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32 .\" @(#) dlopen.3 1.6 90/01/31 SMI
35 .Dd September 10, 2002
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_GLOBALX
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.
148 fails, it returns a null pointer, and sets an error condition which may
155 returns the address binding of the symbol described in the null-terminated
158 as it occurs in the shared object identified by
160 The symbols exported by objects added to the address space by
162 can be accessed only through calls to
164 Such symbols do not supersede any definition of those symbols already present
165 in the address space when the object is loaded, nor are they available to
166 satisfy normal dynamic linking references.
170 is called with the special
173 it is interpreted as a reference to the executable or shared object
176 Thus a shared object can reference its own symbols.
180 is called with the special
183 the search for the symbol follows the algorithm used for resolving
184 undefined symbols when objects are loaded.
185 The objects searched are
186 as follows, in the given order:
189 The referencing object itself (or the object from which the call to
191 is made), if that object was linked using the
196 All objects loaded at program start-up.
198 All objects loaded via
206 All objects loaded via
208 which are in needed-object DAGs that also contain the referencing object.
213 is called with the special
216 then the search for the symbol is limited to the shared objects
217 which were loaded after the one issuing the call to
219 Thus, if the function is called from the main program, all
220 the shared libraries are searched.
221 If it is called from a shared library, all subsequent shared
222 libraries are searched.
224 is useful for implementing wrappers around library functions.
225 For example, a wrapper function
231 .Li dlsym(RTLD_NEXT, \&"getpid\&") .
234 interface, below, should be used, since
236 is a function and not a data object.)
240 is called with the special
243 then the search for the symbol is limited to the shared object
246 and those shared objects which were loaded after it.
251 returns a null pointer if the symbol cannot be found, and sets an error
252 condition which may be queried with
258 implements all of the behavior of
260 but has a return type which can be cast to a function pointer without
261 triggering compiler diagnostics.
265 returns a data pointer; in the C standard, conversions between
266 data and function pointer types are undefined.
269 utilities warn about such casts.)
270 The precise return type of
272 is unspecified; applications must cast it to an appropriate function pointer
278 returns a null-terminated character string describing the last error that
279 occurred during a call to
287 If no such error has occurred,
289 returns a null pointer.
292 the error indication is reset.
293 Thus in the case of two calls
296 where the second call follows the first immediately, the second call
297 will always return a null pointer.
302 deletes a reference to the shared object referenced by
304 If the reference count drops to 0, the object is removed from the
308 Just before removing a shared object in this way, the dynamic linker
311 function, if such a function is defined by the object.
314 is successful, it returns a value of 0.
315 Otherwise it returns -1, and sets an error condition that can be
319 The object-intrinsic functions
323 are called with no arguments, and are not expected to return values.
325 ELF executables need to be linked
330 for symbols defined in the executable to become visible to
333 In previous implementations, it was necessary to prepend an underscore
334 to all external symbols in order to gain symbol
335 compatibility with object code compiled from the C language.
337 still the case when using the (obsolete)
339 option to the C language compiler.
347 return a null pointer in the event of errors.
351 returns 0 on success, or -1 if an error occurred.
352 Whenever an error has been detected, a message detailing it can be
353 retrieved via a call to