DLADDR(3) Linux Programmer's Manual DLADDR(3)
NAME
dladdr, dladdr1 - translate address to symbolic information
SYNOPSIS
#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 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
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 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().
SEE ALSO
dl_iterate_phdr(3), dlinfo(3), dlopen(3), dlsym(3), ld.so(8)
COLOPHON
This page is part of release 4.15 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
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 DLADDR(3)
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