ld.so(1) manual page

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.

When resolving dependencies for the loaded objects, ld-elf.so.1 translates dynamic token strings in rpath and soname. If the -z origin option of the static linker was set when linking the binary, the token expansion is performed at the object load time, see ld(1). The following strings are recognized now:

$ORIGIN Translated to the full path of the loaded object.

$OSNAME Translated to the name of the operating system implementation.

$OSREL Translated to the release level of the operating system.

$PLATFORM

Translated to the machine hardware platform.

$LIB Translated to the system library path component on the platform. It is lib for native binaries, and typically lib32 for compat32 binaries. Other translations might exist for other ABIs supported on the platform.

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. The following search order is used to locate required shared objects:

DT_RPATH of the referencing object unless that object also contains a DT_RUNPATH tag
DT_RPATH of the program unless the referencing object contains a DT_RUNPATH tag
Path indicated by LD_LIBRARY_PATH environment variable
DT_RUNPATH of the referencing object
Hints file produced by the ldconfig(8) utility
The /lib and /usr/lib directories, unless the referencing object was linked using the "-z nodefaultlib" option

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. If the activated image is setuid or setgid, the variables are ignored.

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_DYNAMIC_WEAK If set, use the ELF standard-compliant symbol lookup behavior: resolve to the first found symbol definition.
By default, FreeBSD provides the non-standard symbol lookup behavior: when a weak symbol definition is found, remember the definition and keep searching in the remaining shared objects for a non-weak definition. If found, the non-weak definition is preferred, otherwise the remembered weak definition is returned.
Symbols exported by dynamic linker itself (see dlfcn(3)) are always resolved using FreeBSD rules regardless of the presence of the variable. This variable is unset for set-user-ID and set-group-ID programs.

LD_LIBMAP A library replacement list in the same format as libmap.conf(5). For convenience, the characters ‘=’ and ‘amp;,’ can be used instead of a space and a newline. This variable is parsed after libmap.conf(5), and will override its entries. This variable is unset for set-user-ID and set-group-ID programs.

LD_LIBMAP_DISABLE

If set, disables the use of libmap.conf(5) and LD_LIBMAP. This variable is unset for set-user-ID and set-group-ID programs.

LD_ELF_HINTS_PATH

This variable will override the default location of "hints" file. This variable is unset for set-user-ID and set-group-ID programs.

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

LD_LIBRARY_PATH_RPATH

If the variable is specified and has a value starting with any of y, Y or 1 symbols, the path specified by LD_LIBRARY_PATH variable is allowed to override the path from DT_RPATH for binaries which does not contain DT_RUNPATH tag. For such binaries, when the variable LD_LIBRARY_PATH_RPATH is set, "-z nodefaultlib" link-time option is ignored as well.

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 variable is unset for set-user-ID and set-group-ID programs.

LD_PRELOAD_FDS A colon separated list of file descriptor numbers for libraries. This is intended for preloading libraries in which we already have a file descriptor. This may optimize the process of loading libraries because we do not have to look for them in directories. It may also be useful in a capability base system where we do not have access to global namespaces such as the filesystem.

LD_LIBRARY_PATH_FDS

A colon separated list of file descriptor numbers for library directories. This is intended for use within capsicum(4) sandboxes, when global namespaces such as the filesystem are unavailable. It is consulted just after LD_LIBRARY_PATH. This variable is unset for set-user-ID and set-group-ID programs.

LD_BIND_NOT When set to a nonempty string, prevents modifications of the PLT slots when doing bindings. As result, each call of the PLT-resolved function is resolved. In combination with debug output, this provides complete account of all bind actions at runtime. This variable is unset 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. If the dependency name starts with string lib, LD_TRACE_LOADED_OBJECTS_FMT1 is used, otherwise LD_TRACE_LOADED_OBJECTS_FMT2 is used. The following conversions can be used:

%a The main program's name (also known as "__progname").

amp;%A The value of the environment variable LD_TRACE_LOADED_OBJECTS_PROGNAME. Typically used to print both the names of programs and shared libraries being inspected using ldd(1).

