mirror of https://github.com/mkerrisk/man-pages
290 lines
7.6 KiB
Groff
290 lines
7.6 KiB
Groff
.\" Copyright (C) 2015 Michael Kerrisk <mtk.manpages@gmail.com>
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.\" and Copyright (C) 2008 Petr Baudis <pasky@suse.cz> (dladdr caveat)
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.TH DLADDR 3 2020-08-13 "Linux" "Linux Programmer's Manual"
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.SH NAME
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dladdr, dladdr1 \- translate address to symbolic information
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.SH SYNOPSIS
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.nf
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.B #define _GNU_SOURCE
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.B #include <dlfcn.h>
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.PP
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.BI "int dladdr(void *" addr ", Dl_info *" info );
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.PP
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.BI "int dladdr1(void *" addr ", Dl_info *" info ", void **" \
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extra_info ", int " flags );
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.PP
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Link with \fI\-ldl\fP.
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.fi
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.SH DESCRIPTION
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The function
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.BR dladdr ()
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determines whether the address specified in
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.IR addr
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is located in one of the shared objects loaded by the calling application.
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If it is, then
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.BR dladdr ()
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returns information about the shared object and symbol that overlaps
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.IR addr .
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This information is returned in a
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.I Dl_info
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structure:
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.PP
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.in +4n
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.EX
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typedef struct {
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const char *dli_fname; /* Pathname of shared object that
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contains address */
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void *dli_fbase; /* Base address at which shared
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object is loaded */
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const char *dli_sname; /* Name of symbol whose definition
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overlaps \fIaddr\fP */
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void *dli_saddr; /* Exact address of symbol named
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in \fIdli_sname\fP */
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} Dl_info;
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.EE
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.in
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.PP
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If no symbol matching
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.I addr
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could be found, then
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.I dli_sname
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and
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.I dli_saddr
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are set to NULL.
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.PP
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The function
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.BR dladdr1 ()
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is like
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.BR dladdr (),
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but returns additional information via the argument
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.IR extra_info .
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The information returned depends on the value specified in
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.IR flags ,
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which can have one of the following values:
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.TP
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.B RTLD_DL_LINKMAP
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Obtain a pointer to the link map for the matched file.
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The
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.IR extra_info
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argument points to a pointer to a
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.I link_map
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structure (i.e.,
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.IR "struct link_map\ **" ),
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defined in
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.I <link.h>
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as:
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.IP
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.in +4n
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.EX
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struct link_map {
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ElfW(Addr) l_addr; /* Difference between the
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address in the ELF file and
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the address in memory */
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char *l_name; /* Absolute pathname where
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object was found */
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ElfW(Dyn) *l_ld; /* Dynamic section of the
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shared object */
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struct link_map *l_next, *l_prev;
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/* Chain of loaded objects */
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/* Plus additional fields private to the
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implementation */
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};
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.EE
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.in
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.TP
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.B RTLD_DL_SYMENT
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Obtain a pointer to the ELF symbol table entry of the matching symbol.
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The
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.IR extra_info
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argument is a pointer to a symbol pointer:
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.IR "const ElfW(Sym) **" .
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The
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.IR ElfW ()
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macro definition turns its argument into the name of an ELF data
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type suitable for the hardware architecture.
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For example, on a 64-bit platform,
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.I ElfW(Sym)
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yields the data type name
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.IR Elf64_Sym ,
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which is defined in
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.IR <elf.h>
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as:
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.IP
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.in +4n
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.EX
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typedef struct {
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Elf64_Word st_name; /* Symbol name */
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unsigned char st_info; /* Symbol type and binding */
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unsigned char st_other; /* Symbol visibility */
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Elf64_Section st_shndx; /* Section index */
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Elf64_Addr st_value; /* Symbol value */
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Elf64_Xword st_size; /* Symbol size */
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} Elf64_Sym;
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.EE
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.in
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.IP
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The
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.I st_name
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field is an index into the string table.
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.IP
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The
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.I st_info
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field encodes the symbol's type and binding.
