DLSYM(P) POSIX Programmer's Manual DLSYM(P)
dlsym - obtain the address of a symbol from a dlopen object
void *dlsym(void *restrict handle, const char *restrict name);
The dlsym() function shall obtain the address of a symbol defined within an object made
accessible through a dlopen() call. The handle argument is the value returned from a call
to dlopen() (and which has not since been released via a call to dlclose()), and name is
the symbol's name as a character string.
The dlsym() function shall search for the named symbol in all objects loaded automatically
as a result of loading the object referenced by handle (see dlopen() ). Load ordering is
used in dlsym() operations upon the global symbol object. The symbol resolution algorithm
used shall be dependency order as described in dlopen() .
The RTLD_DEFAULT and RTLD_NEXT flags are reserved for future use.
If handle does not refer to a valid object opened by dlopen(), or if the named symbol can-
not be found within any of the objects associated with handle, dlsym() shall return NULL.
More detailed diagnostic information shall be available through dlerror() .
No errors are defined.
The following sections are informative.
The following example shows how dlopen() and dlsym() can be used to access either function
or data objects. For simplicity, error checking has been omitted.
int *iptr, (*fptr)(int);
/* open the needed object */
handle = dlopen("/usr/home/me/libfoo.so", RTLD_LOCAL | RTLD_LAZY);
/* find the address of function and data objects */
*(void **)(&fptr) = dlsym(handle, "my_function");
iptr = (int *)dlsym(handle, "my_object");
/* invoke function, passing value of integer as a parameter */
Special purpose values for handle are reserved for future use. These values and their
The symbol lookup happens in the normal global scope; that is, a search for a sym-
bol using this handle would find the same definition as a direct use of this symbol
in the program code.
Specifies the next object after this one that defines name. This one refers to the
object containing the invocation of dlsym(). The next object is the one found upon
the application of a load order symbol resolution algorithm (see dlopen() ). The
next object 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 an object that was included in the same dlopen() oper-
ation 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 alloca-
tions, 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
The ISO C standard does not require that pointers to functions can be cast back and forth
to pointers to data. Indeed, the ISO C standard does not require that an object of type
void * can hold a pointer to a function. Implementations supporting the XSI extension,
however, do require that an object of type void * can hold a pointer to a function. The
result of converting a pointer to a function into a pointer to another data type (except
void *) is still undefined, however. Note that compilers conforming to the ISO C standard
are required to generate a warning if a conversion from a void * pointer to a function
pointer is attempted as in:
fptr = (int (*)(int))dlsym(handle, "my_function");
Due to the problem noted here, a future version may either add a new function to return
function pointers, or the current interface may be deprecated in favor of two new func-
tions: one that returns data pointers and the other that returns function pointers.
dlclose() , dlerror() , dlopen() , the Base Definitions volume of IEEE Std 1003.1-2001,
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System
Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by
the Institute of Electrical and Electronics Engineers, Inc and The Open Group. 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 orig-
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 DLSYM(P)