%o The library name.

%p The full pathname as determined by rtld library search rules.

%x The library's load address.

Additionally, ‘\n’ and ‘\t’ are recognized and have their usual meaning.

LD_UTRACE

If set, ld-elf.so.1 will log events such as the loading and unloading of shared objects via utrace(2).

LD_LOADFLTR

If set, ld-elf.so.1 will process the filtee dependencies of the loaded objects immediately, instead of postponing it until required. Normally, the filtees are opened at the time of the first symbol resolution from the filter object.

LD_SHOW_AUXV

If set, causes ld-elf.so.1 to dump content of the aux vector to standard output, before passing control to any user code.

LD_STATIC_TLS_EXTRA

If the variable is specified and has a numeric value, ld-elf.so.1 will set the size of the static TLS extra space to the specified number of bytes. The static TLS extra space is used when loading objects compiled for initial-exec TLS code model with dlopen(3). The minimum value that can be specified is 128.

LD_NO_DL_ITERATE_PHDR_AFTER_FORK

Allow dl_iterate_phdr(3) to block in callback, without causing deadlock with the fork(2). The drawback is that the image started in this mode cannot use dl_iterate_phdr(3) after fork.

-b exe
	Use the executable exe instead of image_path for activation. If this option is specified, image_path is only used to provide the argv[0] value to the program.
-d
	Turn off the emulation of the binary execute permission.
-f fd
	File descriptor fd references the binary to be activated by ld-elf.so.1. It must already be opened in the process when executing ld-elf.so.1. If this option is specified, image_path is only used to provide the argv[0] value to the program.
-o OPT=VALUE
	Set the OPT configuration variable to the value VALUE. The possible variable names are listed above as LD_ prefixed environment variables, but here are referenced without the LD_ prefix. A configuration variable set this way does not leak into the activated image's environment. The option can be repeated as many times as needed to set all configuration parameters. The parameters set using this option have priority over the same parameters assigned via environment.
-p
	If the `image_path` argument specifies a name which does not contain a slash "`/`" character, ld-elf.so.1 uses the search path provided by the environment variable `PATH` to find the binary to execute.
-u
	Ignore all LD_ environment variables and previous command line -o options that otherwise affect the dynamic linker behavior.
-v
	Display information about this run-time linker binary, then exit.
--
	Ends the ld-elf.so.1 options. The argument following -- is interpreted as the path of the binary to execute.

`_rtld_version__FreeBSD_version`
	Symbol exports the value of the `__FreeBSD_version` definition as it was provided during the ld-elf.so.1 build. The symbol is always present since the `_rtld_version` facility was introduced.
`_rtld_version_laddr_offset`
	The l_addr member of the link_map structure contains the load offset of the shared object. Before that, l_addr contained the base address of the library. See dlinfo(3). Also it indicates the presence of l_refname member of the structure.
`_rtld_version_dlpi_tls_data`
	The dlpi_tls_data member of the structure dl_phdr_info contains the address of the module TLS segment for the calling thread, and not the address of the initialization segment.

`/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.

Manual Pages — RTLD

NAME

CONTENTS

DESCRIPTION

DIRECT EXECUTION MODE

VERSIONING

FILES

SEE ALSO

`$ORIGIN`	Translated to the full path of the loaded object.
`$OSNAME`	Translated to the name of the operating system implementation.
`$OSREL`	Translated to the release level of the operating system.
`$PLATFORM`
	Translated to the machine hardware platform.
`$LIB`	Translated to the system library path component on the platform. It is `lib` for native binaries, and typically `lib32` for compat32 binaries. Other translations might exist for other ABIs supported on the platform.

