RTLD(1)

NAME
ld.so, rtld - 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' 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. This is useful for C++ libraries that contain static constructors.

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 ld.so '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 behaviour as follows:

LD_LIBRARY_PATH
    A colon separated list of directories, overriding the default search path for shared libraries. This is ignored for set-user-ID and set-group-ID programs.

LD_PRELOAD
    A colon separated list of shared libraries, to be linked in before any other shared libraries. If the directory is not specified then the directories specified by LD_LIBRARY_PATH will be searched first followed by the set of built-in standard directories. This is ignored for set-user-ID and set-group-ID programs.

LD_BIND_NOW
    When set to a nonempty string, causes ld.so to relocate all external function calls before starting execution of the program. Normally, function calls are bound lazily, at the first call of each function. LD_BIND_NOW increases the start-up time of a program, but it avoids run-time surprises caused by unexpectedly undefined functions.

LD_WARN_NON_PURE_CODE
    When set to a nonempty string, 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 to a nonempty string, no warning messages of any kind are issued. Normally, a warning is given if satisfactorily versioned library could not be found.

LD_IGNORE_MISSING_OBJECTS
    When set to a nonempty string, makes it a nonfatal condition if one or more required shared objects cannot be loaded. Loading and execution proceeds using the objects that are available. A warning is produced for each missing object, unless the environment variable LD_SUPPRESS_WARNINGS is set to a nonempty string.

    This is ignored for set-user-ID and set-group-ID programs.

    Missing shared objects can be ignored without errors if all the following conditions hold:

    •   They do not supply definitions for any required data symbols.

    •   No functions defined by them are called during program execution.

    •   The environment variable LD_BIND_NOW is unset or is set to the empty string.

LD_TRACE_LOADED_OBJECTS
    When set to a nonempty string, 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

    %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.

FILES
/var/run/ld.so.hints

SEE ALSO
ld(1), link(5), ldconfig(8)

HISTORY
The shared library model employed first appeared in SunOS 4.0