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DLADDR(3) Linux Programmer's Manual DLADDR(3)


     dladdr, dladdr1 - translate address to symbolic information


     #define _GNU_SOURCE
     #include <dlfcn.h>
     int dladdr(void *addr, Dl_info *info);
     int dladdr1(void *addr, Dl_info *info, void **extra_info, int flags);
     Link with -ldl.


     The  function dladdr() determines whether the address specified in addr
     is located in one of the shared objects loaded by the calling  applica-
     tion.   If  it  is,  then dladdr() returns information about the shared
     object and symbol that overlaps addr.  This information is returned  in
     a Dl_info structure:
         typedef struct {
             const char *dli_fname;  /* Pathname of shared object that
                                        contains address */
             void       *dli_fbase;  /* Base address at which shared
                                        object is loaded */
             const char *dli_sname;  /* Name of symbol whose definition
                                        overlaps addr */
             void       *dli_saddr;  /* Exact address of symbol named
                                        in dli_sname */ } Dl_info;
     If no symbol matching addr could be found, then dli_sname and dli_saddr
     are set to NULL.
     The function dladdr1() is like dladdr(), but returns additional  infor-
     mation  via  the argument extra_info.  The information returned depends
     on the value specified in flags, which can have one  of  the  following
            Obtain  a  pointer  to  the  link map for the matched file.  The
            extra_info argument points to a pointer to a link_map  structure
            (i.e., struct link_map **), defined in <link.h> as:
                struct link_map {
                    ElfW(Addr) l_addr;  /* Difference between the
                                           address in the ELF file and
                                           the address in memory */
                    char      *l_name;  /* Absolute pathname where
                                           object was found */
                    ElfW(Dyn) *l_ld;    /* Dynamic section of the
                                           shared object */
                    struct link_map *l_next, *l_prev;
                                        /* Chain of loaded objects */
                    /* Plus additional fields private to the
                       implementation */ };
            Obtain  a  pointer to the ELF symbol table entry of the matching
            symbol.  The extra_info  argument  is  a  pointer  to  a  symbol
            pointer:  const ElfW(Sym) **.  The ElfW() macro definition turns
            its argument into the name of an ELF data type suitable for  the
            hardware  architecture.   For  example,  on  a  64-bit platform,
            ElfW(Sym) yields the data type name Elf64_Sym, which is  defined
            in <elf.h> as:
                typedef struct  {
                    Elf64_Word    st_name;     /* Symbol name */
                    unsigned char st_info;     /* Symbol type and binding */
                    unsigned char st_other;    /* Symbol visibility */
                    Elf64_Section st_shndx;    /* Section index */
                    Elf64_Addr    st_value;    /* Symbol value */
                    Elf64_Xword    st_size;      /*   Symbol   size   */   }
            The st_name field is an index into the string table.
            The  st_info  field  encodes the symbol's type and binding.  The
            type can be extracted using the macro ELF64_ST_TYPE(st_info) (or
            ELF32_ST_TYPE()  on  32-bit  platforms), which yields one of the
            following values:
                Value           Description
                STT_NOTYPE      Symbol type is unspecified
                STT_OBJECT      Symbol is a data object
                STT_FUNC        Symbol is a code object
                STT_SECTION     Symbol associated with a section
                STT_FILE        Symbol's name is file name
                STT_COMMON      Symbol is a common data object
                STT_TLS         Symbol is thread-local data object
                STT_GNU_IFUNC   Symbol is indirect code object
            The symbol binding can be extracted from the st_info field using
            the  macro  ELF64_ST_BIND(st_info) (or ELF32_ST_BIND() on 32-bit
            platforms), which yields one of the following values:
                Value            Description
                STB_LOCAL        Local symbol
                STB_GLOBAL       Global symbol
                STB_WEAK         Weak symbol
                STB_GNU_UNIQUE   Unique symbol
            The st_other field contains the symbol's visibility,  which  can
            be  extracted  using  the macro ELF64_ST_VISIBILITY(st_info) (or
            ELF32_ST_VISIBILITY() on 32-bit platforms), which yields one  of
            the following values:
                Value           Description
                STV_DEFAULT     Default symbol visibility rules
                STV_INTERNAL    Processor-specific hidden class
                STV_HIDDEN      Symbol unavailable in other modules
                STV_PROTECTED   Not preemptible, not exported


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


     dladdr() is present in glibc 2.0 and later.  dladdr1()  first  appeared
     in glibc 2.3.3.


     For   an   explanation   of   the  terms  used  in  this  section,  see
     |Interface           | Attribute     | Value   |
     |dladdr(), dladdr1() | Thread safety | MT-Safe |


     These functions are nonstandard GNU extensions that are also present on


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


     dl_iterate_phdr(3), dlinfo(3), dlopen(3), dlsym(3),


     This  page  is  part of release 4.16 of the Linux man-pages project.  A
     description of the project, information about reporting bugs,  and  the
     latest     version     of     this    page,    can    be    found    at

Linux 2017-09-15 DLADDR(3)

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