1dladdr(3)                  Library Functions Manual                  dladdr(3)
2
3
4

NAME

6       dladdr, dladdr1 - translate address to symbolic information
7

LIBRARY

9       Dynamic linking library (libdl, -ldl)
10

SYNOPSIS

12       #define _GNU_SOURCE
13       #include <dlfcn.h>
14
15       int dladdr(const void *addr, Dl_info *info);
16       int dladdr1(const void *addr, Dl_info *info, void **extra_info,
17                   int flags);
18

DESCRIPTION

20       The  function dladdr() determines whether the address specified in addr
21       is located in one of the shared objects loaded by the calling  applica‐
22       tion.  If it is, then dladdr() returns information about the shared ob‐
23       ject and symbol that overlaps addr.  This information is returned in  a
24       Dl_info structure:
25
26           typedef struct {
27               const char *dli_fname;  /* Pathname of shared object that
28                                          contains address */
29               void       *dli_fbase;  /* Base address at which shared
30                                          object is loaded */
31               const char *dli_sname;  /* Name of symbol whose definition
32                                          overlaps addr */
33               void       *dli_saddr;  /* Exact address of symbol named
34                                          in dli_sname */
35           } Dl_info;
36
37       If no symbol matching addr could be found, then dli_sname and dli_saddr
38       are set to NULL.
39
40       The function dladdr1() is like dladdr(), but returns additional  infor‐
41       mation  via  the argument extra_info.  The information returned depends
42       on the value specified in flags, which can have one  of  the  following
43       values:
44
45       RTLD_DL_LINKMAP
46              Obtain  a pointer to the link map for the matched file.  The ex‐
47              tra_info argument points to a pointer to  a  link_map  structure
48              (i.e., struct link_map **), defined in <link.h> as:
49
50                  struct link_map {
51                      ElfW(Addr) l_addr;  /* Difference between the
52                                             address in the ELF file and
53                                             the address in memory */
54                      char      *l_name;  /* Absolute pathname where
55                                             object was found */
56                      ElfW(Dyn) *l_ld;    /* Dynamic section of the
57                                             shared object */
58                      struct link_map *l_next, *l_prev;
59                                          /* Chain of loaded objects */
60
61                      /* Plus additional fields private to the
62                         implementation */
63                  };
64
65       RTLD_DL_SYMENT
66              Obtain  a  pointer to the ELF symbol table entry of the matching
67              symbol.  The extra_info  argument  is  a  pointer  to  a  symbol
68              pointer:  const ElfW(Sym) **.  The ElfW() macro definition turns
69              its argument into the name of an ELF data type suitable for  the
70              hardware  architecture.   For  example,  on  a  64-bit platform,
71              ElfW(Sym) yields the data type name Elf64_Sym, which is  defined
72              in <elf.h> as:
73
74                  typedef struct  {
75                      Elf64_Word    st_name;     /* Symbol name */
76                      unsigned char st_info;     /* Symbol type and binding */
77                      unsigned char st_other;    /* Symbol visibility */
78                      Elf64_Section st_shndx;    /* Section index */
79                      Elf64_Addr    st_value;    /* Symbol value */
80                      Elf64_Xword   st_size;     /* Symbol size */
81                  } Elf64_Sym;
82
83              The st_name field is an index into the string table.
84
85              The  st_info  field  encodes the symbol's type and binding.  The
86              type can be extracted using the macro ELF64_ST_TYPE(st_info) (or
87              ELF32_ST_TYPE()  on  32-bit  platforms), which yields one of the
88              following values:
89
90                  Value           Description
91                  STT_NOTYPE      Symbol type is unspecified
92                  STT_OBJECT      Symbol is a data object
93                  STT_FUNC        Symbol is a code object
94                  STT_SECTION     Symbol associated with a section
95                  STT_FILE        Symbol's name is filename
96                  STT_COMMON      Symbol is a common data object
97                  STT_TLS         Symbol is thread-local data object
98                  STT_GNU_IFUNC   Symbol is indirect code object
99
100              The symbol binding can be extracted from the st_info field using
101              the  macro  ELF64_ST_BIND(st_info) (or ELF32_ST_BIND() on 32-bit
102              platforms), which yields one of the following values:
103
104                  Value            Description
105                  STB_LOCAL        Local symbol
106                  STB_GLOBAL       Global symbol
107                  STB_WEAK         Weak symbol
108                  STB_GNU_UNIQUE   Unique symbol
109
110              The st_other field contains the symbol's visibility,  which  can
111              be  extracted  using  the macro ELF64_ST_VISIBILITY(st_info) (or
112              ELF32_ST_VISIBILITY() on 32-bit platforms), which yields one  of
113              the following values:
114
115                  Value           Description
116                  STV_DEFAULT     Default symbol visibility rules
117                  STV_INTERNAL    Processor-specific hidden class
118                  STV_HIDDEN      Symbol unavailable in other modules
119                  STV_PROTECTED   Not preemptible, not exported
120

RETURN VALUE

122       On  success,  these  functions  return a nonzero value.  If the address
123       specified in addr could be matched to a shared object,  but  not  to  a
124       symbol   in   the   shared   object,   then   the  info->dli_sname  and
125       info->dli_saddr fields are set to NULL.
126
127       If the address specified in addr could not be matched to a  shared  ob‐
128       ject, then these functions return 0.  In this case, an error message is
129       not available via dlerror(3).
130

ATTRIBUTES

132       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
133       tributes(7).
134
135       ┌────────────────────────────────────────────┬───────────────┬─────────┐
136Interface                                   Attribute     Value   
137       ├────────────────────────────────────────────┼───────────────┼─────────┤
138dladdr(), dladdr1()                         │ Thread safety │ MT-Safe │
139       └────────────────────────────────────────────┴───────────────┴─────────┘
140

STANDARDS

142       GNU.
143

HISTORY

145       dladdr()
146              glibc 2.0.
147
148       dladdr1()
149              glibc 2.3.3.
150
151       Solaris.
152

BUGS

154       Sometimes, the function pointers you pass to dladdr() may surprise you.
155       On  some  architectures  (notably  i386  and  x86-64),  dli_fname   and
156       dli_fbase  may end up pointing back at the object from which you called
157       dladdr(), even if the function used as an argument should come  from  a
158       dynamically linked library.
159
160       The  problem  is  that  the  function pointer will still be resolved at
161       compile time, but merely point to the  plt  (Procedure  Linkage  Table)
162       section  of the original object (which dispatches the call after asking
163       the dynamic linker to resolve the symbol).  To work  around  this,  you
164       can  try  to  compile  the  code  to be position-independent: then, the
165       compiler cannot prepare the pointer at compile time any more and gcc(1)
166       will  generate  code  that just loads the final symbol address from the
167       got (Global Offset Table) at run time before passing it to dladdr().
168

SEE ALSO

170       dl_iterate_phdr(3), dlinfo(3), dlopen(3), dlsym(3), ld.so(8)
171
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173
174Linux man-pages 6.05              2023-07-20                         dladdr(3)
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