dlsym(3X)

NAME

dlsym − get the address of a symbol in a shared object

SYNOPSIS

cc [ flag ... ] file ... −ldl [ library ... ]

#include <dlfcn.h>

void ∗dlsym(void ∗handle, const char ∗name);

MT-LEVEL

MT-Safe

DESCRIPTION

dlsym() is one of a family of routines that give the user direct access to the dynamic linking facilities. (SEE Linker and Libraries Guide ). These routines are made available via the library loaded when the option -ldl is passed to the link-editor.

These routines are available to dynamically linked processes ONLY.

dlsym() allows a process to obtain the address of a symbol defined within a shared object. handle is either the value returned from a call to dlopen(), or the special flag RTLD_NEXT. name is the symbol’s name as a character string.

In the former case the corresponding shared object must not have been closed using dlclose(). dlsym() will search for the named symbol in all shared objects loaded automatically as a result of loading the object referenced by handle (see dlopen()).

In the latter case dlsym() will search for the named symbol in the objects that were loaded following the object from which the dlsym() call is being made. If these subsequent objects were loaded from dlopen() calls, dlsym() will search the object only if the caller is part of the same dlopen() dependency hierarchy, or the object was given global search access (see dlopen() with reference to the mode RTLD_GLOBAL).

RETURN VALUES

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

EXAMPLES

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);

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