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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 dif-
       fer (consult 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 load-
       ing 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 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 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()  defini-
       tion  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 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, Stan-
       dard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base  Specifica-
       tions  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 Stan-
       dard,  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)