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 — get the address of a symbol from a symbol table handle

SYNOPSIS

       #include <dlfcn.h>

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

DESCRIPTION

       The  dlsym()  function  shall  obtain  the  address  of  a symbol (a function identifier or a data object
       identifier) defined in the symbol table identified by the handle  argument.  The  handle  argument  is  a
       symbol  table handle returned from a call to dlopen() (and which has not since been released by a call to
       dlclose()), and name is the symbol's name as a character string. The return value from dlsym(), cast to a
       pointer  to  the  type of the named symbol, can be used to call (in the case of a function) or access the
       contents of (in the case of a data object) the named symbol.

       The dlsym() function shall search for the named symbol in the symbol table referenced by handle.  If  the
       symbol  table  was  created with lazy loading (see RTLD_LAZY in dlopen()), load ordering shall be used in
       dlsym() operations to relocate  executable  object  files  needed  to  resolve  the  symbol.  The  symbol
       resolution algorithm used shall be dependency order as described in dlopen().

       The  RTLD_DEFAULT  and  RTLD_NEXT symbolic constants (which may be defined in <dlfcn.h>) are reserved for
       future use as special values that applications may be allowed to use for handle.

RETURN VALUE

       Upon successful completion, if name names a function identifier, dlsym() shall return the address of  the
       function converted from type pointer to function to type pointer to void; otherwise, dlsym() shall return
       the address of the data object associated with the data object identifier named by name converted from  a
       pointer  to the type of the data object to a pointer to void.  If handle does not refer to a valid symbol
       table handle or if the symbol named by name cannot be found in the symbol table associated  with  handle,
       dlsym() shall return a null pointer.

       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 a function or a data
       object. For simplicity, error checking has been omitted.

           void *handle;
           int (*fptr)(int), *iptr, result;
           /* open the needed symbol table */
           handle = dlopen("/usr/home/me/libfoo.so", RTLD_LOCAL | RTLD_LAZY);
           /* find the address of the function my_function */
           fptr = (int (*)(int))dlsym(handle, "my_function");
           /* find the address of the data object my_object */
           iptr = (int *)dlsym(handle, "my_OBJ");
           /* invoke my_function, passing the value of my_OBJ as the parameter */
           result = (*fptr)(*iptr);

APPLICATION USAGE

       The following special purpose values for handle are reserved  for  future  use  and  have  the  indicated
       meanings:

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

       RTLD_NEXT   Specifies  the next executable object file after this one that defines name.  This one refers
                   to the executable object file containing the invocation  of  dlsym().   The  next  executable
                   object file is the one found upon the application of a load order symbol resolution algorithm
                   (see dlopen()).  The next symbol 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 in an executable object file 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.

       Note that conversion from a void * pointer to a function pointer as in:

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

       is  not  defined  by  the  ISO C  standard.  This  standard requires this conversion to work correctly on
       conforming implementations.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       dlclose(), dlerror(), dlopen()

       The Base Definitions volume of POSIX.1‐2008, <dlfcn.h>

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition,
       Standard  for  Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers,  Inc
       and  The  Open Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .

       Any typographical or formatting errors that appear in this page are most likely to have  been  introduced
       during   the   conversion  of  the  source  files  to  man  page  format.  To  report  such  errors,  see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .