trusty (3) dlsym.3posix.gz

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 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() 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 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, 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 .