1DLSYM(P) POSIX Programmer's Manual DLSYM(P)
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6 dlsym - obtain the address of a symbol from a dlopen object
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9 #include <dlfcn.h>
10 void *dlsym(void *restrict handle, const char *restrict name);
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15 The dlsym() function shall obtain the address of a symbol defined
16 within an object made accessible through a dlopen() call. The handle
17 argument is the value returned from a call to dlopen() (and which has
18 not since been released via a call to dlclose()), and name is the sym‐
19 bol's name as a character string.
20 The dlsym() function shall search for the named symbol in all objects
22 loaded automatically as a result of loading the object referenced by
23 handle (see dlopen() ). Load ordering is used in dlsym() operations
24 upon the global symbol object. The symbol resolution algorithm used
25 shall be dependency order as described in dlopen() .
26 The RTLD_DEFAULT and RTLD_NEXT flags are reserved for future use.
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30 If handle does not refer to a valid object opened by dlopen(), or if
31 the named symbol cannot be found within any of the objects associated
32 with handle, dlsym() shall return NULL. More detailed diagnostic infor‐
33 mation shall be available through dlerror() .
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36 No errors are defined.
37 The following sections are informative.
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41 The following example shows how dlopen() and dlsym() can be used to
42 access either function or data objects. For simplicity, error checking
43 has been omitted.
44 void *handle;
47 int *iptr, (*fptr)(int);
48 /* open the needed object */
51 handle = dlopen("/usr/home/me/libfoo.so", RTLD_LOCAL | RTLD_LAZY);
52 /* find the address of function and data objects */
55 *(void **)(&fptr) = dlsym(handle, "my_function");
56 iptr = (int *)dlsym(handle, "my_object");
57 /* invoke function, passing value of integer as a parameter */
60 (*fptr)(*iptr);
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63 Special purpose values for handle are reserved for future use. These
64 values and their meanings are:
65 RTLD_DEFAULT
67 The symbol lookup happens in the normal global scope; that is, a
68 search for a symbol using this handle would find the same defi‐
69 nition as a direct use of this symbol in the program code.
70 RTLD_NEXT
72 Specifies the next object after this one that defines name.
73 This one refers to the object containing the invocation of
74 dlsym(). The next object is the one found upon the application
75 of a load order symbol resolution algorithm (see dlopen() ). The
76 next object is either one of global scope (because it was intro‐
77 duced as part of the original process image or because it was
78 added with a dlopen() operation including the RTLD_GLOBAL flag),
79 or is an object that was included in the same dlopen() operation
80 that loaded this one.
81 The RTLD_NEXT flag is useful to navigate an intentionally created hier‐
83 archy of multiply-defined symbols created through interposition. For
84 example, if a program wished to create an implementation of malloc()
85 that embedded some statistics gathering about memory allocations, such
86 an implementation could use the real malloc() definition to perform the
87 memory allocation-and itself only embed the necessary logic to imple‐
88 ment the statistics gathering function.
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92 The ISO C standard does not require that pointers to functions can be
93 cast back and forth to pointers to data. Indeed, the ISO C standard
94 does not require that an object of type void * can hold a pointer to a
95 function. Implementations supporting the XSI extension, however, do
96 require that an object of type void * can hold a pointer to a function.
97 The result of converting a pointer to a function into a pointer to
98 another data type (except void *) is still undefined, however. Note
99 that compilers conforming to the ISO C standard are required to gener‐
100 ate a warning if a conversion from a void * pointer to a function
101 pointer is attempted as in:
102 fptr = (int (*)(int))dlsym(handle, "my_function");
105 Due to the problem noted here, a future version may either add a new
107 function to return function pointers, or the current interface may be
108 deprecated in favor of two new functions: one that returns data point‐
109 ers and the other that returns function pointers.
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112 None.
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115 dlclose() , dlerror() , dlopen() , the Base Definitions volume of
116 IEEE Std 1003.1-2001, <dlfcn.h>
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119 Portions of this text are reprinted and reproduced in electronic form
120 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
121 -- Portable Operating System Interface (POSIX), The Open Group Base
122 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
123 Electrical and Electronics Engineers, Inc and The Open Group. In the
124 event of any discrepancy between this version and the original IEEE and
125 The Open Group Standard, the original IEEE and The Open Group Standard
126 is the referee document. The original Standard can be obtained online
127 at http://www.opengroup.org/unix/online.html .
128IEEE/The Open Group 2003 DLSYM(P)