This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
The dlsym() function shall obtain the address of a symbol (a function
identifier or a data object identifier) defined in the symbol table
identified by the handle argument. The handle argument is a symbol
table handle returned from a call to dlopen() (and which has not
since been released by a call to dlclose()), and name is the symbol's
name as a character string. The return value from dlsym(), cast to a
pointer to the type of the named symbol, can be used to call (in the
case of a function) or access the contents of (in the case of a data
object) the named symbol.
The dlsym() function shall search for the named symbol in the symbol
table referenced by handle. If the symbol table was created with
lazy loading (see RTLD_LAZY in dlopen()), load ordering shall be used
in dlsym() operations to relocate executable object files needed to
resolve the symbol. The symbol resolution algorithm used shall be
dependency order as described in dlopen().
The RTLD_DEFAULT and RTLD_NEXT symbolic constants (which may be
defined in <dlfcn.h>) are reserved for future use as special values
that applications may be allowed to use for handle.
Upon successful completion, if name names a function identifier,
dlsym() shall return the address of the function converted from type
pointer to function to type pointer to void; otherwise, dlsym() shall
return the address of the data object associated with the data object
identifier named by name converted from a pointer to the type of the
data object to a pointer to void. If handle does not refer to a
valid symbol table handle or if the symbol named by name cannot be
found in the symbol table associated with handle, dlsym() shall
return a null pointer.
More detailed diagnostic information shall be available through
The following example shows how dlopen() and dlsym() can be used to
access either a function or a data object. For simplicity, error
checking has been omitted.
void *handle;int (*fptr)(int), *iptr, result;/* open the needed symbol table */handle = dlopen("/usr/home/me/libfoo.so", RTLD_LOCAL | RTLD_LAZY);/* find the address of the function my_function */fptr = (int (*)(int))dlsym(handle, "my_function");/* find the address of the data object my_object */iptr = (int *)dlsym(handle, "my_OBJ");/* invoke my_function, passing the value of my_OBJ as the parameter */result = (*fptr)(*iptr);
The following special purpose values for handle are reserved for
future use and have the indicated meanings:
The identifier lookup happens in the normal global scope;
that is, a search for an identifier using handle would
find the same definition as a direct use of this
identifier in the program code.
RTLD_NEXT Specifies the next executable object file after this one
that defines name. This one refers to the executable
object file containing the invocation of dlsym(). The
next executable object file is the one found upon the
application of a load order symbol resolution algorithm
(see dlopen()). The next symbol 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 in an
executable object file 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.
Note that conversion from a void * pointer to a function pointer as
fptr = (int (*)(int))dlsym(handle, "my_function");
is not defined by the ISO C standard. This standard requires this
conversion to work correctly on conforming implementations.
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
IEEE/The Open Group 2013 DLSYM(3P)