dlsym(3C) dlsym(3C)
NAME
dlsym() - get the address of a symbol in shared library
SYNOPSIS
[flag]... file... [library]...
Multithread Usage
This routine is thread-safe.
DESCRIPTION
is one of a family of routines that give the user direct access to the dynamic linking facilities (using the option on the compiler or com-
mand line). allows a process to obtain the address of a symbol defined within a shared object previously opened by handle is a either the
value returned by a call to or one of the special flags and In the former case, the corresponding shared object must not have been closed
using name is the symbol's name as a character string.
searches for the named symbol in all shared objects loaded automatically as a result of loading the object referenced by handle (see
dlopen(3C)).
If handle is the search begins with the "next" object after the object from which was invoked. Objects are searched using a load order
symbol resolution algorithm (see dlopen(3C)). The "next" object, and all other objects searched, are either of global scope (because they
were loaded at startup or as part of a operation with the flag) or are objects loaded by the same operation that loaded the caller of
If handle is the search begins with the object from which was invoked. Objects are searched using the load order symbol resolution algo-
rithm.
If handle is then the symbol search is done in the scope of the object that invoked For example, if the caller object was loaded as a
result of with (see dlopen(3C)), it does not search symbols in objects that were not loaded in same invocation as the caller object.
RETURN VALUE
If handle does not refer to a valid object opened by or if the named symbol cannot be found within any of the objects associated with han-
dle, returns NULL. More detailed diagnostic information is available through
ERRORS
If fails, a subsequent call to returns one of the following values:
Cannot apply relocation in library.
Internal error encountered in
Invalid
handle.
End of liblist, invalid argument.
Out of memory.
failed on entry to or exit from
failed on exit from
failed on entry to
Unknown symbol.
APPLICATION USAGE
can be used to navigate an intentionally created hierarchy of multiply defined symbols created through interposition. For example, if a
program wished to create an implementation of that embedded some statistics gathering about memory allocations, such an implementation
could define its own which would gather the necessary information, and use with to find the "real" which would perform the actual memory
allocation. Of course, this "real" could be another user-defined interface that added its own value and then used to find the system
EXAMPLES
The following example shows how you can use and to access either function or data objects. For simplicity, error checking has been omit-
ted.
void *handle;
int i, *iptr;
int (*fptr)(int);
/* open the needed object */
handle = dlopen("/usr/mydir/mylib.sl", RTLD_LAZY);
/* find address of function and data objects */
fptr = (int (*)(int))dlsym(handle, "some_function");
iptr = (int *)dlsym(handle, "int_object");
/* invoke function, passing value of integer as a parameter */
i = (*fptr)(*iptr);
The next example shows how one can use with to add functionality to an existing interface. Again, error checking has been omitted.
extern void record_malloc(void *, size_t);
void *
malloc(size_t sz)
{
void *ptr;
void *(*real_malloc)(size_t);
real_malloc = (void * (*) (size_t))
dlsym(RTLD_NEXT, "malloc");
ptr = (*real_malloc)(sz);
record_malloc(ptr, sz);
return ptr;
}
SEE ALSO
dlclose(3C), dlerrno(3C), dlerror(3C), dlopen(3C).
Texts and Tutorials
(See the option)
(See manuals(5) for ordering information)
dlsym(3C)