'\" t

.\" Copyright (C) 2015 Michael Kerrisk <mtk.manpages@gmail.com>

.\" and Copyright (C) 2008 Petr Baudis <pasky@suse.cz> (dladdr caveat)

.\"

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.\"

.TH DLADDR 3 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

dladdr, dladdr1 \- translate address to symbolic information

.SH SYNOPSIS

.nf

.B #define _GNU_SOURCE

.B #include <dlfcn.h>

.PP

.BI "int dladdr(void *" addr ", Dl_info *" info );

.PP

.BI "int dladdr1(void *" addr ", Dl_info *" info ", void **" \

        extra_info ", int " flags );

.PP

Link with \fI\-ldl\fP.

.fi

.SH DESCRIPTION

The function

.BR dladdr ()

determines whether the address specified in

.IR addr

is located in one of the shared objects loaded by the calling application.

If it is, then

.BR dladdr ()

returns information about the shared object and symbol that overlaps

.IR addr .

This information is returned in a

.I Dl_info

structure:

.PP

.in +4n

.EX

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 \fIaddr\fP */

    void       *dli_saddr;  /* Exact address of symbol named

                               in \fIdli_sname\fP */

} Dl_info;

.EE

.in

.PP

If no symbol matching

.I addr

could be found, then

.I dli_sname

and

.I dli_saddr

are set to NULL.

.PP

The function

.BR dladdr1 ()

is like

.BR dladdr (),

but returns additional information via the argument

.IR extra_info .

The information returned depends on the value specified in

.IR flags ,

which can have one of the following values:

.TP

.B RTLD_DL_LINKMAP

Obtain a pointer to the link map for the matched file.

The

.IR extra_info

argument points to a pointer to a

.I link_map

structure (i.e.,

.IR "struct link_map\ **" ),

defined in

.I <link.h>

as:

.IP

.in +4n

.EX

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 */

};

.EE

.in

.TP

.B RTLD_DL_SYMENT

Obtain a pointer to the ELF symbol table entry of the matching symbol.

The

.IR extra_info

argument is a pointer to a symbol pointer:

.IR "const ElfW(Sym) **" .

The

.IR 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,

.I ElfW(Sym)

yields the data type name

.IR Elf64_Sym ,

which is defined in

.IR <elf.h>

as:

.IP

.in +4n

.EX

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;

.EE

.in

.IP

The

.I st_name

field is an index into the string table.

.IP

The

.I st_info

field encodes the symbol's type and binding.

The type can be extracted using the macro

.BR ELF64_ST_TYPE(st_info)

(or

.BR ELF32_ST_TYPE()

on 32-bit platforms), which yields one of the following values:

.in +4n

.TS

lb lb

lb l.

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

.TE

.in

.IP

The symbol binding can be extracted from the

.I st_info

field using the macro

.BR ELF64_ST_BIND(st_info)

(or

.BR ELF32_ST_BIND()

on 32-bit platforms), which yields one of the following values:

.in +4n

.TS

lb lb

lb l.

Value	Description

STB_LOCAL	Local symbol

STB_GLOBAL	Global symbol

STB_WEAK	Weak symbol

STB_GNU_UNIQUE	Unique symbol

.TE

.in

.IP

The

.I st_other

field contains the symbol's visibility, which can be extracted using the macro

.BR ELF64_ST_VISIBILITY(st_info)

(or

.BR ELF32_ST_VISIBILITY()

on 32-bit platforms), which yields one of the following values:

.in +4n

.TS

lb lb

lb l.

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

.TE

.in

.SH RETURN VALUE

On success, these functions return a nonzero value.

If the address specified in

.I addr

could be matched to a shared object,

but not to a symbol in the shared object, then the

.I info->dli_sname

and

.I info->dli_saddr

fields are set to NULL.

.PP

If the address specified in

.I addr

could not be matched to a shared object, then these functions return 0.

In this case, an error message is

.I not

.\" According to the FreeBSD man page, dladdr1() does signal an

.\" error via dlerror() for this case.

available via

.BR dlerror (3).

.SH VERSIONS

.BR dladdr ()

is present in glibc 2.0 and later.

.BR dladdr1 ()

first appeared in glibc 2.3.3.

.SH ATTRIBUTES

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

.BR attributes (7).

.TS

allbox;

lbw19 lb lb

l l l.

Interface	Attribute	Value

T{

.BR dladdr (),

.BR dladdr1 ()

T}	Thread safety	MT-Safe

.TE

.SH CONFORMING TO

These functions are nonstandard GNU extensions

that are also present on Solaris.

.SH BUGS

Sometimes, the function pointers you pass to

.BR dladdr ()

may surprise you.

On some architectures (notably i386 and x86-64),

.I dli_fname

and

.I dli_fbase

may end up pointing back at the object from which you called

.BR dladdr (),

even if the function used as an argument should come from

a dynamically linked library.

.PP

The problem is that the function pointer will still be resolved

at compile time, but merely point to the

.I 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

.BR gcc (1)

will generate code that just loads the final symbol address from the

.I got

(Global Offset Table) at run time before passing it to

.BR dladdr ().

.SH SEE ALSO

.BR dl_iterate_phdr (3),

.BR dlinfo (3),

.BR dlopen (3),

.BR dlsym (3),

.BR ld.so (8)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I 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/.