LD.AOUT_SO(1) | General Commands Manual | LD.AOUT_SO(1) |
After all shared libraries have been successfully loaded, ld.aout_so proceeds to resolve external references from both the main program and all objects loaded. A mechanism is provided for initialization routines to be called, on a per-object basis, giving a shared object an opportunity to perform any extra set-up, before execution of the program proper begins. ld.aout_so looks for a symbol named .init in each object's symbol table. If present, this symbol is assumed to represent a C-function declared as void .init(void), which is then called. Similarly, a void .fini(void) function is called just before an object is unloaded from the process address space as a result of calling dlclose(3). Note that while an object's .init is always called, whether the object is loaded automatically at program startup or programmatically by using dlopen(3), the .fini function is called only on ‘last dlclose(3)'.
This mechanism is exploited by the system-supplied C++ constructor initialization code located in /usr/lib/c++rt.o. This file should be included in the list of object-code files passed to ld(1) when building a shared C++ library.
ld.aout_so is itself a shared object that is initially loaded by the startup module crt0. Since a.out(5) formats do not provide easy access to the file header from within a running process, crt0 uses the special symbol _DYNAMIC to determine whether a program is in fact dynamically linked or not. Whenever the linker ld(1) has relocated this symbol to a location other than 0, crt0 assumes the services of ld.aout_so are needed (see link(5) for details). crt0 passes control to rtld's entry point before the program's main() routine is called. Thus, ld.aout_so can complete the link-editing process before the dynamic program calls upon services of any dynamic library.
To quickly locate the required shared objects in the filesystem, ld.aout_so may use a “hints” file, prepared by the ldconfig(8) utility, in which the full path specification of the shared objects can be looked up by hashing on the 3-tuple <library-name, major-version-number, minor-version-number> .
ld.aout_so recognizes a number of environment variables that can be used to modify its behavior as follows:
Additionally, \n and \t are recognized and have their usual meaning.
January 1, 2011 | NetBSD 6.1 |