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, RTLD_PROBE only searches for symbol definitions 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.

• 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.

• 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(5) for descriptions of the following attributes:

See Also

ld(1), ld.so.1(1), dladdr(3C), dlclose(3C), dldump(3C), dlerror(3C), dlinfo(3C), dlopen(3C), attributes(5), standards(5)

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.