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
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 |
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
attributes(7).
Interface |
Attribute |
Value |
dladdr (), dladdr1 () |
Thread safety |
MT-Safe |
These functions are nonstandard GNU extensions that are also present on Solaris.
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().
dl_iterate_phdr(3),
dlinfo(3),
dlopen(3),
dlsym(3),
ld.so(8)