NAME

ld.so — run-time link-editor

DESCRIPTION

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 ⟨library-name, major-version-number, minor-version-number⟩.

ld.so searches for shared objects in the following lists of paths:

1. LD_LIBRARY_PATH environment variable if set
2. DT_RUNPATH of the calling object if set, otherwise:
   1. DT_RPATH of the calling object if set
   2. DT_RPATH of the program if set
   (The use of DT_RPATH is deprecated as its behaviour varies across operating systems.)
3. default search path set by ldconfig(8)
4. /usr/lib

ld.so recognises a number of environment variables that can be used to modify its behaviour as follows:

LD_LIBRARY_PATH

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.

LD_PRELOAD

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.

LD_BIND_NOW

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.

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. LD_TRACE_LOADED_OBJECTS_FMT1 is used for tracing shared libraries; LD_TRACE_LOADED_OBJECTS_FMT2 for dynamically loaded objects, the dynamic linker, and the main executable. 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.

%e

The end address of the object.

%g

The object's group reference count.

%m

The object's major version number.

%n

The object's minor version number.

%O

The object's open count.

%o

The object name.

%p

The full pathname as determined by ld.so's library search rules.

%r

The object's reference count.

%x

The object's load address.

Additionally, \n and \t are recognised and have their usual meaning.

LD_DEBUG

When set, be verbose about what ld.so does. This variable is ignored for set-user-ID and set-group-ID executables.

FILES

/var/run/ld.so.hints

library location hints built by ldconfig(8)

SEE ALSO

ld(1), elf(5), ldconfig(8)

HISTORY

The shared library model employed first appeared in SunOS 4.0.