Originální popis anglicky:
dl_iterate_phdr - walk through list of shared objects
Návod, kniha: Library Functions Manual
#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> 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
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 memory using the formula:
addr == info->dlpi_addr + info->dlpi_phdr[x].p_vaddr;
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);
}
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.
ldd(1),
objdump(1),
readelf(1),
dlopen(3),
ld.so(8), and the
Executable and Linking Format Specification
available at various locations online.
