dlopen(3X)                     DG/UX R4.11MU05                    dlopen(3X)


NAME
       dlopen - open a shared object

SYNOPSIS
       cc [flag ...] file ...  -ldl [library ...]

       #include <dlfcn.h>

       void *dlopen(char *pathname, int mode);

DESCRIPTION
       dlopen is one of a family of routines that give the user direct
       access to the dynamic linking facilities.  These routines are
       available in a library which is loaded if the option -ldl is used
       with cc or ld.

       dlopen makes a shared object available to a running process.  dlopen
       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.  pathname is the path name of
       the object to be opened; it may be an absolute path or relative to
       the current directory.  If the value of pathname is 0, dlopen will
       make the symbols contained in the original a.out, and all of the
       objects that were loaded at program startup with the a.out, available
       through dlsym.

       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:

       RTLDLAZY
              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.

       RTLDNOW
              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.

SEE ALSO
       cc(1), ld(1), sh(1), exec(2), dlclose(3X), dlerror(3X), dlsym(3X).

DIAGNOSTICS
       If pathname cannot be found, cannot be opened for reading, is not a
       shared object, or if an error occurs during the process of loading
       pathname or relocating its symbolic references, dlopen will return
       NULL.  More detailed diagnostic information will be available through
       dlerror.

NOTES
       If other shared objects were link edited with pathname when pathname
       was built, those objects will automatically be loaded by dlopen.  The
       directory search path that will be used to find both pathname and the
       other needed objects may be specified by setting the environment
       variable LDLIBRARYPATH.  This environment variable should contain a
       colon-separated list of directories, in the same format as the PATH
       variable [see sh(1)].  LDLIBRARYPATH will be ignored if the process
       is running setuid or setgid [see exec(2)] or if the name specified is
       not a simple file name (i.e.  contains a / character).  Objects whose
       names resolve to the same absolute or relative path name may be
       opened any number of times using dlopen, however, the object
       referenced will only be loaded once into the address space of the
       current process.  The same object referenced by two different path
       names, however, may be loaded multiple times.  For example, given the
       object /usr/home/me/mylibs/mylib.so, and assuming the current working
       directory is /usr/home/me/workdir,

            ...
            void *handle1;
            void *handle2;

            handle1 = dlopen("../mylibs/mylib.so", RTLDLAZY);
            handle2 = dlopen("/usr/home/me/mylibs/mylib.so", RTLDLAZY);
            ...

       will result in mylibs.so being loaded twice for the current process.
       On the other hand, given the same object and current working
       directory, if LDLIBRARYPATH=/usr/home/me/mylibs, then

            ...
            void *handle1;
            void *handle2;

            handle1 = dlopen("mylib.so", RTLDLAZY);
            handle2 = dlopen("/usr/home/me/mylibs/mylib.so", RTLDLAZY);
            ...

       will result in mylibs.so being loaded only once.

       Objects loaded by a single invocation of dlopen may import symbols
       from one another or from any object loaded automatically during
       program startup, but objects loaded by one dlopen invocation may not
       directly reference symbols from objects loaded by a different dlopen
       invocation.  Those symbols may, however, be referenced indirectly
       using dlsym.

       Users who wish to gain access to the symbol table of the a.out itself
       using dlsym(0, mode) should be aware that some symbols defined in the
       a.out may not be available to the dynamic linker.  The symbol table
       created by ld for use by the dynamic linker might contain only a
       subset of the symbols defined in the a.out: specifically those
       referenced by the shared objects with which the a.out is linked.


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