1DLOPEN(3P) POSIX Programmer's Manual DLOPEN(3P)
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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.
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13 dlopen — open a symbol table handle
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16 #include <dlfcn.h>
17 void *dlopen(const char *file, int mode);
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21 The dlopen() function shall make the symbols (function identifiers and
22 data object identifiers) in the executable object file specified by
23 file available to the calling program.
24 The class of executable object files eligible for this operation and
26 the manner of their construction are implementation-defined, though
27 typically such files are shared libraries or programs.
28 Implementations may permit the construction of embedded dependencies in
30 executable object files. In such cases, a dlopen() operation shall load
31 those dependencies in addition to the executable object file specified
32 by file. Implementations may also impose specific constraints on the
33 construction of programs that can employ dlopen() and its related ser‐
34 vices.
35 A successful dlopen() shall return a symbol table handle which the
37 caller may use on subsequent calls to dlsym() and dlclose().
38 The value of this symbol table handle should not be interpreted in any
40 way by the caller.
41 The file argument is used to construct a pathname to the executable
43 object file. If file contains a <slash> character, the file argument is
44 used as the pathname for the file. Otherwise, file is used in an imple‐
45 mentation-defined manner to yield a pathname.
46 If file is a null pointer, dlopen() shall return a global symbol table
48 handle for the currently running process image. This symbol table han‐
49 dle shall provide access to the symbols from an ordered set of exe‐
50 cutable object files consisting of the original program image file, any
51 executable object files loaded at program start-up as specified by that
52 process file (for example, shared libraries), and the set of executable
53 object files loaded using dlopen() operations with the RTLD_GLOBAL
54 flag. As the latter set of executable object files can change during
55 execution, the set of symbols made available by this symbol table han‐
56 dle can also change dynamically.
57 Only a single copy of an executable object file shall be brought into
59 the address space, even if dlopen() is invoked multiple times in refer‐
60 ence to the executable object file, and even if different pathnames are
61 used to reference the executable object file.
62 The mode parameter describes how dlopen() shall operate upon file with
64 respect to the processing of relocations and the scope of visibility of
65 the symbols provided within file. When an executable object file is
66 brought into the address space of a process, it may contain references
67 to symbols whose addresses are not known until the executable object
68 file is loaded.
69 These references shall be relocated before the symbols can be accessed.
71 The mode parameter governs when these relocations take place and may
72 have the following values:
73 RTLD_LAZY Relocations shall be performed at an implementation-defined
75 time, ranging from the time of the dlopen() call until the
76 first reference to a given symbol occurs. Specifying
77 RTLD_LAZY should improve performance on implementations
78 supporting dynamic symbol binding since a process might not
79 reference all of the symbols in an executable object file.
80 And, for systems supporting dynamic symbol resolution for
81 normal process execution, this behavior mimics the normal
82 handling of process execution.
83 RTLD_NOW All necessary relocations shall be performed when the exe‐
85 cutable object file is first loaded. This may waste some
86 processing if relocations are performed for symbols that
87 are never referenced. This behavior may be useful for
88 applications that need to know that all symbols referenced
89 during execution will be available before dlopen() returns.
90 Any executable object file loaded by dlopen() that requires relocations
92 against global symbols can reference the symbols in the original
93 process image file, any executable object files loaded at program
94 start-up, from the initial process image itself, from any other exe‐
95 cutable object file included in the same dlopen() invocation, and any
96 executable object files that were loaded in any dlopen() invocation and
97 which specified the RTLD_GLOBAL flag. To determine the scope of visi‐
98 bility for the symbols loaded with a dlopen() invocation, the mode
99 parameter should be a bitwise-inclusive OR with one of the following
100 values:
101 RTLD_GLOBAL The executable object file's symbols shall be made avail‐
103 able for relocation processing of any other executable
104 object file. In addition, symbol lookup using
105 dlopen(NULL,mode) and an associated dlsym() allows exe‐
106 cutable object files loaded with this mode to be searched.
