DLSYM(3P)						     POSIX Programmer's Manual							 DLSYM(3P)

PROLOG

       This  manual page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (consult the correspond-
       ing Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

NAME

       dlsym - obtain the address of a symbol from a dlopen object

SYNOPSIS

       #include <dlfcn.h>

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

DESCRIPTION

       The dlsym() function shall obtain the address of a symbol defined within an object made accessible through  a  dlopen()	call.  The  handle
       argument  is  the  value  returned from a call to dlopen() (and which has not since been released via a call to dlclose()), and name is the
       symbol's name as a character string.

       The dlsym() function shall search for the named symbol in all objects loaded automatically as a result of loading the object referenced	by
       handle  (see  dlopen()).  Load  ordering  is used in dlsym() operations upon the global symbol object. The symbol resolution algorithm used
       shall be dependency order as described in dlopen().

       The RTLD_DEFAULT and RTLD_NEXT flags are reserved for future use.

RETURN VALUE

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

ERRORS

       No errors are defined.

       The following sections are informative.

EXAMPLES

       The  following example shows how dlopen() and dlsym() can be used 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", RTLD_LOCAL | RTLD_LAZY);

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

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

APPLICATION USAGE

       Special purpose values for handle are reserved for future use.  These values and their meanings are:

       RTLD_DEFAULT
	      The symbol lookup happens in the normal global scope; that is, a search for a symbol using this handle would find the  same  defini-
	      tion as a direct use of this symbol in the program code.

       RTLD_NEXT
	      Specifies the next object after this one that defines name.  This one refers to the object containing the invocation of dlsym(). The
	      next object is the one found upon the application of a load order symbol resolution algorithm (see dlopen()).  The  next	object	is
	      either  one of global scope (because it was introduced as part of the original process image or because it was added with a dlopen()
	      operation including the RTLD_GLOBAL flag), or is an object that was included in the same dlopen() operation that loaded this one.

       The RTLD_NEXT flag is useful to navigate an intentionally created hierarchy of multiply-defined symbols created through interposition.  For
       example, if a program wished to create an implementation of malloc() that embedded some statistics gathering about memory allocations, such
       an implementation could use the real malloc() definition to perform the memory allocation-and itself only  embed  the  necessary  logic	to
       implement the statistics gathering function.

RATIONALE

       The  ISO C  standard does not require that pointers to functions can be cast back and forth to pointers to data. Indeed, the ISO C standard
       does not require that an object of type void * can hold a pointer to a function. Implementations supporting the XSI extension, however,	do
       require	that an object of type void * can hold a pointer to a function. The result of converting a pointer to a function into a pointer to
       another data type (except void *) is still undefined, however. Note that compilers conforming to the ISO C standard are required to  gener-
       ate a warning if a conversion from a void * pointer to a function pointer is attempted as in:

	      fptr = (int (*)(int))dlsym(handle, "my_function");

       Due  to the problem noted here, a future version may either add a new function to return function pointers, or the current interface may be
       deprecated in favor of two new functions: one that returns data pointers and the other that returns function pointers.

FUTURE DIRECTIONS

       None.

SEE ALSO

       dlclose(), dlerror(), dlopen(), the Base Definitions volume of IEEE Std 1003.1-2001, <dlfcn.h>

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technol-
       ogy -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
       Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and  the  original  IEEE
       and  The  Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained
       online at http://www.opengroup.org/unix/online.html .

IEEE
/The Open Group						       2003								 DLSYM(3P)