dlsym(3DL) (man pages section 3: Basic Library Functions)

man pages section 3: Basic Library Functions

dlsym(3DL)

NAME

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

SYNOPSIS

cc [ flag ... ] file ... -ldl [ library ... ]
#include <dlfcn.h>

handle

name

DESCRIPTION

The dlsym() function is one of a family of functions that provide the user direct access to the dynamic linking facilities. (See Linker and Libraries Guide ). These functions are made available through the library loaded when the option -ldl is passed to the link-editor.

These functions are available to dynamically-linked processes ONLY.

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

In the case of a handle returned from dlopen(), the corresponding shared object must not have been closed using dlclose(). The dlsym() function will search for the named symbol in all shared objects loaded automatically as a result of loading the object referenced by handle. See dlopen(3DL).

In the case of the special handle RLTD_NEXT, dlsym() will search for the named symbol in the objects that were loaded following the object from which the dlsym() call is being made.

In the case of the special handle RTLD_DEFAULT, dlsym() will search for the named symbol starting with the first object loaded and proceeding through the list of loaded objects until a match is found. This search follows the default model employed to relocate all objects within the process.

In the case of both RLTD_NEXT and RTLD_DEFAULT, if the objects being searched have been loaded from dlopen() calls, dlsym() will search the object only if the caller is part of the same dlopen() dependency hierarchy, or if the object was given global search access. See dlopen(3DL) for a discussion of the RTLD_GLOBAL mode.

RETURN VALUES

If handle does not refer to a valid object opened by dlopen(), is not the special handles RTLD_NEXT or RTLD_DEFAULT, or if the named symbol cannot be found within any of the objects associated with handle, dlsym() will return NULL. More detailed diagnostic information is available through dlerror().

EXAMPLES

Example 1 Using `dlopen()` and `dlsym()` to access a function or data objects.

The following example shows how one can 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 Using `dlsym()` to verify that a particular function is defined.

The following code fragment shows how dlsym() can be used to verify that a particular function is defined and to call it only if it is.

int (*fptr)();

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

ATTRIBUTES

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE	ATTRIBUTE VALUE
MT-Level	MT-Safe