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32 .\" @(#) dlopen.3 1.6 90/01/31 SMI
33 .\" $FreeBSD: src/lib/libc/gen/dlopen.3,v 1.8.2.10 2003/03/15 15:11:05 trhodes Exp $
35 .Dd September 24, 1989
43 .Nd programmatic interface to the dynamic linker
49 .Fn dlopen "const char *path" "int mode"
51 .Fn dlsym "void *handle" "const char *symbol"
55 .Fn dlclose "void *handle"
57 These functions provide a simple programmatic interface to the services of the
59 Operations are provided to add new shared objects to a
60 program's address space, to obtain the address bindings of symbols
62 objects, and to remove such objects when their use is no longer required.
67 provides access to the shared object in
69 returning a descriptor that can be used for later
70 references to the object in calls to
76 was not in the address space prior to the call to
78 it is placed in the address space.
79 When an object is first loaded into the address space in this way, its
82 if any, is called by the dynamic linker.
85 has already been placed in the address space in a previous call to
87 it is not added a second time, although a reference count of
92 A null pointer supplied for
94 is interpreted as a reference to the main
95 executable of the process.
99 controls the way in which external function references from the
100 loaded object are bound to their referents.
101 It must contain one of the following values, possibly ORed with
102 additional flags which will be described subsequently:
103 .Bl -tag -width RTLD_LAZYX
105 Each external function reference is resolved when the function is first
108 All external function references are bound immediately by
113 is normally preferred, for reasons of efficiency.
116 is useful to ensure that any undefined symbols are discovered during the
120 One of the following flags may be ORed into the
123 .Bl -tag -width RTLD_GLOBALX
125 Symbols from this shared object and its directed acyclic graph (DAG)
126 of needed objects will be available for resolving undefined references
127 from all other shared objects.
129 Symbols in this shared object and its DAG of needed objects will be
130 available for resolving undefined references only from other objects
132 This is the default, but it may be specified
133 explicitly with this flag.
135 When set, causes dynamic linker to exit after loading all objects
136 needed by this shared object and printing a summary which includes
137 the absolute pathnames of all objects, to standard output.
140 will return to the caller only in the case of error.
145 fails, it returns a null pointer, and sets an error condition which may
152 returns the address binding of the symbol described in the null-terminated
155 as it occurs in the shared object identified by
157 The symbols exported by objects added to the address space by
159 can be accessed only through calls to
161 Such symbols do not supersede any definition of those symbols already present
162 in the address space when the object is loaded, nor are they available to
163 satisfy normal dynamic linking references.
167 is called with the special
170 it is interpreted as a reference to the executable or shared object
173 Thus a shared object can reference its own symbols.
177 is called with the special
180 the search for the symbol follows the algorithm used for resolving
181 undefined symbols when objects are loaded.
182 The objects searched are
183 as follows, in the given order:
186 The referencing object itself (or the object from which the call to
188 is made), if that object was linked using the
193 All objects loaded at program start-up.
195 All objects loaded via
197 which are in needed-object DAGs that also contain the referencing object.
199 All objects loaded via
210 is called with the special
213 then the search for the symbol is limited to the shared objects
214 which were loaded after the one issuing the call to
216 Thus, if the function is called from the main program, all
217 the shared libraries are searched.
218 If it is called from a shared library, all subsequent shared
219 libraries are searched.
221 is useful for implementing wrappers around library functions.
222 For example, a wrapper function
228 .Li dlsym(RTLD_NEXT, \&"getpid\&") .
232 is called with the special
235 then the search for the symbol is limited to the shared object
238 and those shared objects which were loaded after it.
243 returns a null pointer if the symbol cannot be found, and sets an error
244 condition which may be queried with
250 returns a null-terminated character string describing the last error that
251 occurred during a call to
258 If no such error has occurred,
260 returns a null pointer.
263 the error indication is reset.
264 Thus in the case of two calls
267 where the second call follows the first immediately, the second call
268 will always return a null pointer.
273 deletes a reference to the shared object referenced by
275 If the reference count drops to 0, the object is removed from the
279 Just before removing a shared object in this way, the dynamic linker
282 function, if such a function is defined by the object.
285 is successful, it returns a value of 0.
286 Otherwise it returns -1, and sets an error condition that can be
290 The object-intrinsic functions
294 are called with no arguments, and are not expected to return values.
296 ELF executables need to be linked
301 for symbols defined in the executable to become visible to
304 In previous implementations, it was necessary to prepend an underscore
305 to all external symbols in order to gain symbol
306 compatibility with object code compiled from the C language.
308 still the case when using the (obsolete)
310 option to the C language compiler.
317 return a null pointer in the event of errors.
321 returns 0 on success, or -1 if an error occurred.
322 Whenever an error has been detected, a message detailing it can be
323 retrieved via a call to