LD.AOUT_SO(1) NetBSD General Commands Manual LD.AOUT_SO(1)
NAME
ld.aout_so -- run-time link-editor
DESCRIPTION
ld.aout_so is a self-contained, position independent program image pro- viding run-time support for loading and link-editing shared objects into a process' address space. It uses the data structures (see link(5)) con- tained within dynamically linked programs to determine which shared libraries are needed and loads them at a convenient virtual address using the mmap(2) system call. After all shared libraries have been successfully loaded, ld.aout_so pro- ceeds 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 sym- bol 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 automat- ically 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 ini- tialization 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 com- plete the link-editing process before the dynamic program calls upon ser- vices 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: LD_LIBRARY_PATH A colon separated list of directories, overrid- ing the default search path for shared libraries. LD_PRELOAD A colon separated list of shared object file- names to be loaded after the main program but before its shared object dependencies. LD_WARN_NON_PURE_CODE When set, issue a warning whenever a link-edit- ing operation requires modification of the text segment of some loaded object. This is usually indicative of an incorrectly built library. LD_SUPPRESS_WARNINGS When set, no warning messages of any kind are issued. Normally, a warning is given if satis- factorily versioned library could not be found. LD_TRACE_LOADED_OBJECTS When set, causes ld.aout_so to exit after load- ing the shared objects and printing a summary which includes the absolute pathnames of all objects, to standard output. LD_TRACE_LOADED_OBJECTS_FMT1 LD_TRACE_LOADED_OBJECTS_FMT2 When set, these variables are interpreted as format strings a la printf(3) to customize the trace output and are used by ldd(1)'s -f option and allows ldd(1) to be operated as a filter more conveniently. The following conversions can be used: %a The main program's name (also known as ``__progname''). %A The value of the environment variable LD_TRACE_LOADED_OBJECTS_PROGNAME %o The library name. %m The library's major version number. %n The library's minor version number. %p The full pathname as determined by rtld's library search rules. %x The library's load address. Additionally, \n and \t are recognized and have their usual meaning. LD_NO_INTERN_SEARCH When set, ld.aout_so does not process any inter- nal search paths that were recorded in the exe- cutable. LD_NOSTD_PATH When set, do not include a set of built-in stan- dard directory paths for searching. This might be useful when running on a system with a com- pletely non-standard filesystem layout.
FILES
/var/run/ld.so.hints library location hints built by ldconfig(8)
SEE ALSO
ld(1), ld.elf_so(1), ld.so(1), link(5), ldconfig(8)
HISTORY
The shared library model employed first appeared in SunOS 4.0.
SECURITY CONSIDERATIONS
The environment variables LD_LIBRARY_PATH and LD_PRELOAD are not honored when executing in a set-user-ID or set-group-ID environment. This action is taken to prevent malicious substitution of shared object dependencies or interposition of symbols. NetBSD 6.1.2 January 1, 2011 NetBSD 6.1.2
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