DL_ITERATE_PHDR(3) DL_ITERATE_PHDR(3)
NAME
dl_iterate_phdr - walk through list of shared objects
SYNOPSIS
#define _GNU_SOURCE
#include <link.h>
int dl_iterate_phdr(
int (*callback) (struct dl_phdr_info *info,
size_t size, void *data),
void *data);
DESCRIPTION
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 applica-
tion'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. Fur-
ther information on these types can be found in the <elf.h> and
<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 seg-
ments, 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;
EXAMPLE
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);
}
RETURN VALUE
The dl_iterate_phdr() function returns whatever value was returned by
the last call to callback.
CONFORMING TO
The dl_iterate_phdr() function is Linux specific and should be avoided
in portable applications.
SEE ALSO
ldd(1), objdump(1), readelf(1), dlopen(3), ld.so(8), and the Executable
and Linking Format Specification available at various locations online.
Linux Programmer's Manual Linux 2.4.21 DL_ITERATE_PHDR(3)