dlsym(3C)dlsym(3C)NAMEdlsym() - get the address of a symbol in shared library
[flag]... file... [library]...
This routine is thread-safe.
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 command
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 for‐
mer 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 automati‐
cally as a result of loading the object referenced by handle (see
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 algorithm.
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.
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
If fails, a subsequent call to returns one of the following values:
Cannot apply relocation in library.
Internal error encountered in
End of liblist, invalid argument.
Out of memory.
failed on entry to or exit from
failed on exit from
failed on entry to
can be used to navigate an intentionally created hierarchy of multiply
defined symbols created through interposition. For example, if a pro‐
gram wished to create an implementation of that embedded some statis‐
tics 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 alloca‐
tion. Of course, this "real" could be another user-defined interface
that added its own value and then used to find the system
The following example shows how you can use and to access either func‐
tion or data objects. For simplicity, error checking has been omitted.
int i, *iptr;
/* 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);
real_malloc = (void * (*) (size_t))
ptr = (*real_malloc)(sz);
SEE ALSOdlclose(3C), dlerrno(3C), dlerror(3C), dlopen(3C).
Texts and Tutorials
(See the option)
(See manuals(5) for ordering information)