DragonFly On-Line Manual Pages
RTLD(1) DragonFly General Commands Manual RTLD(1)
NAME
rtld, ld-elf.so.2, _rtld_functrace -- run-time link-editor
SYNOPSIS
int
_rtld_functrace(const char *callerso, const char *calleeso,
const char *calleefun, void *stack);
DESCRIPTION
The rtld 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, rtld 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, rtld may be allowed
to translate dynamic token strings in rpath and soname by setting -z
origin option of the static linker 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.
The rtld 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:
1. DT_RPATH of the referencing object unless that object also
contains a DT_RUNPATH tag
2. DT_RPATH of the program unless the referencing object contains
a DT_RUNPATH tag
3. Path indicated by LD_LIBRARY_PATH environment variable
4. DT_RUNPATH of the referencing object
5. Hints file produced by the ldconfig(8) utility
6. The /lib and /usr/lib directories, unless the referencing
object was linked using the ``-z nodefaultlib'' option
The rtld utility recognizes a number of environment variables that can be
used to modify its behaviour. for example:
LD_DUMP_REL_POST If set, rtld will print a table containing all
relocations after symbol binding and relocation.
LD_DUMP_REL_PRE If set, rtld 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. 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_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_LIBRARY_PATH_FDS A colon separated list of file
descriptor numbers for library directories. This is
intended for future use within capsicum 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_NOW When set to a nonempty string, causes rtld 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 rtld 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 rtld 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'').
%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's
library search rules.
%x The library's load address.
Additionally, `\n' and `\t' are recognized and have
their usual meaning.
LD_UTRACE If set, rtld will log events such as the loading and
unloading of shared objects via utrace(2).
LD_LOADFLTR If set, rtld 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.
If a shared object preloaded by the LD_PRELOAD mechanism contains a
public symbol ``_rtld_functrace'', rtld will transfer control to this
function each time it needs to resolve an unbound function symbol. By
returning a non-zero value, _rtld_functrace() can advise the linker to
keep tracing the specified combination of caller shared object and called
function; returning 0 will prevent _rtld_functrace() to be called for
this combination again.
When implementing a custom _rtld_functrace() function, be aware of the
possibility that _rtld_functrace() might be called for functions called
on its behalf, or that multiple threads could enter _rtld_functrace() at
the same time.
DIFFERENCES BETWEEN .1 and .2
ABI changes have been made to support TLS allocation and initialization
and to give threading libraries a chance to complete initialization of
the TCB prior to the calling of the _init() functions for the dynamically
loaded libraries.
FILES
/var/run/ld-elf.so.hints Hints file.
/etc/libmap.conf The libmap configuration file.
EXAMPLES
To set up an _rtld_functrace() for printing out the functions as they are
called, this code can be used:
#include <string.h>
#include <unistd.h>
static int nl = 10;
int
_rtld_functrace(const char *callerso, const char *calleeso,
const char *calleefun, void *stack)
{
write(2, "calling ", 8);
write(2, calleefun, strlen(calleefun));
write(2, &nl, 1);
return 1;
}
If put in a file named ft.c and compiled with
$ cc -shared -fPIC ft.c -o ft.so
setting LD_PRELOAD to the path of ft.so will activate it.
SEE ALSO
ld(1), ldd(1), elf(5), libmap.conf(5), ldconfig(8)
DragonFly 5.5 June 20, 2014 DragonFly 5.5