dlsym(S)

dlsym -- get the address of a symbol in shared object

Synopsis

   
   #include <dlfcn.h>
   

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

Description

dlsym allows a process to obtain the address of a symbol defined within a shared object previously opened by dlopen. handle is either the value returned by a call to dlopen or is the special flag RTLD_NEXT. In the former case, the corresponding shared object must not have been closed using dlclose. name is the symbol's name as a character string.

dlsym searches for the named symbol in all shared objects loaded automatically as a result of loading the object referenced by handle [see dlopen(S)]. The symbol resolution algorithm used is described in dlopen.

If handle is RTLD_NEXT, the search begins with the ``next'' object after the object from which dlsym was invoked. Objects are searched using a load order symbol resolution algorithm [see dlopen(S)]. The ``next'' object, and all other objects searched, are either of global scope (because they were loaded at startup or as part of a dlopen operation with the RTLD_GLOBAL flag) or are objects loaded by the same dlopen operation that loaded the caller of dlsym.

Return values

If handle does not refer to a valid object opened by dlopen or is not the special value RTLD_NEXT, or if the named symbol cannot be found within any of the objects associated with handle, dlsym returns NULL. More detailed diagnostic information is available through dlerror.

Usage

RTLD_NEXT 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 malloc that embedded some statistics gathering about memory allocations, such an implementation could define its own malloc which would gather the necessary information, and use dlsym with RTLD_NEXT to find the ``real'' malloc, which would perform the actual memory allocation. Of course, this ``real'' malloc could be another user-defined interface that added its own value and then used RTLD_NEXT to find the system malloc.

Examples

The following example shows how one can use dlopen and dlsym to access either function or data objects. For simplicity, error checking has been omitted.

   void *handle;
   int i, *iptr;
   int (*fptr)(int);
   
   /* open the needed object */
   handle = dlopen("/usr/mydir/mylib.so", 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 dlsym with RTLD_NEXT 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;
   }

References

dlclose(S), dlerror(S), dlopen(S)