Table 25 Parameters
Parameter | Definition |
|
|
handle | Either the value returned by a call to dlopen or one of the special flags RTLD_NEXT, |
| RTLD_SELF, and RTLD_DEFAULT. In the former case, the corresponding shared library must |
| not have been closed using dlclose. |
|
|
name | The symbol's name as a character string. |
|
|
Return Values
If handle does not refer to a valid shared library opened by dlopen, or if the named symbol cannot be found within any of the shared libraries associated with handle, dlsym returns NULL. The dlerror routine provides more detailed diagnostic information.
Description
The dlsym routine allows a process to obtain the address of a symbol defined within a shared library previously opened by dlopen.
The dlsym routine searches for the named symbol in all shared libraries loaded automatically as a result of loading the shared library referenced by handle [see dlopen(3C)].
If handle is RTLD_NEXT, the search begins with the “"next"” shared library after the shared library from which dlsym was invoked. Shared libraries are searched using a load order symbol resolution algorithm [see dlopen(3C)]. The "next" shared library, and all other shared libraries 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 shared libraries loaded by the same dlopen operation that loaded the caller of dlsym.
If handle is RTLD_SELF, the search begins with the object from which dlsym was invoked. Objects are searched using the load order symbol resolution algorithm. If handle is RTLD_DEFAULT, then the symbol search is done in the scope of the object that invoked dlsym. For example, if the caller object was loaded as a result of dlopen with RTLD_GROUP, it searches symbols in objects that were loaded in the same dlopen invocation as the caller object.
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 gathers the necessary information, and use dlsym with RTLD_NEXT to find the “"real"” malloc, which performs the actual memory allocation. Of course, this “"real" ”malloc can be another
Using dlopen and dlsym to access either function or data objects
The following example shows how to 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);
158 Shared Library Management Routines