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Android Dynamic Linker Design Notes =================================== Introduction: ------------- This document provides several notes related to the design of the Android dynamic linker. Prelinking: ----------- System libraries in Android are internally prelinked, which means that any internal relocations within them are stripped from the corresponding shared object, in order to reduce size and speed up loading. Such libraries can only be loaded at the very specific virtual memory address they have been prelinked to (during the build process). The list of prelinked system libraries and their corresponding virtual memory address is found in the file: build/core/prelink-linux-<arch>.map It should be updated each time a new system library is added to the system. The prelink step happens at build time, and uses the 'soslim' and 'apriori' tools: - 'apriori' is the real prelink tool which removes relocations from the shared object, however, it must be given a list of symbols to remove from the file. - 'soslim' is used to find symbols in an executable ELF file and generate a list that can be passed to 'apriori'. By default, these tools are only used to remove internal symbols from libraries, though they have been designed to allow more aggressive optimizations (e.g. 'global' prelinking and symbol stripping, which prevent replacing individual system libraries though). You can disable prelinking at build time by modifying your Android.mk with a line like: LOCAL_PRELINK_MODULE := false Initialization and Termination functions: ----------------------------------------- The Unix Sys V Binary Interface standard states that an executable can have the following entries in its .dynamic section: DT_INIT Points to the address of an initialization function that must be called when the file is loaded. DT_INIT_ARRAY Points to an array of function addresses that must be called, in-order, to perform initialization. Some of the entries in the array can be 0 or -1, and should be ignored. Note: this is generally stored in a .init_array section DT_INIT_ARRAYSZ The size of the DT_INITARRAY, if any DT_FINI Points to the address of a finalization function which must be called when the file is unloaded or the process terminated. DT_FINI_ARRAY Same as DT_INITARRAY but for finalizers. Note that the functions must be called in reverse-order though Note: this is generally stored in a .fini_array section DT_FINI_ARRAYSZ Size of FT_FINIARRAY DT_PREINIT_ARRAY An array similar to DT_INIT_ARRAY which must *only* be present in executables, not shared libraries, which contains a list of functions that need to be called before any other initialization function (i.e. DT_INIT and/or DT_INIT_ARRAY) in the executable or any of its libraries. Note: this is generally stored in a .preinit_array section DT_PREINIT_ARRAYSZ The size of DT_PREINIT_ARRAY If both a DT_INIT and DT_INITARRAY entry are present, the DT_INIT function must be called before the DT_INITARRAY functions. Consequently, the DT_FINIARRAY must be parsed in reverse order before the DT_FINI function, if both are available. Note that the implementation of static C++ constructors is very much processor dependent, and may use different ELF sections. On the ARM (see "C++ ABI for ARM" document), the static constructors must be called explicitly from the DT_INIT_ARRAY, and each one of them shall register a destructor by calling the special __eabi_atexit() function (provided by the C library). The DT_FINI_ARRAY is not used by static C++ destructors. On x86, the lists of constructors and destructors are placed in special sections named ".ctors" and ".dtors", and the DT_INIT / DT_FINI functions are in charge of calling them explicitly. Debugging: ---------- It is possible to enable debug output in the dynamic linker. To do so, follow these steps: 1/ Modify the line in Android.mk that says: LOCAL_CFLAGS += -DLINKER_DEBUG=0 Into the following: LOCAL_CFLAGS += -DLINKER_DEBUG=1 2/ Force-rebuild the dynamic linker: cd bionic/linker mm -B 3/ Rebuild a new system image. You can increase the verbosity of debug traces by defining the DEBUG environment variable to a numeric value from 0 to 2. This will only affect new processes being launched. By default, traces are sent to logcat, with the "linker" tag. You can change this to go to stdout instead by setting the definition of LINKER_DEBUG_TO_LOG to 0 in "linker_debug.h".