1DLOPEN(3P) POSIX Programmer's Manual DLOPEN(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 dlopen — open a symbol table handle
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15 #include <dlfcn.h>
16 void *dlopen(const char *file, int mode);
18
20 The dlopen() function shall make the symbols (function identifiers and
21 data object identifiers) in the executable object file specified by
22 file available to the calling program.
23 The class of executable object files eligible for this operation and
25 the manner of their construction are implementation-defined, though
26 typically such files are shared libraries or programs.
27 Implementations may permit the construction of embedded dependencies in
29 executable object files. In such cases, a dlopen() operation shall load
30 those dependencies in addition to the executable object file specified
31 by file. Implementations may also impose specific constraints on the
32 construction of programs that can employ dlopen() and its related ser‐
33 vices.
34 A successful dlopen() shall return a symbol table handle which the
36 caller may use on subsequent calls to dlsym() and dlclose().
37 The value of this symbol table handle should not be interpreted in any
39 way by the caller.
40 The file argument is used to construct a pathname to the executable
42 object file. If file contains a <slash> character, the file argument is
43 used as the pathname for the file. Otherwise, file is used in an imple‐
44 mentation-defined manner to yield a pathname.
45 If file is a null pointer, dlopen() shall return a global symbol table
47 handle for the currently running process image. This symbol table han‐
48 dle shall provide access to the symbols from an ordered set of exe‐
49 cutable object files consisting of the original program image file, any
50 executable object files loaded at program start-up as specified by that
51 process file (for example, shared libraries), and the set of executable
52 object files loaded using dlopen() operations with the RTLD_GLOBAL
53 flag. As the latter set of executable object files can change during
54 execution, the set of symbols made available by this symbol table han‐
55 dle can also change dynamically.
56 Only a single copy of an executable object file shall be brought into
58 the address space, even if dlopen() is invoked multiple times in refer‐
59 ence to the executable object file, and even if different pathnames are
60 used to reference the executable object file.
61 The mode parameter describes how dlopen() shall operate upon file with
63 respect to the processing of relocations and the scope of visibility of
64 the symbols provided within file. When an executable object file is
65 brought into the address space of a process, it may contain references
66 to symbols whose addresses are not known until the executable object
67 file is loaded.
68 These references shall be relocated before the symbols can be accessed.
70 The mode parameter governs when these relocations take place and may
71 have the following values:
72 RTLD_LAZY Relocations shall be performed at an implementation-defined
74 time, ranging from the time of the dlopen() call until the
75 first reference to a given symbol occurs. Specifying
76 RTLD_LAZY should improve performance on implementations
77 supporting dynamic symbol binding since a process might not
78 reference all of the symbols in an executable object file.
79 And, for systems supporting dynamic symbol resolution for
80 normal process execution, this behavior mimics the normal
81 handling of process execution.
82 RTLD_NOW All necessary relocations shall be performed when the exe‐
84 cutable object file is first loaded. This may waste some
85 processing if relocations are performed for symbols that
86 are never referenced. This behavior may be useful for
87 applications that need to know that all symbols referenced
88 during execution will be available before dlopen() returns.
89 Any executable object file loaded by dlopen() that requires relocations
91 against global symbols can reference the symbols in the original
92 process image file, any executable object files loaded at program
93 start-up, from the initial process image itself, from any other exe‐
94 cutable object file included in the same dlopen() invocation, and any
95 executable object files that were loaded in any dlopen() invocation and
96 which specified the RTLD_GLOBAL flag. To determine the scope of visi‐
97 bility for the symbols loaded with a dlopen() invocation, the mode
98 parameter should be a bitwise-inclusive OR with one of the following
99 values:
100 RTLD_GLOBAL The executable object file's symbols shall be made avail‐
102 able for relocation processing of any other executable
103 object file. In addition, symbol lookup using
104 dlopen(NULL,mode) and an associated dlsym() allows exe‐
105 cutable object files loaded with this mode to be searched.
