Updated: 2021/Apr/14


DLFCN(3)                   Library Functions Manual                   DLFCN(3)

NAME
     dlopen, dlclose, dlsym, dlvsym, dladdr, dlctl, dlerror - dynamic link
     interface

LIBRARY
     (These functions are not in a library.  They are included in every
     dynamically linked program automatically.)

SYNOPSIS
     #include <dlfcn.h>

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

     int
     dlclose(void *handle);

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

     void *
     dlvsym(void * restrict handle, const char * restrict symbol,
         const char *version);

     int
     dladdr(void * restrict addr, Dl_info * restrict dli);

     int
     dlctl(void *handle, int cmd, void *data);

     char *
     dlerror(void);

DESCRIPTION
     These functions provide an interface to the run-time linker ld.so(1).
     They allow new shared objects to be loaded into the process' address
     space under program control.

     The dlopen() function takes the name of a shared object as the first
     argument.  The path argument can be specified as either an absolute
     pathname to a shared object or just the name of the shared object itself.
     When an absolute pathname is specified, only the path provided will be
     searched.  When just a shared object name is specified, the same search
     rules apply that are used for "intrinsic" shared object searches.  (see
     ld.elf_so(1))

     Shared libraries take the following form: "lib<name>.so[.xx[.yy]]".

     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.

     If the first argument is NULL, dlopen() returns a handle on the global
     symbol object.  This object provides access to all symbols from an
     ordered set of objects consisting of the original program image and any
     dependencies loaded during startup.

     The mode parameter specifies symbol resolution time and symbol
     visibility.  One of the following values may be used to specify symbol
     resolution time:

           RTLD_NOW           Symbols are resolved immediately.

           RTLD_LAZY          Symbols are resolved when they are first
                              referred to.  This is the default value if
                              resolution time is unspecified.

     One of the following values may be used to specify symbol visibility:

           RTLD_GLOBAL        The object's symbols and the symbols of its
                              dependencies will be visible to other objects.

           RTLD_LOCAL         The object's symbols and the symbols of its
                              dependencies will not be visible to other
                              objects.  This is the default value if
                              visibility is unspecified.

     To specify both resolution time and visibility, bitwise inclusive OR one
     of each of the above values together.  If an object was opened with
     RTLD_LOCAL and later opened with RTLD_GLOBAL, then it is promoted to
     RTLD_GLOBAL.

     Additionally, one of the following flags may be ORed into the mode
     argument:

           RTLD_NODELETE      Prevents unload of the loaded object on
                              dlclose().  The same behaviour may be requested
                              by -z nodelete option of the static linker
                              ld(1).

           RTLD_NOLOAD        Only return valid handle for the object if it is
                              already loaded in the process address space,
                              otherwise do not load the object and return
                              NULL.

     dlopen() returns a handle to be used in calls to dlclose(), dlsym(),
     dlvsym(), 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 one loaded at startup time, or the object
     is a dependency of another object then the object is removed when its
     reference count drops to zero.  dlclose() returns 0 on success and non-
     zero on failure.

     dlsym() looks for a definition of symbol in the shared object designated
     by handle, and all shared objects that are listed as dependencies.  The
     symbol's address is returned.  If the symbol cannot be resolved, NULL is
     returned.

     dlsym() may also be called with special handle values.  dlsym() respects
     symbol visibility as specified by the dlopen() mode parameter.  However,
     the symbols of an object's dependencies are always visible to it.  All
     shared objects loaded at program startup are globally visible.  Only the
     symbols in the main executable that are referenced by a shared object at
     link time will be visible unless it has been linked with the
     --export-dynamic option where all of its symbols will be visible.  The
     following special handle values may be used with dlsym():

           NULL              Interpreted as a reference to the executable or
                             shared object from which the call is being made.
                             Thus an object can reference its own symbols and
                             the symbols of its dependencies without calling
                             dlopen().

           RTLD_DEFAULT      All the visible shared objects and the executable
                             will be searched in the order they were loaded.

           RTLD_NEXT         The search for symbol is limited to the visible
                             shared objects which were loaded after the one
                             issuing the call to dlsym().  Thus, if dlsym() is
                             called from the main program, all the visible
                             shared libraries are searched.  If it is called
                             from a shared library, all subsequently visible
                             shared libraries are searched.

           RTLD_SELF         The search for symbol is limited to the shared
                             object issuing the call to dlsym() and those
                             shared objects which were loaded after it that
                             are visible.

     dlvsym() does the same as dlsym() but takes a version string as an
     additional argument.  Both the symbol and the version must match in order
     for the symbol to be resolved.

     dladdr() examines all currently mapped shared objects for a symbol whose
     address -- as mapped in the process address space -- is closest to but
     not exceeding the value passed in the first argument addr.  The symbols
     of a shared object are only eligible if addr is between the base address
     of the shared object and the value of the symbol _end in the same shared
     object.  If no object for which this condition holds true can be found,
     dladdr() will return 0.  Otherwise, a non-zero value is returned and the
     dli argument will be used to provide information on the selected symbol
     and the shared object it is contained in.  The dli argument points at a
     caller-provided Dl_info structure defined as follows:

           typedef struct {
                   const char  *dli_fname;     /* File defining the symbol */
                   void        *dli_fbase;     /* Base address */
                   const char  *dli_sname;     /* Symbol name */
                   const void  *dli_saddr;     /* Symbol address */
           } Dl_info;

     The structure members are further described as follows:

     dli_fname     The pathname of the shared object containing the address
                   addr.

     dli_fbase     The base address at which this shared object is loaded in
                   the process address space.  This may be zero if the symbol
                   was found in the internally generated "copy" section (see
                   link(5)) which is not associated with a file.

     dli_sname     points at the nul-terminated name of the selected symbol

     dli_saddr     is the actual address (as it appears in the process address
                   space) of the symbol.

     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
     aspects 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
     invocation of dlerror(), dlerror() returns NULL.  Thus, invoking
     dlerror() a second time, immediately following a prior invocation, will
     result in NULL being returned.

ERRORS
     The error "Cannot dlopen non-loadable /usr/lib/libpthread.so.1" is
     generated when a program dlopen()s a module that needs libpthread but
     isn't linked against it itself.

SEE ALSO
     ld(1), rtld(1), dlinfo(3), link(5)

HISTORY
     Some of the dl* functions first appeared in SunOS 4.

NetBSD 9.99                    January 13, 2020                    NetBSD 9.99