1DLSYM(3)                   Linux Programmer's Manual                  DLSYM(3)
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NAME

6       dlsym,  dlvsym  - obtain address of a symbol in a shared object or exe‐
7       cutable
8

SYNOPSIS

10       #include <dlfcn.h>
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12       void *dlsym(void *restrict handle, const char *restrict symbol);
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14       #define _GNU_SOURCE
15       #include <dlfcn.h>
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17       void *dlvsym(void *restrict handle, const char *restrict symbol,
18                    const char *restrict version);
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20       Link with -ldl.
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DESCRIPTION

23       The function dlsym() takes a "handle" of a dynamic loaded shared object
24       returned by dlopen(3) along with a null-terminated symbol name, and re‐
25       turns the address where that symbol is loaded into memory.  If the sym‐
26       bol  is not found, in the specified object or any of the shared objects
27       that were automatically  loaded  by  dlopen(3)  when  that  object  was
28       loaded,  dlsym()  returns  NULL.   (The  search performed by dlsym() is
29       breadth first through the dependency tree of these shared objects.)
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31       In unusual cases (see NOTES) the value of the symbol could actually  be
32       NULL.   Therefore,  a NULL return from dlsym() need not indicate an er‐
33       ror.  The correct way to distinguish an error from a symbol whose value
34       is  NULL  is to call dlerror(3) to clear any old error conditions, then
35       call dlsym(), and then call dlerror(3) again, saving its  return  value
36       into a variable, and check whether this saved value is not NULL.
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38       There are two special pseudo-handles that may be specified in handle:
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40       RTLD_DEFAULT
41              Find  the  first  occurrence of the desired symbol using the de‐
42              fault shared object  search  order.   The  search  will  include
43              global  symbols  in the executable and its dependencies, as well
44              as symbols in shared objects that were dynamically  loaded  with
45              the RTLD_GLOBAL flag.
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47       RTLD_NEXT
48              Find the next occurrence of the desired symbol in the search or‐
49              der after the current object.  This  allows  one  to  provide  a
50              wrapper around a function in another shared object, so that, for
51              example, the definition of a function in a preloaded shared  ob‐
52              ject (see LD_PRELOAD in ld.so(8)) can find and invoke the "real"
53              function provided in another shared object (or for that  matter,
54              the  "next"  definition of the function in cases where there are
55              multiple layers of preloading).
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57       The _GNU_SOURCE feature test macro must be defined in order  to  obtain
58       the definitions of RTLD_DEFAULT and RTLD_NEXT from <dlfcn.h>.
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60       The  function  dlvsym()  does  the  same as dlsym() but takes a version
61       string as an additional argument.
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RETURN VALUE

64       On success, these functions return the address associated with  symbol.
65       On  failure,  they return NULL; the cause of the error can be diagnosed
66       using dlerror(3).
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VERSIONS

69       dlsym() is present in glibc 2.0 and later.  dlvsym() first appeared  in
70       glibc 2.1.
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ATTRIBUTES

73       For  an  explanation  of  the  terms  used  in  this  section,  see at‐
74       tributes(7).
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76       ┌────────────────────────────────────────────┬───────────────┬─────────┐
77Interface                                   Attribute     Value   
78       ├────────────────────────────────────────────┼───────────────┼─────────┤
79dlsym(), dlvsym()                           │ Thread safety │ MT-Safe │
80       └────────────────────────────────────────────┴───────────────┴─────────┘
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CONFORMING TO

83       POSIX.1-2001 describes dlsym().  The dlvsym() function is a GNU  exten‐
84       sion.
85

NOTES

87       There  are  several  scenarios  when  the address of a global symbol is
88       NULL.  For example, a symbol can be placed at zero address by the link‐
89       er, via a linker script or with --defsym command-line option. Undefined
90       weak symbols also have NULL value.  Finally, the symbol  value  may  be
91       the  result  of  a GNU indirect function (IFUNC) resolver function that
92       returns NULL as the resolved value. In the latter  case,  dlsym()  also
93       returns  NULL  without error. However, in the former two cases, the be‐
94       havior of GNU dynamic linker  is  inconsistent:  relocation  processing
95       succeeds and the symbol can be observed to have NULL value, but dlsym()
96       fails and dlerror() indicates a lookup error.
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98   History
99       The dlsym() function is part of the dlopen  API,  derived  from  SunOS.
100       That system does not have dlvsym().
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EXAMPLES

103       See dlopen(3).
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SEE ALSO

106       dl_iterate_phdr(3),   dladdr(3),   dlerror(3),   dlinfo(3),  dlopen(3),
107       ld.so(8)
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COLOPHON

110       This page is part of release 5.12 of the Linux  man-pages  project.   A
111       description  of  the project, information about reporting bugs, and the
112       latest    version    of    this    page,    can     be     found     at
113       https://www.kernel.org/doc/man-pages/.
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117Linux                             2021-03-22                          DLSYM(3)
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