106 RTLD_LOCAL The executable object file's symbols shall not be made
108 available for relocation processing of any other executable
109 object file.
110 If neither RTLD_GLOBAL nor RTLD_LOCAL is specified, the default behav‐
112 ior is unspecified.
113 If an executable object file is specified in multiple dlopen() invoca‐
115 tions, mode is interpreted at each invocation.
116 If RTLD_NOW has been specified, all relocations shall have been com‐
118 pleted rendering further RTLD_NOW operations redundant and any further
119 RTLD_LAZY operations irrelevant.
120 If RTLD_GLOBAL has been specified, the executable object file shall
122 maintain the RTLD_GLOBAL status regardless of any previous or future
123 specification of RTLD_LOCAL, as long as the executable object file
124 remains in the address space (see dlclose()).
125 Symbols introduced into the process image through calls to dlopen() may
127 be used in relocation activities. Symbols so introduced may duplicate
128 symbols already defined by the program or previous dlopen() operations.
129 To resolve the ambiguities such a situation might present, the resolu‐
130 tion of a symbol reference to symbol definition is based on a symbol
131 resolution order. Two such resolution orders are defined: load order
132 and dependency order. Load order establishes an ordering among symbol
133 definitions, such that the first definition loaded (including defini‐
134 tions from the process image file and any dependent executable object
135 files loaded with it) has priority over executable object files added
136 later (by dlopen()). Load ordering is used in relocation processing.
137 Dependency ordering uses a breadth-first order starting with a given
138 executable object file, then all of its dependencies, then any depen‐
139 dents of those, iterating until all dependencies are satisfied. With
140 the exception of the global symbol table handle obtained via a dlopen()
141 operation with a null pointer as the file argument, dependency ordering
142 is used by the dlsym() function. Load ordering is used in dlsym() oper‐
143 ations upon the global symbol table handle.
144 When an executable object file is first made accessible via dlopen(),
146 it and its dependent executable object files are added in dependency
147 order. Once all the executable object files are added, relocations are
148 performed using load order. Note that if an executable object file or
149 its dependencies had been previously loaded, the load and dependency
150 orders may yield different resolutions.
151 The symbols introduced by dlopen() operations and available through
153 dlsym() are at a minimum those which are exported as identifiers of
154 global scope by the executable object file. Typically, such identifiers
155 shall be those that were specified in (for example) C source code as
156 having extern linkage. The precise manner in which an implementation
157 constructs the set of exported symbols for an executable object file is
158 implementation-defined.
159
161 Upon successful completion, dlopen() shall return a symbol table han‐
162 dle. If file cannot be found, cannot be opened for reading, is not of
163 an appropriate executable object file format for processing by
164 dlopen(), or if an error occurs during the process of loading file or
165 relocating its symbolic references, dlopen() shall return a null
166 pointer. More detailed diagnostic information shall be available
167 through dlerror().
168
170 No errors are defined.
171 The following sections are informative.
173
175 Refer to dlsym().
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178 None.
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181 None.
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184 None.
185
187 dlclose(), dlerror(), dlsym()
188 The Base Definitions volume of POSIX.1‐2017, <dlfcn.h>
190
192 Portions of this text are reprinted and reproduced in electronic form
193 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
194 table Operating System Interface (POSIX), The Open Group Base Specifi‐
195 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
196 Electrical and Electronics Engineers, Inc and The Open Group. In the
197 event of any discrepancy between this version and the original IEEE and
198 The Open Group Standard, the original IEEE and The Open Group Standard
199 is the referee document. The original Standard can be obtained online
200 at http://www.opengroup.org/unix/online.html .
201 Any typographical or formatting errors that appear in this page are
203 most likely to have been introduced during the conversion of the source
204 files to man page format. To report such errors, see https://www.ker‐
205 nel.org/doc/man-pages/reporting_bugs.html .
206IEEE/The Open Group 2017 DLOPEN(3P)