xref: /freebsd/lib/libc/gen/dlopen.3 (revision 195ebc7e9e4b129de810833791a19dfb4349d6a9)
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32.\" @(#) dlopen.3 1.6 90/01/31 SMI
33.\" $FreeBSD$
34.\"
35.Dd April 1, 2009
36.Os
37.Dt DLOPEN 3
38.Sh NAME
39.Nm dlopen ,
40.Nm dlsym ,
41.Nm dlfunc ,
42.Nm dlerror ,
43.Nm dlclose
44.Nd programmatic interface to the dynamic linker
45.Sh LIBRARY
46.Lb libc
47.Sh SYNOPSIS
48.In dlfcn.h
49.Ft void *
50.Fn dlopen "const char *path" "int mode"
51.Ft void *
52.Fn dlsym "void * restrict handle" "const char * restrict symbol"
53.Ft dlfunc_t
54.Fn dlfunc "void * restrict handle" "const char * restrict symbol"
55.Ft const char *
56.Fn dlerror "void"
57.Ft int
58.Fn dlclose "void *handle"
59.Sh DESCRIPTION
60These functions provide a simple programmatic interface to the services of the
61dynamic linker.
62Operations are provided to add new shared objects to a
63program's address space, to obtain the address bindings of symbols
64defined by such
65objects, and to remove such objects when their use is no longer required.
66.Pp
67The
68.Fn dlopen
69function
70provides access to the shared object in
71.Fa path ,
72returning a descriptor that can be used for later
73references to the object in calls to
74.Fn dlsym
75and
76.Fn dlclose .
77If
78.Fa path
79was not in the address space prior to the call to
80.Fn dlopen ,
81it is placed in the address space.
82When an object is first loaded into the address space in this way, its
83function
84.Fn _init ,
85if any, is called by the dynamic linker.
86If
87.Fa path
88has already been placed in the address space in a previous call to
89.Fn dlopen ,
90it is not added a second time, although a reference count of
91.Fn dlopen
92operations on
93.Fa path
94is maintained.
95A null pointer supplied for
96.Fa path
97is interpreted as a reference to the main
98executable of the process.
99The
100.Fa mode
101argument
102controls the way in which external function references from the
103loaded object are bound to their referents.
104It must contain one of the following values, possibly ORed with
105additional flags which will be described subsequently:
106.Bl -tag -width RTLD_LAZYX
107.It Dv RTLD_LAZY
108Each external function reference is resolved when the function is first
109called.
110.It Dv RTLD_NOW
111All external function references are bound immediately by
112.Fn dlopen .
113.El
114.Pp
115.Dv RTLD_LAZY
116is normally preferred, for reasons of efficiency.
117However,
118.Dv RTLD_NOW
119is useful to ensure that any undefined symbols are discovered during the
120call to
121.Fn dlopen .
122.Pp
123One of the following flags may be ORed into the
124.Fa mode
125argument:
126.Bl -tag -width RTLD_NODELETE
127.It Dv RTLD_GLOBAL
128Symbols from this shared object and its directed acyclic graph (DAG)
129of needed objects will be available for resolving undefined references
130from all other shared objects.
131.It Dv RTLD_LOCAL
132Symbols in this shared object and its DAG of needed objects will be
133available for resolving undefined references only from other objects
134in the same DAG.
135This is the default, but it may be specified
136explicitly with this flag.
137.It Dv RTLD_TRACE
138When set, causes dynamic linker to exit after loading all objects
139needed by this shared object and printing a summary which includes
140the absolute pathnames of all objects, to standard output.
141With this flag
142.Fn dlopen
143will return to the caller only in the case of error.
144.It Dv RTLD_NODELETE
145Prevents unload of the loaded object on
146.Fn dlclose .
147The same behaviour may be requested by
148.Fl "z nodelete"
149option of the static linker
150.Xr ld 1 .
151.El
152.Pp
153If
154.Fn dlopen
155fails, it returns a null pointer, and sets an error condition which may
156be interrogated with
157.Fn dlerror .
158.Pp
159The
160.Fn dlsym
161function
162returns the address binding of the symbol described in the null-terminated
163character string
164.Fa symbol ,
165as it occurs in the shared object identified by
166.Fa handle .
167The symbols exported by objects added to the address space by
168.Fn dlopen
169can be accessed only through calls to
170.Fn dlsym .
171Such symbols do not supersede any definition of those symbols already present
172in the address space when the object is loaded, nor are they available to
173satisfy normal dynamic linking references.
174.Pp
175If
176.Fn dlsym
177is called with the special
178.Fa handle
179.Dv NULL ,
180it is interpreted as a reference to the executable or shared object
181from which the call
182is being made.
183Thus a shared object can reference its own symbols.
