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dlopen(3)

NAME

     dlopen, dlsym, dlfunc, dlerror, dlclose -- programmatic interface to the
     dynamic linker

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <dlfcn.h>

     void *
     dlopen(const char *path, int mode);

     void *
     dlsym(void * restrict handle, const char * restrict symbol);

     dlfunc_t
     dlfunc(void * restrict handle, const char * restrict symbol);

     const char *
     dlerror(void);

     int
     dlclose(void *handle);

DESCRIPTION

     These functions provide a simple programmatic interface to the services
     of the dynamic linker.  Operations are provided to add new shared objects
     to a program's address space, to obtain the address bindings of symbols
     defined by such objects, and to remove such objects when their use is no
     longer required.

     The dlopen() function provides access to the shared object in path,
     returning a descriptor that can be used for later references to the
     object in calls to dlsym() and dlclose().	If path was not in the address
     space prior to the call to dlopen(), it is placed in the address space.
     When an object is first loaded into the address space in this way, its
     function _init(), if any, is called by the dynamic linker.  If path has
     already been placed in the address space in a previous call to dlopen(),
     it is not added a second time, although a reference count of dlopen()
     operations on path is maintained.	A null pointer supplied for path is
     interpreted as a reference to the main executable of the process.	The
     mode argument controls the way in which external function references from
     the loaded object are bound to their referents.  It must contain one of
     the following values, possibly ORed with additional flags which will be
     described subsequently:

     RTLD_LAZY	 Each external function reference is resolved when the func-
		 tion is first called.

     RTLD_NOW	 All external function references are bound immediately by
		 dlopen().

     RTLD_LAZY is normally preferred, for reasons of efficiency.  However,
     RTLD_NOW is useful to ensure that any undefined symbols are discovered
     during the call to dlopen().

		   from other objects in the same DAG.	This is the default,
		   but it may be specified explicitly with this flag.

     RTLD_TRACE    When set, causes dynamic linker to exit after loading all
		   objects needed by this shared object and printing a summary
		   which includes the absolute pathnames of all objects, to
		   standard output.  With this flag dlopen() will return to
		   the caller only in the case of error.

     If dlopen() fails, it returns a null pointer, and sets an error condition
     which may be interrogated with dlerror().

     The dlsym() function returns the address binding of the symbol described
     in the null-terminated character string symbol, as it occurs in the
     shared object identified by handle.  The symbols exported by objects
     added to the address space by dlopen() can be accessed only through calls
     to dlsym().  Such symbols do not supersede any definition of those sym-
     bols already present in the address space when the object is loaded, nor
     are they available to satisfy normal dynamic linking references.

     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, the search for
     the symbol follows the algorithm used for resolving undefined symbols
     when objects are loaded.  The objects searched are as follows, in the
     given order:

     1.   The referencing object itself (or the object from which the call to
	  dlsym() is made), if that object was linked using the -Wsymbolic
	  option to ld(1).

     2.   All objects loaded at program start-up.

     3.   All objects loaded via dlopen() with the RTLD_GLOBAL flag set in the
	  mode argument.

     4.   All objects loaded via dlopen() which are in needed-object DAGs that
	  also contain the referencing object.

     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.	RTLD_NEXT is useful for implementing wrappers around library
     functions.  For example, a wrapper function getpid() could access the
     ``real'' getpid() with dlsym(RTLD_NEXT, "getpid").  (Actually, the
     dlfunc() interface, below, should be used, since getpid() is a function
     and not a data object.)

     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.

     The dlsym() function returns a null pointer if the symbol cannot be
     found, and sets an error condition which may be queried with dlerror().

     The dlerror() function returns a null-terminated character string
     describing the last error that occurred during a call to dlopen(),
     dladdr(), dlinfo(), dlsym(), dlfunc(), or dlclose().  If no such error
     has occurred, dlerror() returns a null pointer.  At each call to
     dlerror(), the error indication is reset.	Thus in the case of two calls
     to dlerror(), where the second call follows the first immediately, the
     second call will always return a null pointer.

     The dlclose() function deletes a reference to the shared object refer-
     enced by handle.  If the reference count drops to 0, the object is
     removed from the address space, and handle is rendered invalid.  Just
     before removing a shared object in this way, the dynamic linker calls the
     object's _fini() function, if such a function is defined by the object.
     If dlclose() is successful, it returns a value of 0.  Otherwise it
     returns -1, and sets an error condition that can be interrogated with
     dlerror().

     The object-intrinsic functions _init() and _fini() are called with no
     arguments, and are not expected to return values.

NOTES

     ELF executables need to be linked using the -export-dynamic option to
     ld(1) for symbols defined in the executable to become visible to dlsym().

     In previous implementations, it was necessary to prepend an underscore to
     all external symbols in order to gain symbol compatibility with object
     code compiled from the C language.  This is still the case when using the
     (obsolete) -aout option to the C language compiler.

ERRORS

     The dlopen(), dlsym(), and dlfunc() functions return a null pointer in
     the event of errors.  The dlclose() function returns 0 on success, or -1
     if an error occurred.  Whenever an error has been detected, a message
     detailing it can be retrieved via a call to dlerror().

SEE ALSO

     ld(1), rtld(1), dladdr(3), dlinfo(3), link(5)

FreeBSD 5.4		      September 10, 2002		   FreeBSD 5.4

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