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_DYNAMIC_WEAK	If set, use the ELF standard-compliant symbol lookup behavior: resolve to the first found symbol definition. By default, FreeBSD provides the non-standard symbol lookup behavior: when a weak symbol definition is found, remember the definition and keep searching in the remaining shared objects for a non-weak definition. If found, the non-weak definition is preferred, otherwise the remembered weak definition is returned. Symbols exported by dynamic linker itself (see dlfcn(3)) are always resolved using FreeBSD rules regardless of the presence of the variable. This variable is unset for set-user-ID and set-group-ID programs.
LD_LIBMAP	A library replacement list in the same format as libmap.conf(5). For convenience, the characters ‘`=`’ and ‘`amp;,`’ can be used instead of a space and a newline. This variable is parsed after libmap.conf(5), and will override its entries. This variable is unset for set-user-ID and set-group-ID programs.
LD_LIBMAP_DISABLE
	If set, disables the use of libmap.conf(5) and LD_LIBMAP. This variable is unset for set-user-ID and set-group-ID programs.
LD_ELF_HINTS_PATH
	This variable will override the default location of "hints" file. This variable is unset for set-user-ID and set-group-ID programs.
LD_LIBRARY_PATH	A colon separated list of directories, overriding the default search path for shared libraries. This variable is unset for set-user-ID and set-group-ID programs.
LD_LIBRARY_PATH_RPATH
	If the variable is specified and has a value starting with any of y, Y or 1 symbols, the path specified by LD_LIBRARY_PATH variable is allowed to override the path from `DT_RPATH` for binaries which does not contain `DT_RUNPATH` tag. For such binaries, when the variable LD_LIBRARY_PATH_RPATH is set, "-z nodefaultlib" link-time option is ignored as well.
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 variable is unset for set-user-ID and set-group-ID programs.
LD_PRELOAD_FDS	A colon separated list of file descriptor numbers for libraries. This is intended for preloading libraries in which we already have a file descriptor. This may optimize the process of loading libraries because we do not have to look for them in directories. It may also be useful in a capability base system where we do not have access to global namespaces such as the filesystem.
LD_LIBRARY_PATH_FDS
	A colon separated list of file descriptor numbers for library directories. This is intended for use within capsicum(4) sandboxes, when global namespaces such as the filesystem are unavailable. It is consulted just after LD_LIBRARY_PATH. This variable is unset for set-user-ID and set-group-ID programs.
LD_BIND_NOT	When set to a nonempty string, prevents modifications of the PLT slots when doing bindings. As result, each call of the PLT-resolved function is resolved. In combination with debug output, this provides complete account of all bind actions at runtime. This variable is unset 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. If the dependency name starts with string `lib`, LD_TRACE_LOADED_OBJECTS_FMT1 is used, otherwise LD_TRACE_LOADED_OBJECTS_FMT2 is used. The following conversions can be used:

`%a`	The main program's name (also known as "__progname").
`amp;%A`	The value of the environment variable LD_TRACE_LOADED_OBJECTS_PROGNAME. Typically used to print both the names of programs and shared libraries being inspected using ldd(1).
`%o`	The library name.
`%p`	The full pathname as determined by rtld library search rules.
`%x`	The library's load address.

LD_UTRACE
	If set, ld-elf.so.1 will log events such as the loading and unloading of shared objects via utrace(2).
LD_LOADFLTR
	If set, ld-elf.so.1 will process the filtee dependencies of the loaded objects immediately, instead of postponing it until required. Normally, the filtees are opened at the time of the first symbol resolution from the filter object.
LD_SHOW_AUXV
	If set, causes ld-elf.so.1 to dump content of the aux vector to standard output, before passing control to any user code.
LD_STATIC_TLS_EXTRA
	If the variable is specified and has a numeric value, ld-elf.so.1 will set the size of the static TLS extra space to the specified number of bytes. The static TLS extra space is used when loading objects compiled for initial-exec TLS code model with dlopen(3). The minimum value that can be specified is 128.
LD_NO_DL_ITERATE_PHDR_AFTER_FORK
	Allow dl_iterate_phdr(3) to block in callback, without causing deadlock with the fork(2). The drawback is that the image started in this mode cannot use dl_iterate_phdr(3) after fork.