dladdr (3)
Leading comments
Copyright (C) 2015 Michael Kerrisk <mtk.manpages@gmail.com> and Copyright (C) 2008 Petr Baudis <pasky@suse.cz> (dladdr caveat) %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distrib...
NAME
dladdr, dladdr1 - translate address to symbolic informationSYNOPSIS
#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
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:
- RTLD_DL_LINKMAP
-
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 */ }; - RTLD_DL_SYMENT
-
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 */ } 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 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
RETURN VALUE
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
dladdr() is present in glibc 2.0 and later. dladdr1() first appeared in glibc 2.3.3.ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).| Interface | Attribute | Value |
| dladdr(), dladdr1() | Thread safety | MT-Safe |
CONFORMING TO
These functions are nonstandard GNU extensions that are also present on Solaris.BUGS
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().