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 .