ld.so is a self-contained, position independent program image providing run-time support for loading and link-editing shared objects into a process' address space. It uses the data structures (see
link(5)) contained 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.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.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.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.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.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.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.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, overriding the default search path for shared libraries.
LD_PRELOAD
A colon separated list of shared object filenames 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-editing 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 satisfactorily versioned library could not be found.
LD_TRACE_LOADED_OBJECTS
When set, causes ld.so to exit after loading 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
%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.so does not process any internal search paths that were recorded in the executable.
LD_NOSTD_PATH
When set, do not include a set of built-in standard directory paths for searching. This might be useful when running on a system with a completely non-standard filesystem layout.