Originální popis anglicky:
dlsym - obtain the address of a symbol from a dlopen object
Návod, kniha: POSIX Programmer's Manual
#include <dlfcn.h>
void *dlsym(void *restrict
handle, const char
*restrict name);
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.
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() .
No errors are defined.
The following sections are informative.
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);
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.
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.
None.
dlclose() ,
dlerror() ,
dlopen() , the Base Definitions
volume of IEEE Std 1003.1-2001,
<dlfcn.h>
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
.
