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 (con-
       sult the corresponding 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 simplic-
       ity, 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 definition 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 invoca-
              tion 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 sup-
       porting 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 generate 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 Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6,  Copy-
       right (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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                     DLSYM(3P)