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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.
166 Symbols from the loaded library are put before global symbols when
167 resolving symbolic references originated from the library.
172 fails, it returns a null pointer, and sets an error condition which may
178 function is similar to
180 but it takes the file descriptor argument
182 which is used for the file operations needed to load an object
183 into the address space.
186 is not closed by the function regardless a result of execution,
187 but a duplicate of the file descriptor is.
188 This may be important if a
190 lock is held on the passed descriptor.
193 argument -1 is interpreted as a reference to the main
194 executable of the process, similar to
202 function can be used by the code that needs to perform
203 additional checks on the loaded objects, to prevent races with
204 symlinking or renames.
209 returns the address binding of the symbol described in the null-terminated
212 as it occurs in the shared object identified by
214 The symbols exported by objects added to the address space by
216 can be accessed only through calls to
218 Such symbols do not supersede any definition of those symbols already present
219 in the address space when the object is loaded, nor are they available to
220 satisfy normal dynamic linking references.
224 is called with the special
227 it is interpreted as a reference to the executable or shared object
230 Thus a shared object can reference its own symbols.
234 is called with the special
237 the search for the symbol follows the algorithm used for resolving
238 undefined symbols when objects are loaded.
239 The objects searched are
240 as follows, in the given order:
243 The referencing object itself (or the object from which the call to
245 is made), if that object was linked using the
250 All objects loaded at program start-up.
252 All objects loaded via
260 All objects loaded via
262 which are in needed-object DAGs that also contain the referencing object.
267 is called with the special
270 then the search for the symbol is limited to the shared objects
271 which were loaded after the one issuing the call to
273 Thus, if the function is called from the main program, all
274 the shared libraries are searched.
275 If it is called from a shared library, all subsequent shared
276 libraries are searched.
278 is useful for implementing wrappers around library functions.
279 For example, a wrapper function
285 .Li dlsym(RTLD_NEXT, \&"getpid\&") .
288 interface, below, should be used, since
290 is a function and not a data object.)
294 is called with the special
297 then the search for the symbol is limited to the shared object
300 and those shared objects which were loaded after it.
305 returns a null pointer if the symbol cannot be found, and sets an error
306 condition which may be queried with
311 function behaves like
313 but takes an extra argument
315 a null-terminated character string which is used to request a specific version
322 implements all of the behavior of
324 but has a return type which can be cast to a function pointer without
325 triggering compiler diagnostics.
329 returns a data pointer; in the C standard, conversions between
330 data and function pointer types are undefined.
333 utilities warn about such casts.)
334 The precise return type of
336 is unspecified; applications must cast it to an appropriate function pointer
342 returns a null-terminated character string describing the last error that
343 occurred during a call to
352 If no such error has occurred,
354 returns a null pointer.
357 the error indication is reset.
358 Thus in the case of two calls
361 where the second call follows the first immediately, the second call
362 will always return a null pointer.
367 deletes a reference to the shared object referenced by
369 If the reference count drops to 0, the object is removed from the
373 Just before removing a shared object in this way, the dynamic linker
376 function, if such a function is defined by the object.
379 is successful, it returns a value of 0.
380 Otherwise it returns -1, and sets an error condition that can be
384 The object-intrinsic functions
388 are called with no arguments, and are not expected to return values.
390 ELF executables need to be linked
395 for symbols defined in the executable to become visible to
401 Other ELF platforms require linking with
407 does not require linking with the library, but supports it for compatibility.
409 In previous implementations, it was necessary to prepend an underscore
410 to all external symbols in order to gain symbol
411 compatibility with object code compiled from the C language.
413 still the case when using the (obsolete)
415 option to the C language compiler.
425 return a null pointer in the event of errors.
429 returns 0 on success, or -1 if an error occurred.
430 Whenever an error has been detected, a message detailing it can be
431 retrieved via a call to