RTLD(1) BSD General Commands Manual RTLD(1)NAME
ld-elf.so.1, ld-elf.so.2, rtld, _rtld_functrace — run-time link-editor
SYNOPSIS
int
_rtld_functrace(const char *callerso, const char *calleeso,
const char *calleefun, void *stack);
DESCRIPTION
ld-elf.so.1 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 construc‐
tors.
ld-elf.so.1 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, relo‐
cating, 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 filesystem, ld-elf.so.1 may
use a “hints” file prepared by the ldconfig(8) utility.
ld-elf.so.1 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 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_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.
If a shared object preloaded by the LD_PRELOAD mechanism contains a pub‐
lic symbol “_rtld_functrace”, ld-elf.so.1 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
EXAMPLES
To set up an _rtld_functrace() for printing out the functions as they are
called, this code can be used:
#include <string.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 ft.c -o ft.so
setting LD_PRELOAD to the path of ft.so will activate it.
SEE ALSOld(1), ldd(1), elf(5), ldconfig(8)BSD February 21, 2011 BSD