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32 .\" @(#) dlopen.3 1.6 90/01/31 SMI
44 .Nd programmatic interface to the dynamic linker
50 .Fn dlopen "const char *path" "int mode"
52 .Fn dlsym "void * restrict handle" "const char * restrict symbol"
54 .Fn dlfunc "void * restrict handle" "const char * restrict symbol"
58 .Fn dlclose "void *handle"
60 These functions provide a simple programmatic interface to the services of the
62 Operations are provided to add new shared objects to a
63 program's address space, to obtain the address bindings of symbols
65 objects, and to remove such objects when their use is no longer required.
70 provides access to the shared object in
72 returning a descriptor that can be used for later
73 references to the object in calls to
79 was not in the address space prior to the call to
81 it is placed in the address space.
82 When an object is first loaded into the address space in this way, its
85 if any, is called by the dynamic linker.
88 has already been placed in the address space in a previous call to
90 it is not added a second time, although a reference count of
95 A null pointer supplied for
97 is interpreted as a reference to the main
98 executable of the process.
102 controls the way in which external function references from the
103 loaded object are bound to their referents.
104 It must contain one of the following values, possibly ORed with
105 additional flags which will be described subsequently:
106 .Bl -tag -width RTLD_LAZYX
108 Each external function reference is resolved when the function is first
111 All external function references are bound immediately by
116 is normally preferred, for reasons of efficiency.
119 is useful to ensure that any undefined symbols are discovered during the
123 One of the following flags may be ORed into the
126 .Bl -tag -width RTLD_NODELETE
128 Symbols from this shared object and its directed acyclic graph (DAG)
129 of needed objects will be available for resolving undefined references
130 from all other shared objects.
132 Symbols in this shared object and its DAG of needed objects will be
133 available for resolving undefined references only from other objects
135 This is the default, but it may be specified
136 explicitly with this flag.
138 When set, causes dynamic linker to exit after loading all objects
139 needed by this shared object and printing a summary which includes
140 the absolute pathnames of all objects, to standard output.
143 will return to the caller only in the case of error.
145 Prevents unload of the loaded object on
147 The same behaviour may be requested by
149 option of the static linker
152 Only return valid handle for the object if it is already loaded in
153 the process address space, otherwise
156 Other mode flags may be specified, which will be applied for promotion
157 for the found object.
162 fails, it returns a null pointer, and sets an error condition which may
169 returns the address binding of the symbol described in the null-terminated
172 as it occurs in the shared object identified by
174 The symbols exported by objects added to the address space by
176 can be accessed only through calls to
178 Such symbols do not supersede any definition of those symbols already present
179 in the address space when the object is loaded, nor are they available to
180 satisfy normal dynamic linking references.
184 is called with the special
187 it is interpreted as a reference to the executable or shared object
190 Thus a shared object can reference its own symbols.
194 is called with the special
197 the search for the symbol follows the algorithm used for resolving
198 undefined symbols when objects are loaded.
199 The objects searched are
200 as follows, in the given order:
203 The referencing object itself (or the object from which the call to
205 is made), if that object was linked using the
210 All objects loaded at program start-up.
212 All objects loaded via
220 All objects loaded via
222 which are in needed-object DAGs that also contain the referencing object.
227 is called with the special
230 then the search for the symbol is limited to the shared objects
231 which were loaded after the one issuing the call to
233 Thus, if the function is called from the main program, all
234 the shared libraries are searched.
235 If it is called from a shared library, all subsequent shared
236 libraries are searched.
238 is useful for implementing wrappers around library functions.
239 For example, a wrapper function
245 .Li dlsym(RTLD_NEXT, \&"getpid\&") .
248 interface, below, should be used, since
250 is a function and not a data object.)
254 is called with the special
257 then the search for the symbol is limited to the shared object
260 and those shared objects which were loaded after it.
265 returns a null pointer if the symbol cannot be found, and sets an error
266 condition which may be queried with
272 implements all of the behavior of
274 but has a return type which can be cast to a function pointer without
275 triggering compiler diagnostics.
279 returns a data pointer; in the C standard, conversions between
280 data and function pointer types are undefined.
283 utilities warn about such casts.)
284 The precise return type of
286 is unspecified; applications must cast it to an appropriate function pointer
292 returns a null-terminated character string describing the last error that
293 occurred during a call to
301 If no such error has occurred,
303 returns a null pointer.
306 the error indication is reset.
307 Thus in the case of two calls
310 where the second call follows the first immediately, the second call
311 will always return a null pointer.
316 deletes a reference to the shared object referenced by
318 If the reference count drops to 0, the object is removed from the
322 Just before removing a shared object in this way, the dynamic linker
325 function, if such a function is defined by the object.
328 is successful, it returns a value of 0.
329 Otherwise it returns -1, and sets an error condition that can be
333 The object-intrinsic functions
337 are called with no arguments, and are not expected to return values.
339 ELF executables need to be linked
344 for symbols defined in the executable to become visible to
347 In previous implementations, it was necessary to prepend an underscore
348 to all external symbols in order to gain symbol
349 compatibility with object code compiled from the C language.
351 still the case when using the (obsolete)
353 option to the C language compiler.
361 return a null pointer in the event of errors.
365 returns 0 on success, or -1 if an error occurred.
366 Whenever an error has been detected, a message detailing it can be
367 retrieved via a call to