107 RTLD_LOCAL The executable object file's symbols shall not be made
109 available for relocation processing of any other executable
110 object file.
111 If neither RTLD_GLOBAL nor RTLD_LOCAL is specified, the default behav‐
113 ior is unspecified.
114 If an executable object file is specified in multiple dlopen() invoca‐
116 tions, mode is interpreted at each invocation.
117 If RTLD_NOW has been specified, all relocations shall have been com‐
119 pleted rendering further RTLD_NOW operations redundant and any further
120 RTLD_LAZY operations irrelevant.
121 If RTLD_GLOBAL has been specified, the executable object file shall
123 maintain the RTLD_GLOBAL status regardless of any previous or future
124 specification of RTLD_LOCAL, as long as the executable object file
125 remains in the address space (see dlclose()).
126 Symbols introduced into the process image through calls to dlopen() may
128 be used in relocation activities. Symbols so introduced may duplicate
129 symbols already defined by the program or previous dlopen() operations.
130 To resolve the ambiguities such a situation might present, the resolu‐
131 tion of a symbol reference to symbol definition is based on a symbol
132 resolution order. Two such resolution orders are defined: load order
133 and dependency order. Load order establishes an ordering among symbol
134 definitions, such that the first definition loaded (including defini‐
135 tions from the process image file and any dependent executable object
136 files loaded with it) has priority over executable object files added
137 later (by dlopen()). Load ordering is used in relocation processing.
138 Dependency ordering uses a breadth-first order starting with a given
139 executable object file, then all of its dependencies, then any depen‐
140 dents of those, iterating until all dependencies are satisfied. With
141 the exception of the global symbol table handle obtained via a dlopen()
142 operation with a null pointer as the file argument, dependency ordering
143 is used by the dlsym() function. Load ordering is used in dlsym() oper‐
144 ations upon the global symbol table handle.
145 When an executable object file is first made accessible via dlopen(),
147 it and its dependent executable object files are added in dependency
148 order. Once all the executable object files are added, relocations are
149 performed using load order. Note that if an executable object file or
150 its dependencies had been previously loaded, the load and dependency
151 orders may yield different resolutions.
152 The symbols introduced by dlopen() operations and available through
154 dlsym() are at a minimum those which are exported as identifiers of
155 global scope by the executable object file. Typically, such identifiers
156 shall be those that were specified in (for example) C source code as
157 having extern linkage. The precise manner in which an implementation
158 constructs the set of exported symbols for an executable object file is
159 implementation-defined.
160
162 Upon successful completion, dlopen() shall return a symbol table han‐
163 dle. If file cannot be found, cannot be opened for reading, is not of
164 an appropriate executable object file format for processing by
165 dlopen(), or if an error occurs during the process of loading file or
166 relocating its symbolic references, dlopen() shall return a null
167 pointer. More detailed diagnostic information shall be available
168 through dlerror().
169
171 No errors are defined.
172 The following sections are informative.
174
176 Refer to dlsym().
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179 None.
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182 None.
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185 None.
186
188 dlclose(), dlerror(), dlsym()
189 The Base Definitions volume of POSIX.1‐2008, <dlfcn.h>
191
193 Portions of this text are reprinted and reproduced in electronic form
194 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
195 -- Portable Operating System Interface (POSIX), The Open Group Base
196 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
197 cal and Electronics Engineers, Inc and The Open Group. (This is
198 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
199 event of any discrepancy between this version and the original IEEE and
200 The Open Group Standard, the original IEEE and The Open Group Standard
201 is the referee document. The original Standard can be obtained online
202 at http://www.unix.org/online.html .
203 Any typographical or formatting errors that appear in this page are
205 most likely to have been introduced during the conversion of the source
206 files to man page format. To report such errors, see https://www.ker‐
207 nel.org/doc/man-pages/reporting_bugs.html .
208IEEE/The Open Group 2013 DLOPEN(3P)