ld-elf.so.1, ld.so, rtld - run-time link-editor

CONTENTS

Description
Files
See Also

The ld-elf.so.1 utility is a self-contained shared object providing run-time support for loading and link-editing shared objects into a process’ address space. It is also commonly known as the dynamic linker. It uses the data structures contained within dynamically linked programs to determine which shared libraries are needed and loads them using the mmap(2) system call.

After all shared libraries have been successfully loaded, ld-elf.so.1 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.

The ld-elf.so.1 utility itself is loaded by the kernel together with any dynamically-linked program that is to be executed. The kernel transfers control to the dynamic linker. After the dynamic linker has finished loading, relocating, and initializing the program and its required shared objects, it transfers control to the entry point of the program.

To locate the required shared objects in the file system, ld-elf.so.1 may use a "hints" file prepared by the ldconfig(8) utility.

The ld-elf.so.1 utility recognizes a number of environment variables that can be used to modify its behaviour. On 64-bit architectures, the linker for 32-bit objects recognizes all the environment variables listed below, but is being prefixed with LD_32_, for example: LD_32_TRACE_LOADED_OBJECTS.

LD_DUMP_REL_POST	If set ld-elf.so.1 will print a table containing all relocations after symbol binding and relocation.
LD_DUMP_REL_PRE	If set ld-elf.so.1 will print a table containing all relocations before symbol binding and relocation.
LD_LIBMAP	A library replacement list in the same format as libmap.conf(5). For convenience, the characters ‘=’ and ‘,’ can be used instead of a space and a newline. This variable is parsed after libmap.conf(5), and will override its entries.
LD_LIBMAP_DISABLE
	If set, disables the use of libmap.conf(5) and LD_LIBMAP.
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 list of shared libraries, separated by colons and/or white space, 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-elf.so.1 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_TRACE_LOADED_OBJECTS
	When set to a nonempty string, causes ld-elf.so.1 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_ALL
	When set to a nonempty string, causes ld-elf.so.1 to expand the summary to indicate which objects caused each object to be loaded.
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.
%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.

/var/run/ld-elf.so.hints	Hints file.
/var/run/ld-elf32.so.hints
	Hints file for 32-bit binaries on 64-bit system.
/etc/libmap.conf	The libmap configuration file.
/etc/libmap32.conf	The libmap configuration file for 32-bit binaries on 64-bit system.

ld(1), ldd(1), elf(5), libmap.conf(5), ldconfig(8)

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