Module java.base

Interface SymbolLookup

Functional Interface:
This is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference.

@FunctionalInterface public interface SymbolLookup
A symbol lookup retrieves the address of a symbol in one or more libraries. A symbol is a named entity, such as a function or a global variable.

A symbol lookup is created with respect to a particular library (or libraries). Subsequently, the find(String) method takes the name of a symbol and returns the address of the symbol in that library.

The address of a symbol is modeled as a zero-length memory segment. The segment can be used in different ways:

  • It can be passed to a Linker to create a downcall method handle, which can then be used to call the foreign function at the segment's address.
  • It can be passed to an existing downcall method handleRESTRICTED, as an argument to the underlying foreign function.
  • It can be stored inside another memory segment.
  • It can be used to access the region of memory backing a global variable (this requires resizingRESTRICTED the segment first).

Obtaining a symbol lookup

The factory methods libraryLookup(String, Arena)RESTRICTED and libraryLookup(Path, Arena)RESTRICTED create a symbol lookup for a library known to the operating system. The library is specified by either its name or a path. The library is loaded if not already loaded. The symbol lookup, which is known as a library lookup, and its lifetime is controlled by an arena. For instance, if the provided arena is a confined arena, the library associated with the symbol lookup is unloaded when the confined arena is closed:
 try (Arena arena = Arena.ofConfined()) {
     SymbolLookup libGL = SymbolLookup.libraryLookup("", arena); // loaded here
     MemorySegment glGetString = libGL.find("glGetString").orElseThrow();
 } // unloaded here

If a library was previously loaded through JNI, i.e., by System.load(String) or System.loadLibrary(String), then the library was also associated with a particular class loader. The factory method loaderLookup() creates a symbol lookup for all the libraries associated with the caller's class loader:

System.loadLibrary("GL"); // loaded here
SymbolLookup libGL = SymbolLookup.loaderLookup();
MemorySegment glGetString = libGL.find("glGetString").orElseThrow();
This symbol lookup, which is known as a loader lookup, is dynamic with respect to the libraries associated with the class loader. If other libraries are subsequently loaded through JNI and associated with the class loader, then the loader lookup will expose their symbols automatically.

Note that a loader lookup only exposes symbols in libraries that were previously loaded through JNI, i.e., by System.load(String) or System.loadLibrary(String). A loader lookup does not expose symbols in libraries that were loaded in the course of creating a library lookup:

 libraryLookup("", arena).find("glGetString").isPresent(); // true
 loaderLookup().find("glGetString").isPresent(); // false
Note also that a library lookup for library L exposes symbols in L even if L was previously loaded through JNI (the association with a class loader is immaterial to the library lookup):
 System.loadLibrary("GL"); // loaded here
 libraryLookup("", arena).find("glGetString").isPresent(); // true

Finally, each Linker provides a symbol lookup for libraries that are commonly used on the OS and processor combination supported by that Linker. This symbol lookup, which is known as a default lookup, helps clients to quickly find addresses of well-known symbols. For example, a Linker for Linux/x64 might choose to expose symbols in libc through the default lookup:

 Linker nativeLinker = Linker.nativeLinker();
 SymbolLookup stdlib = nativeLinker.defaultLookup();
 MemorySegment malloc = stdlib.find("malloc").orElseThrow();