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The type can be extracted using the macro
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.BR ELF64_ST_TYPE(st_info)
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(or
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.BR ELF32_ST_TYPE()
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on 32-bit platforms), which yields one of the following values:
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.in +4n
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.TS
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lb lb
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lb l.
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Value Description
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STT_NOTYPE Symbol type is unspecified
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STT_OBJECT Symbol is a data object
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STT_FUNC Symbol is a code object
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STT_SECTION Symbol associated with a section
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STT_FILE Symbol's name is filename
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STT_COMMON Symbol is a common data object
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STT_TLS Symbol is thread-local data object
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STT_GNU_IFUNC Symbol is indirect code object
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.TE
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.in
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.IP
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The symbol binding can be extracted from the
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.I st_info
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field using the macro
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.BR ELF64_ST_BIND(st_info)
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(or
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.BR ELF32_ST_BIND()
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on 32-bit platforms), which yields one of the following values:
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.in +4n
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.TS
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lb lb
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lb l.
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Value Description
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STB_LOCAL Local symbol
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STB_GLOBAL Global symbol
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STB_WEAK Weak symbol
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STB_GNU_UNIQUE Unique symbol
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.TE
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.in
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.IP
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The
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.I st_other
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field contains the symbol's visibility, which can be extracted using the macro
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.BR ELF64_ST_VISIBILITY(st_info)
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(or
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.BR ELF32_ST_VISIBILITY()
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on 32-bit platforms), which yields one of the following values:
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.in +4n
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.TS
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lb lb
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lb l.
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Value Description
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STV_DEFAULT Default symbol visibility rules
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STV_INTERNAL Processor-specific hidden class
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STV_HIDDEN Symbol unavailable in other modules
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STV_PROTECTED Not preemptible, not exported
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.TE
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.in
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.SH RETURN VALUE
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On success, these functions return a nonzero value.
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If the address specified in
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.I addr
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could be matched to a shared object,
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but not to a symbol in the shared object, then the
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.I info->dli_sname
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and
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.I info->dli_saddr
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fields are set to NULL.
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.PP
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If the address specified in
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.I addr
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could not be matched to a shared object, then these functions return 0.
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In this case, an error message is
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.I not
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.\" According to the FreeBSD man page, dladdr1() does signal an
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.\" error via dlerror() for this case.
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available via
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.BR dlerror (3).
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.SH VERSIONS
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.BR dladdr ()
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is present in glibc 2.0 and later.
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.BR dladdr1 ()
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first appeared in glibc 2.3.3.
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.SH ATTRIBUTES
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For an explanation of the terms used in this section, see
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.BR attributes (7).
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.TS
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allbox;
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lbw19 lb lb
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l l l.
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Interface Attribute Value
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T{
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.BR dladdr (),
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.BR dladdr1 ()
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T} Thread safety MT-Safe
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.TE
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.SH CONFORMING TO
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These functions are nonstandard GNU extensions
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that are also present on Solaris.
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.SH BUGS
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Sometimes, the function pointers you pass to
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.BR dladdr ()
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may surprise you.
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On some architectures (notably i386 and x86-64),
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.I dli_fname
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and
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.I dli_fbase
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may end up pointing back at the object from which you called
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.BR dladdr (),
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even if the function used as an argument should come from
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a dynamically linked library.
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.PP
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The problem is that the function pointer will still be resolved
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at compile time, but merely point to the
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.I plt
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(Procedure Linkage Table)
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section of the original object (which dispatches the call after
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asking the dynamic linker to resolve the symbol).
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To work around this,
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you can try to compile the code to be position-independent:
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then, the compiler cannot prepare the pointer
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at compile time any more and
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.BR gcc (1)
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will generate code that just loads the final symbol address from the
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.I got
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(Global Offset Table) at run time before passing it to
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.BR dladdr ().
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.SH SEE ALSO
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.BR dl_iterate_phdr (3),
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.BR dlinfo (3),
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.BR dlopen (3),
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.BR dlsym (3),
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.BR ld.so (8)
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