DLSYM(3C) Standard C Library Functions DLSYM(3C)

NAME

dlsym - get the address of a symbol in a shared object or executable

SYNOPSIS

#include <dlfcn.h>

void *dlsym(void *restrict handle, const char *restrict name);

DESCRIPTION

The dlsym() function allows a process to obtain the address of a symbol
that is defined within a shared object or executable. The handle argument
is either the value returned from a call to dlopen() or one of a family
of special handles. The name argument is the symbol's name as a character
string.


If handle is returned from dlopen(), the associated shared object must
not have been closed using dlclose(). A handle can be obtained from
dlopen() using the RTLD_FIRST mode. With this mode, the dlsym() function
searches for the named symbol in the initial object referenced by handle.
Without this mode, the dlsym() function searches for the named symbol in
the group of shared objects loaded automatically as a result of loading
the object referenced by handle. See dlopen(3C) and NOTES.


The following special handles are supported.

RTLD_DEFAULT
Instructs dlsym() to search for the named symbol starting
with the first object loaded, typically the dynamic
executable. The search continues through the list of
initial dependencies that are loaded with the process,
followed by any objects obtained with dlopen(3C). This
search follows the default model that is used to relocate
all objects within the process.

This model also provides for transitioning into a lazy
loading environment. If a symbol can not be found in the
presently loaded objects, any pending lazy loaded objects
are processed in an attempt to locate the symbol. This
loading compensates for objects that have not fully
defined their dependencies. However, this compensation
can undermine the advantages of lazy loading.


RTLD_PROBE
Instructs dlsym() to search for the named symbol in the
same manner as occurs with a handle of RTLD_DEFAULT.
However, this model only searches for symbols in the
presently loaded objects, together with any lazy loadable
objects specifically identified by the caller to provide
the named symbol. This handle does not trigger an
exhaustive load of any lazy loadable symbols in an
attempt to find the named symbol. This handle can provide
a more optimal search than would occur using
RTLD_DEFAULT.


RTLD_NEXT
Instructs dlsym() to search for the named symbol in the
objects that were loaded following the object from which
the dlsym() call is being made.


RTLD_SELF
Instructs dlsym() to search for the named symbol in the
objects that were loaded starting with the object from
which the dlsym() call is being made.


When used with a special handle, dlsym() is selective in searching
objects that have been loaded using dlopen(). These objects are searched
for symbols if one of the following conditions are true.

o The object is part of the same local dlopen() dependency
hierarchy as the calling object. See the Linker and Libraries
Guide for a description of dlopen() dependency hierarchies.

o The object has global search access. See dlopen(3C) for a
discussion of the RTLD_GLOBAL mode.

RETURN VALUES

The dlsym() function returns NULL if handle does not refer to a valid
object opened by dlopen() or is not one of the special handles. The
function also returns NULL if the named symbol cannot be found within any
of the objects associated with handle. Additional diagnostic information
is available through dlerror(3C).

EXAMPLES

Example 1: Use dlopen() and dlsym() to access a function or data objects.

The following code fragment demonstrates how to use dlopen() and dlsym()
to access either function or data objects. For simplicity, error checking
has been omitted.


void *handle;
int *iptr, (*fptr)(int);

/* open the needed object */
handle = dlopen("/usr/home/me/libfoo.so.1", RTLD_LAZY);

/* find the address of function and data objects */
fptr = (int (*)(int))dlsym(handle, "my_function");
iptr = (int *)dlsym(handle, "my_object");

/* invoke function, passing value of integer as a parameter */
(*fptr)(*iptr);

Example 2: Use dlsym() to verify that a particular function is defined.

The following code fragment shows how to use dlsym() to verify that a
function is defined. If the function exists, the function is called.


int (*fptr)();

if ((fptr = (int (*)())dlsym(RTLD_DEFAULT,
"my_function")) != NULL) {
(*fptr)();
}

USAGE

The dlsym() function is one of a family of functions that give the user
direct access to the dynamic linking facilities. These facilities are
available to dynamically-linked processes only. See the Linker and
Libraries Guide.

ATTRIBUTES

See attributes(7) for descriptions of the following attributes:


+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | Standard |
+--------------------+-----------------+
|MT-Level | MT-Safe |
+--------------------+-----------------+

SEE ALSO

ld(1), ld.so.1(1), dladdr(3C), dlclose(3C), dldump(3C), dlerror(3C),
dlinfo(3C), dlopen(3C), attributes(7), standards(7)


Linker and Libraries Guide

NOTES

If an object is acting as a filter, care should be taken when
interpreting the address of any symbol obtained using a handle to this
object. For example, using dlsym(3C) to obtain the symbol _end for this
object, results in returning the address of the symbol _end within the
filtee, not the filter. For more information on filters see the Linker
and Libraries Guide.

September 26, 2005 DLSYM(3C)