The Single UNIX ® Specification, Version 2
Copyright © 1997 The Open Group


dlsym - obtain the address of a symbol from a dlopen() object


#include <dlfcn.h>

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


dlsym() allows a process to obtain the address of a symbol defined within an object made accessible through a dlopen() call. handle is the value returned from a call to dlopen() (and which has not since been released via a call to dlclose()), name is the symbol's name as a character string.

dlsym() will 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 will be dependency order as described in dlopen().


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


No errors are defined.


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     *iptr, (*fptr)(int);

/* open the needed object */
handle = dlopen("/usr/home/me/", RTLD_LAZY);

/* find the address of function and data objects */
fptr = (int (*)(int))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 meanings are:
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() 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.




dlclose(), dlerror(), dlopen().

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Copyright © 1997 The Open Group
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