dl_iterate_phdr — walk through list of shared objects
#define _GNU_SOURCE #include <link.h>
int
dl_iterate_phdr( |
int | (*callback)( struct
dl_phdr_info *info, size_t size, void
*data) , |
void * | data) ; |
The dl_iterate_phdr
()
function allows an application to inquire at run-time to find
out which shared objects it has loaded.
The dl_iterate_phdr
()
function walks through the list of an application's shared
objects and calls the function callback
once for each object,
until either all shared objects have been processed or
callback
returns a
non-zero value.
Each call to callback
receives three
arguments: info
,
which is a pointer to a structure containing information
about the shared object; size
, which is the size of
the structure pointed to by info
; and data
, which is a copy of
whatever value was passed by the calling program as the
second argument (also named data
) in the call to
dl_iterate_phdr
().
The info
argument is a structure of the following type:
struct dl_phdr_info { ElfW(Addr) dlpi_addr; /* Base address of object */ const char *dlpi_name; /* (Null-terminated) name of object */ const ElfW(Phdr) *dlpi_phdr; /* Pointer to array of ELF program headers for this object */ ElfW(Half) dlpi_phnum; /* # of items in 'dlpi_phdr' */ };
(The ElfW
()
macro definition turns its argument into the name of an ELF
data type suitable for the hardware architecture. For
example, on a 32-bit platform, ElfW(Addr) yields the data
type name Elf32_Addr. Further information on these types can
be found in the <
elf.h
>
<
link.h
>
header files.)
The dlpi_addr
field indicates the base address of the shared object (i.e.,
the difference between the virtual memory address of the
shared object and the offset of that object in the file from
which it was loaded). The dlpi_name
field is a
null-terminated string giving the pathname from which the
shared object was loaded.
To understand the meaning of the dlpi_phdr
and dlpi_phnum
fields, we need to
be aware that an ELF shared object consists of a number of
segments, each of which has a corresponding program header
describing the segment. The dlpi_phdr
field is a pointer
to an array of the program headers for this shared object.
The dlpi_phnum
field indicates the size of this array.
These program headers are structures of the following form:
typedef | struct { | |||
Elf32_Word |
p_type ; |
/* Segment type */
|
||
Elf32_Off |
p_offset ; |
/* Segment file offset */
|
||
Elf32_Addr |
p_vaddr ; |
/* Segment virtual address */
|
||
Elf32_Addr |
p_paddr ; |
/* Segment physical address */
|
||
Elf32_Word |
p_filesz ; |
/* Segment size in file */
|
||
Elf32_Word |
p_memsz ; |
/* Segment size in memory */
|
||
Elf32_Word |
p_flags ; |
/* Segment flags */
|
||
Elf32_Word |
p_align ; |
/* Segment alignment */
|
||
} Elf32_Phdr; |
Note that we can calculate the location of a particular
program header, x
,
in virtual memory using the formula:
addr == info->dlpi_addr + info->dlpi_phdr[x].p_vaddr;
The dl_iterate_phdr
()
function returns whatever value was returned by the last call
to callback
.
The dl_iterate_phdr
()
function is Linux specific and should be avoided in portable
applications.
The following program displays a list of pathnames of the shared objects it has loaded. For each shared object, the program lists the virtual addresses at which the object's ELF segments are loaded.
#define _GNU_SOURCE #include <link.h> #include <stdlib.h> #include <stdio.h> static int callback(struct dl_phdr_info *info, size_t size, void *data) { int j; printf("name=%s (%d segments)\n", info−>dlpi_name, info−>dlpi_phnum); for (j = 0; j < info−>dlpi_phnum; j++) printf("\t\t header %2d: address=%10p\n", j, (void *) (info−>dlpi_addr + info−>dlpi_phdr[j].p_vaddr)); return 0; } int main(int argc, char *argv[]) { dl_iterate_phdr(callback, NULL); exit(EXIT_SUCCESS); }
ldd(1), objdump(1), readelf(1), dlopen(3), feature_test_macros(7), ld.so(8), and the Executable and Linking Format Specification available at various locations online.
Copyright (c) 2003 by Michael Kerrisk <mtk-manpagesgmx.net> 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 distributed under the terms of a permission notice identical to this one Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. License. |