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25 .TH DLSYM 3 2017-07-13 "Linux" "Linux Programmer's Manual"
27 dlsym, dlvsym \- obtain address of a symbol in a shared object or executable
31 .BI "void *dlsym(void *" handle ", const char *" symbol );
33 .B #define _GNU_SOURCE
37 .BI "void *dlvsym(void *" handle ", char *" symbol ", char *" version );
39 Link with \fI\-ldl\fP.
43 takes a "handle" of a dynamic loaded shared object returned by
45 along with a null-terminated symbol name,
46 and returns the address where that symbol is
48 If the symbol is not found, in the specified
49 object or any of the shared objects that were automatically loaded by
51 when that object was loaded,
54 (The search performed by
56 is breadth first through the dependency tree of these shared objects.)
58 Since the value of the symbol could actually be NULL (so that a
61 need not indicate an error), the correct way to test for an error
64 to clear any old error conditions, then call
68 again, saving its return value into a variable, and check whether
69 this saved value is not NULL.
71 There are two special pseudo-handles that may be specified in
75 Find the first occurrence of the desired symbol
76 using the default shared object search order.
77 The search will include global symbols in the executable
79 as well as symbols in shared objects that were dynamically loaded with the
84 Find the next occurrence of the desired symbol in the search order
85 after the current object.
86 This allows one to provide a wrapper
87 around a function in another shared object, so that, for example,
88 the definition of a function in a preloaded shared object
93 can find and invoke the "real" function provided in another shared object
94 (or for that matter, the "next" definition of the function in cases
95 where there are multiple layers of preloading).
99 feature test macro must be defined in order to obtain the
112 but takes a version string as an additional argument.
115 these functions return the address associated with
117 On failure, they return NULL;
118 the cause of the error can be diagnosed using
122 is present in glibc 2.0 and later.
124 first appeared in glibc 2.1.
126 For an explanation of the terms used in this section, see
132 Interface Attribute Value
136 T} Thread safety MT-Safe
139 POSIX.1-2001 describes
143 function is a GNU extension.
148 function is part of the dlopen API, derived from SunOS.
149 That system does not have
155 .BR dl_iterate_phdr (3),