
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ARCHITECTURAL PERSPECTIVE:
Hollywood
PRINCIPLE
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DON'T CALL us,
WE'LL CALL you!
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TOOLING PERSPECTIVE:
DIFFERENT
BUILD PRODUCTS
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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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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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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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FRAMEWORK
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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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⾠
CLASSES OF STATIC
FRAMEWORKS ARE PART OF
THE MAIN BUNDLE
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A DYNAMIC LIBRARY (AKA DYLIB OR FRAMEWORK)
… is a final linked image.
— the man page of ld
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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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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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DYLIBS ON LINK TIME
Exported symbols from the dynamic library are used to resolve
references.
— the man page of ld
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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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MACH-O FILE FORMAT BASIC STRUCTURE
SOURCE: OS X ABI MACH-O FILE FORMAT REFERENCE
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$ 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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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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@rpath?
▸ Should be used as prefix in LC_ID_DYLIB /
LC_LOAD_DYLIB
▸ Searched by dyld at runtime.
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WRAP UP
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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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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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DISADVANTAGES OF DYNAMIC LINKING
▸ Limits dead-code stripping
▸ Increases load times
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RESTRICTIONS
▸ Dynamic Frameworks are needed for @IBDesignable.
▸ Dynamic Linking is needed for Swift.
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WHEN ALTERING BUILD SETTINGS ACROSS TARGETS
CLEAN WON'T DO THE JOB!
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GOLDEN RULE #1
WHEN ALTERING BUILD SETTINGS ACROSS TARGETS
NUKE DERIVEDDATA !
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GOLDEN RULE #1
WHEN ALTERING BUILD SETTINGS ACROSS TARGETS
Always NUKE DERIVEDDATA !
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for question in questions {
question.ask()!
}
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