DLFCN(3) BSD Programmer's Manual DLFCN(3)
dlopen, dlclose, dlsym, dladdr, dlctl, dlerror - dynamic link interface
#include <dlfcn.h>
void *
dlopen(const char *path, int mode);
int
dlclose(void *handle);
void *
dlsym(void *handle, const char *symbol);
int
dladdr(const void *addr, Dl_info *info);
int
dlctl(void *handle, int cmd, void *data);
const char *
dlerror(void);
These functions provide an interface to the run-time linker ld.so(1).
They allow new shared objects to be loaded into a process's address space
under program control.
The dlopen() function takes a name of a shared object as its first argu-
ment. The shared object is mapped into the address space, relocated, and
its external references are resolved in the same way as is done with the
implicitly loaded shared libraries at program startup.
The path argument can either be an absolute pathname or it can be of the
form "lib<name>.so[.xx[.yy]]" in which case the same library search rules
apply that are used for "intrinsic" shared library searches. A null
pointer supplied for path is interpreted as a reference to the main exe-
cutable of the process. The second argument currently has no effect, but
should be set to DL_LAZY for future compatibility.
dlopen() returns a handle to be used in calls to dlclose(), dlsym(), and
dlctl(). If the named shared object has already been loaded by a previous
call to dlopen() (and not yet unloaded by dlclose()), a handle referring
to the resident copy is returned.
dlclose() unlinks and removes the object referred to by handle from the
process address space. If multiple calls to dlopen() have been done on
this object (or the object was once loaded at startup time) the object is
removed when its reference count drops to zero.
dlsym() looks for a definition of symbol in the shared object designated
by handle. The symbol's address is returned. If the symbol cannot be
resolved, NULL is returned.
If dlsym() is called with the special handle NULL, it is interpreted as a
reference to the executable or shared object from which the call is being
made. Thus a shared object can reference its own symbols.
If dlsym() is called with the special handle RTLD_DEFAULT, all the shared
objects will be searched in the order they were loaded.
If dlsym() is called with the special handle RTLD_NEXT, then the search
for the symbol is limited to the shared objects which were loaded after
the one issuing the call to dlsym(). Thus, if the function is called from
the main program, all the shared libraries are searched. If it is called
from a shared library, all subsequent shared libraries are searched.
If dlsym() is called with the special handle RTLD_SELF, then the search
for the symbol is limited to the shared object issuing the call to
dlsym() and those shared objects which were loaded after it.
dladdr() queries the dynamic linker for information about the shared ob-
ject containing the address addr. The information is returned in the
structure specified by info. The structure contains at least the follow-
ing members:
const char *dli_fname The pathname of the shared object containing
the address addr.
void *dli_fbase The base address at which the shared object is
mapped into the address space of the calling
process.
const char *dli_sname The name of the nearest run-time symbol with a
address less than or equal to addr.
If no symbol with a suitable address is found,
both this field and dli_saddr are set to NULL.
void *dli_saddr The address of the symbol returned in
dli_sname.
If a mapped shared object containing addr cannot be found, dladdr() re-
turns 0. In that case, a message detailing the failure can be retrieved
by calling dlerror(). On success, a non-zero value is returned. Note:
both strings pointed at by dli_fname and dli_sname reside in memory
private to the run-time linker module and should not be modified by the
caller.
In dynamically linked programs, the address of a global function will
point to its program linkage table entry, rather than to the entry point
of the function itself. This causes most global functions to appear to be
defined within the main executable, rather than in the shared libraries
where the actual code resides.
dlctl() provides an interface similar to ioctl(2) to control several as-
pects of the run-time linker's operation. This interface is currently
under development.
dlerror() returns a character string representing the most recent error
that has occurred while processing one of the other functions described
here. If no dynamic linking errors have occurred since the last invoca-
tion of dlerror(), dlerror() returns NULL. Thus, invoking dlerror() a
second time, immediately following a prior invocation, will result in
NULL being returned.
ld(1), ld.so(1), link(5)
Some of the dl* functions first appeared in SunOS 4.
An error that occurs while processing a dlopen() request results in the
termination of the program.
MirBSD #10-current September 30, 1995 1