dladdr1(3) — Linux manual page


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

NAME         top

       dladdr, dladdr1 - translate address to symbolic information

SYNOPSIS         top

       #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.

DESCRIPTION         top

       The function dladdr() determines whether the address specified in
       addr is located in one of the shared objects loaded by the calling
       application.  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
       information via the argument extra_info.  The information returned
       depends on the value specified in flags, which can have one of the
       following values:

              Obtain a pointer to the link map for the matched file.  The
              extra_info argument points to a pointer to a link_map struc‐
              ture (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 plat‐
              form, 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 */
                  } Elf64_Sym;

              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 filename
                  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

RETURN VALUE         top

       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).

VERSIONS         top

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

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │Interface           Attribute     Value   │
       │dladdr(), dladdr1() │ Thread safety │ MT-Safe │

CONFORMING TO         top

       These functions are nonstandard GNU extensions that are also present
       on Solaris.

BUGS         top

       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
       compile time, but merely point to the plt (Procedure Linkage Table)
       section 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().

SEE ALSO         top

       dl_iterate_phdr(3), dlinfo(3), dlopen(3), dlsym(3), ld.so(8)

COLOPHON         top

       This page is part of release 5.08 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                            2020-08-13                        DLADDR(3)