ld.so is a self-contained, position
independent program image providing run-time support for loading and
link-editing shared objects into a process's address space. It uses the data
structures (see elf(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 is itself a shared object that is
initially loaded by the kernel.
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
ld.so searches for shared objects in the
following lists of paths:
LD_LIBRARY_PATHenvironment variable if set
DT_RUNPATHof the calling object if set, otherwise:
DT_RPATHof the calling object if set
DT_RPATHof the program if set
DT_RPATHis deprecated as its behaviour varies across operating systems.)
- default search path set by ldconfig(8)
ld.so recognises a number of environment
variables that can be used to modify its behaviour as follows:
- A colon separated list of directories, prepending the default search path for shared libraries. This variable is ignored for set-user-ID and set-group-ID executables.
- A colon separated list of library names to load before any of the regular libraries are loaded. This variable is ignored for set-user-ID and set-group-ID executables.
- Specifies that the dynamic linker should process all relocations before transferring control to the program. Normally, the procedure linkage table entries are handled lazily, avoiding symbol lookup and relocation for unused functions. This variable is ignored for set-user-ID and set-group-ID executables.
- When set, causes
ld.soto exit after loading the shared objects and printing a summary which includes the absolute pathnames of all objects, to standard output.
- When set, these variables are interpreted as format strings a la
printf(3) to customize the trace output and are used by
-foption and allows ldd(1) to be operated as a filter more conveniently.
LD_TRACE_LOADED_OBJECTS_FMT1is used for tracing shared libraries;
LD_TRACE_LOADED_OBJECTS_FMT2for dynamically loaded objects, the dynamic linker, and the main executable. The following conversions can be used:
- The main program's name (also known as “__progname”).
- The value of the environment variable
- The end address of the object.
- The object's group reference count.
- The object's major version number.
- The object's minor version number.
- The object's open count.
- The object name.
- The full pathname as determined by
ld.so's library search rules.
- The object's reference count.
- The object's load address.
Additionally, \n and \t are recognised and have their usual meaning.
- When set, be verbose about what
ld.sodoes. This variable is ignored for set-user-ID and set-group-ID executables.
- library location hints built by ldconfig(8)
ld(1), elf(5), ldconfig(8)
The shared library model employed first appeared in SunOS 4.0.