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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 differ (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 sym-
       bol'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 infor-
       mation 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 defi-
              nition 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 intro-
              duced  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 hier-
       archy  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 imple-
       ment 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 point-
       ers 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, 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)