HP-UX Reference (11i v1 05/09) - 3 Library Functions A-M (vol 6)

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dlsym(3C) dlsym(3C)

[RTLD_ERR_SETCANCELSTATE_FAILED]

__thread_setcancelstate

failed on entry to or exit from

dlsym().

APPLICATION 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

mal-

loc()

that embedded some statistics gathering about memory allocations, such an implementation could

deﬁne its own malloc() which would gather the necessary information, and use

dlsym() with

RTLD_NEXT to ﬁnd the "real"

malloc(), which would perform the actual memory allocation. Of course,

this "real"

malloc()

could be another user-deﬁned interface that added its own value and then used

RTLD_NEXT to ﬁnd the system malloc().

EXAMPLES

The following example shows how you can 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.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

dlsym() with RTLD_NEXT to add functionality to an existing

interface. Again, error checking has been omitted.

extern void record_malloc(void *, size_t);

void *

malloc(size_t sz)

{

void *ptr;

void *(*real_malloc)(size_t);

real_malloc = (void * (*) (size_t))

dlsym(RTLD_NEXT, "malloc");

ptr = (*real_malloc)(sz);

record_malloc(ptr, sz);

return ptr;

}