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Specifying the Search Order of External Symbols

The order in which frameworks and libraries are listed in the Link Binary With Libraries build phase specifies the order in which external symbols are resolved by the static linker at build time and the dynamic linker at runtime. When either of the linkers encounters an undefined external symbol, they look for the symbol starting with the first framework or library listed in the build phase.

When a program is built, the static linker replaces references to external symbols with the addresses of the symbols in the referenced libraries (this is called prebinding), or tells the dynamic linker to resolve the references when a program is loaded or when a symbol is referenced. Having the dynamic linker resolve references to external symbols at runtime provides the most flexibility, as a program can link with new versions of the symbols as they become available. However, this approach is not recommended for all situations, as linking with newer versions of a method or a function may cause problems during a program’s execution.

In addition, how frameworks and libraries are listed in a Link Binary With Libraries build phase, tells the static linker the approach (or the semantics) to use when binding or resolving references to external symbols defined in libraries.

Placing static libraries after dynamic libraries in the Link Binary With Libraries build phase, ensures that the static linker binds references to external symbols defined in static libraries at build time, even when newer versions of the static libraries the application originally was linked with are present in the user’s system.

When static libraries are listed before a dynamic library in the Link Binary With Libraries build phase, the static linker doesn’t resolve references to symbols in those static libraries. Instead, those symbols are resolved by the dynamic linker at runtime. This may cause problems when the static libraries are updated, as the application links to the new, potentially incompatible versions of the symbols instead of the ones the developer intended.

For details on how symbols are resolved, see “Finding Imported Symbols” in “Executing Mach-O Files” in Mac OS X ABI Mach-O File Format Reference and the ld man page.

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