HP-UX Reference (11i v2 03/08) - 3 Library Functions A-M (vol 6)

d
dlopen(3C) dlopen(3C)
NAME
dlopen - open a shared library
SYNOPSIS
cc [flag ... ] cfile ...
-ldl [library ] ...
#include <dlfcn.h>
void *dlopen(const char *file, int mode);
DESCRIPTION
dlopen is one of a family of routines that give the user direct access to the dynamic linking facilities
(using the -ldl option on the compiler or
ld command line). dlopen makes a shared object specified
by a file available to a running process. A shared object may specify other objects that it ‘‘needs’’ in order
to execute properly. These dependencies are specified by
DT_NEEDED entries int the .dynamic section
of the original object. Each needed object may, in turn, specify other needed objects. All such objects are
loaded along with the original object as a result of the call to
dlopen.
A successful
dlopen call returns to the process a handle which the process may use on subsequent calls
to
dlsym and dlclose. This value should not be interpreted in any way by the process.
file is used to construct a pathname to the object file. If file contains a slash character, the file argument
itself is used as the pathname. Otherwise a series of directories is searched for file. First, any directories
specified by the environment variable
LD_LIBRARY_PATH
are searched. Next, any directories specified
by
SHLIB_PATH are searched. Then, any directories specified by a DT_RPATH entry in the .dynamic
section of the original program object are searched. Finally, the directories /usr/lib/hpux32
in 32-
bit mode and
/usr/lib/hpux64
in 64-bit mode are searched.
If the value of file is 0,
dlopen provides a handle on a ‘‘global symbol object.’’ This object provides access
to the symbols from an ordered set of objects consisting of the original a.out, all of the objects that were
loaded at program startup along with the a.out, and all objects loaded using a dlopen operation along
with the
RTLD_GLOBAL flag. As the latter set of objects can change during execution, the set identified
by handle can also change dynamically.
Only a single copy of an object file is brought into the address space, even if
dlopen is invoked multiple
times in reference to the file, and even if different pathnames are used to reference the file.
When a shared object is brought into the address space of a process, it may contain references to symbols
whose addresses are not known until the object is loaded. These references must be relocated before the
symbols can be accessed. The mode parameter governs when these relocations take place and may have
the following values:
RTLD_LAZY Under this mode, only references to data symbols are relocated when the object is loaded.
References to functions are not relocated until a given function is invoked for the first
time. This mode should result in better performance, since a process may not reference
all of the functions in any given shared object.
RTLD_NOW Under this mode, all necessary relocations are performed when the object is first loaded.
This may result in some wasted effort, if relocations are performed for functions that are
never referenced, but is useful for applications that need to know as soon as an object is
loaded that all symbols referenced during execution will be available.
Any object loaded by
dlopen that requires relocations against global symbols can reference the symbols
in the original a.out, any objects loaded at program startup, from the object itself as well as any other
object included in the same dlopen invocation, and any objects that were loaded in any dlopen invoca-
tion that specified the RTLD_GLOBAL flag. To determine the scope of visibility for the symbols loaded
with a dlopen invocation, the mode parameter should be bitwise or’ed with one of the following values:
RTLD_GLOBAL
The object’s symbols are made available for the relocation processing of any other object.
In addition, symbol lookup using dlopen(0, mode) and an associated dlsym() allows
objects loaded with RTLD_GLOBAL to be searched.
RTLD_LOCAL The object’s symbols are made available for relocation processing only to objects loaded in
the same dlopen invocation.
If neither
RTLD_GLOBAL nor RTLD_LOCAL are specified, the default is RTLD_LOCAL.
If a file is specified in multiple
dlopen invocations, mode is interpreted at each invocation. Note, how-
ever, that once RTLD_NOW has been specified, all relocations will have been completed, rendering any
HP-UX 11i Version 2: August 2003 − 1 − Hewlett-Packard Company Section 3−−203