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25 .TH DLSYM 3 2019-03-06 "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 In unusual cases (see NOTES) the value of the symbol could actually be NULL.
59 Therefore, a NULL return from
61 need not indicate an error.
62 The correct way to distinguish an error from a symbol whose value is NULL
65 to clear any old error conditions, then call
69 again, saving its return value into a variable, and check whether
70 this saved value is not NULL.
72 There are two special pseudo-handles that may be specified in
76 Find the first occurrence of the desired symbol
77 using the default shared object search order.
78 The search will include global symbols in the executable
80 as well as symbols in shared objects that were dynamically loaded with the
85 Find the next occurrence of the desired symbol in the search order
86 after the current object.
87 This allows one to provide a wrapper
88 around a function in another shared object, so that, for example,
89 the definition of a function in a preloaded shared object
94 can find and invoke the "real" function provided in another shared object
95 (or for that matter, the "next" definition of the function in cases
96 where there are multiple layers of preloading).
100 feature test macro must be defined in order to obtain the
113 but takes a version string as an additional argument.
116 these functions return the address associated with
118 On failure, they return NULL;
119 the cause of the error can be diagnosed using
123 is present in glibc 2.0 and later.
125 first appeared in glibc 2.1.
127 For an explanation of the terms used in this section, see
133 Interface Attribute Value
137 T} Thread safety MT-Safe
140 POSIX.1-2001 describes
144 function is a GNU extension.
146 The value of a symbol returned by
148 will never be NULL if the shared object is the result of normal compilation,
149 since a global symbol is never placed at the NULL address.
150 There are nevertheless cases where a lookup using
152 may return NULL as the value of a symbol.
153 For example, the symbol value may be the result of
154 a GNU indirect function (IFUNC) resolver function that returns
155 NULL as the resolved value.
160 function is part of the dlopen API, derived from SunOS.
161 That system does not have
167 .BR dl_iterate_phdr (3),