184.Pp
185If
186.Fn dlsym
187is called with the special
188.Fa handle
189.Dv RTLD_DEFAULT ,
190the search for the symbol follows the algorithm used for resolving
191undefined symbols when objects are loaded.
192The objects searched are
193as follows, in the given order:
194.Bl -enum
195.It
196The referencing object itself (or the object from which the call to
197.Fn dlsym
198is made), if that object was linked using the
199.Fl Wsymbolic
200option to
201.Xr ld 1 .
202.It
203All objects loaded at program start-up.
204.It
205All objects loaded via
206.Fn dlopen
207with the
208.Dv RTLD_GLOBAL
209flag set in the
210.Fa mode
211argument.
212.It
213All objects loaded via
214.Fn dlopen
215which are in needed-object DAGs that also contain the referencing object.
216.El
217.Pp
218If
219.Fn dlsym
220is called with the special
221.Fa handle
222.Dv RTLD_NEXT ,
223then the search for the symbol is limited to the shared objects
224which were loaded after the one issuing the call to
225.Fn dlsym .
226Thus, if the function is called from the main program, all
227the shared libraries are searched.
228If it is called from a shared library, all subsequent shared
229libraries are searched.
230.Dv RTLD_NEXT
231is useful for implementing wrappers around library functions.
232For example, a wrapper function
233.Fn getpid
234could access the
235.Dq real
236.Fn getpid
237with
238.Li dlsym(RTLD_NEXT, \&"getpid\&") .
239(Actually, the
240.Fn dlfunc
241interface, below, should be used, since
242.Fn getpid
243is a function and not a data object.)
244.Pp
245If
246.Fn dlsym
247is called with the special
248.Fa handle
249.Dv RTLD_SELF ,
250then the search for the symbol is limited to the shared object
251issuing the call to
252.Fn dlsym
253and those shared objects which were loaded after it.
254.Pp
255The
256.Fn dlsym
257function
258returns a null pointer if the symbol cannot be found, and sets an error
259condition which may be queried with
260.Fn dlerror .
261.Pp
262The
263.Fn dlfunc
264function
265implements all of the behavior of
266.Fn dlsym ,
267but has a return type which can be cast to a function pointer without
268triggering compiler diagnostics.
269(The
270.Fn dlsym
271function
272returns a data pointer; in the C standard, conversions between
273data and function pointer types are undefined.
274Some compilers and
275.Xr lint 1
276utilities warn about such casts.)
277The precise return type of
278.Fn dlfunc
279is unspecified; applications must cast it to an appropriate function pointer
280type.
281.Pp
282The
283.Fn dlerror
284function
285returns a null-terminated character string describing the last error that
286occurred during a call to
287.Fn dlopen ,
288.Fn dladdr ,
289.Fn dlinfo ,
290.Fn dlsym ,
291.Fn dlfunc ,
292or
293.Fn dlclose .
294If no such error has occurred,
295.Fn dlerror
296returns a null pointer.
297At each call to
298.Fn dlerror ,
299the error indication is reset.
300Thus in the case of two calls
301to
302.Fn dlerror ,
303where the second call follows the first immediately, the second call
304will always return a null pointer.
305.Pp
306The
307.Fn dlclose
308function
309deletes a reference to the shared object referenced by
310.Fa handle .
311If the reference count drops to 0, the object is removed from the
312address space, and
313.Fa handle
314is rendered invalid.
315Just before removing a shared object in this way, the dynamic linker
316calls the object's
317.Fn _fini
318function, if such a function is defined by the object.
319If
320.Fn dlclose
321is successful, it returns a value of 0.
322Otherwise it returns -1, and sets an error condition that can be
323interrogated with
324.Fn dlerror .
325.Pp
326The object-intrinsic functions
327.Fn _init
328and
329.Fn _fini
330are called with no arguments, and are not expected to return values.
331.Sh NOTES
332ELF executables need to be linked
333using the
334.Fl export-dynamic
335option to
336.Xr ld 1
337for symbols defined in the executable to become visible to
338.Fn dlsym .
339.Pp
340In previous implementations, it was necessary to prepend an underscore
341to all external symbols in order to gain symbol
342compatibility with object code compiled from the C language.
343This is
344still the case when using the (obsolete)
345.Fl aout
346option to the C language compiler.
347.Sh ERRORS
348The
349.Fn dlopen ,
350.Fn dlsym ,
351and
352.Fn dlfunc
353functions
354return a null pointer in the event of errors.
355The
356.Fn dlclose
357function
358returns 0 on success, or -1 if an error occurred.
359Whenever an error has been detected, a message detailing it can be
360retrieved via a call to
361.Fn dlerror .
362.Sh SEE ALSO
363.Xr ld 1 ,
364.Xr rtld 1 ,
365.Xr dladdr 3 ,
366.Xr dlinfo 3 ,
367.Xr link 5
368