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32 .\" @(#) dlopen.3 1.6 90/01/31 SMI
46 .Nd programmatic interface to the dynamic linker
52 .Fn dlopen "const char *path" "int mode"
54 .Fn fdlopen "int fd" "int mode"
56 .Fn dlsym "void * restrict handle" "const char * restrict symbol"
58 .Fn dlvsym "void * restrict handle" "const char * restrict symbol" "const char * restrict version"
60 .Fn dlfunc "void * restrict handle" "const char * restrict symbol"
64 .Fn dlclose "void *handle"
66 These functions provide a simple programmatic interface to the services of the
68 Operations are provided to add new shared objects to a
69 program's address space, to obtain the address bindings of symbols
71 objects, and to remove such objects when their use is no longer required.
76 provides access to the shared object in
78 returning a descriptor that can be used for later
79 references to the object in calls to
86 was not in the address space prior to the call to
88 it is placed in the address space.
89 When an object is first loaded into the address space in this way, its
92 if any, is called by the dynamic linker.
95 has already been placed in the address space in a previous call to
97 it is not added a second time, although a reference count of
102 A null pointer supplied for
104 is interpreted as a reference to the main
105 executable of the process.
109 controls the way in which external function references from the
110 loaded object are bound to their referents.
111 It must contain one of the following values, possibly ORed with
112 additional flags which will be described subsequently:
113 .Bl -tag -width RTLD_LAZYX
115 Each external function reference is resolved when the function is first
118 All external function references are bound immediately by
123 is normally preferred, for reasons of efficiency.
126 is useful to ensure that any undefined symbols are discovered during the
130 One of the following flags may be ORed into the
133 .Bl -tag -width RTLD_NODELETE
135 Symbols from this shared object and its directed acyclic graph (DAG)
136 of needed objects will be available for resolving undefined references
137 from all other shared objects.
139 Symbols in this shared object and its DAG of needed objects will be
140 available for resolving undefined references only from other objects
142 This is the default, but it may be specified
143 explicitly with this flag.
145 When set, causes dynamic linker to exit after loading all objects
146 needed by this shared object and printing a summary which includes
147 the absolute pathnames of all objects, to standard output.
150 will return to the caller only in the case of error.
152 Prevents unload of the loaded object on
154 The same behaviour may be requested by
156 option of the static linker
159 Only return valid handle for the object if it is already loaded in
160 the process address space, otherwise
163 Other mode flags may be specified, which will be applied for promotion
164 for the found object.
169 fails, it returns a null pointer, and sets an error condition which may
175 function is similar to
177 but it takes the file descriptor argument
179 which is used for the file operations needed to load an object
180 into the address space.
183 is not closed by the function regardless a result of execution,
184 but a duplicate of the file descriptor is.
185 This may be important if a
187 lock is held on the passed descriptor.
190 argument -1 is interpreted as a reference to the main
191 executable of the process, similar to
199 function can be used by the code that needs to perform
200 additional checks on the loaded objects, to prevent races with
201 symlinking or renames.
206 returns the address binding of the symbol described in the null-terminated
209 as it occurs in the shared object identified by
211 The symbols exported by objects added to the address space by
213 can be accessed only through calls to
215 Such symbols do not supersede any definition of those symbols already present
216 in the address space when the object is loaded, nor are they available to
217 satisfy normal dynamic linking references.
221 is called with the special
224 it is interpreted as a reference to the executable or shared object
227 Thus a shared object can reference its own symbols.
231 is called with the special
234 the search for the symbol follows the algorithm used for resolving
235 undefined symbols when objects are loaded.
236 The objects searched are
237 as follows, in the given order:
240 The referencing object itself (or the object from which the call to
242 is made), if that object was linked using the
247 All objects loaded at program start-up.
249 All objects loaded via
257 All objects loaded via
259 which are in needed-object DAGs that also contain the referencing object.
264 is called with the special
267 then the search for the symbol is limited to the shared objects
268 which were loaded after the one issuing the call to
270 Thus, if the function is called from the main program, all
271 the shared libraries are searched.
272 If it is called from a shared library, all subsequent shared
273 libraries are searched.
275 is useful for implementing wrappers around library functions.
276 For example, a wrapper function
282 .Li dlsym(RTLD_NEXT, \&"getpid\&") .
285 interface, below, should be used, since
287 is a function and not a data object.)
291 is called with the special
294 then the search for the symbol is limited to the shared object
297 and those shared objects which were loaded after it.
302 returns a null pointer if the symbol cannot be found, and sets an error
303 condition which may be queried with
308 function behaves like
310 but takes an extra argument
312 a null-terminated character string which is used to request a specific version
319 implements all of the behavior of
321 but has a return type which can be cast to a function pointer without
322 triggering compiler diagnostics.
326 returns a data pointer; in the C standard, conversions between
327 data and function pointer types are undefined.
330 utilities warn about such casts.)
331 The precise return type of
333 is unspecified; applications must cast it to an appropriate function pointer
339 returns a null-terminated character string describing the last error that
340 occurred during a call to
349 If no such error has occurred,
351 returns a null pointer.
354 the error indication is reset.
355 Thus in the case of two calls
358 where the second call follows the first immediately, the second call
359 will always return a null pointer.
364 deletes a reference to the shared object referenced by
366 If the reference count drops to 0, the object is removed from the
370 Just before removing a shared object in this way, the dynamic linker
373 function, if such a function is defined by the object.
376 is successful, it returns a value of 0.
377 Otherwise it returns -1, and sets an error condition that can be
381 The object-intrinsic functions
385 are called with no arguments, and are not expected to return values.
387 ELF executables need to be linked
392 for symbols defined in the executable to become visible to
398 Other ELF platforms require linking with
404 does not require linking with the library, but supports it for compatibility.
406 In previous implementations, it was necessary to prepend an underscore
407 to all external symbols in order to gain symbol
408 compatibility with object code compiled from the C language.
410 still the case when using the (obsolete)
412 option to the C language compiler.
422 return a null pointer in the event of errors.
426 returns 0 on success, or -1 if an error occurred.
427 Whenever an error has been detected, a message detailing it can be
428 retrieved via a call to