xenial (3) dl_iterate_phdr.3.gz
NAME
dl_iterate_phdr - walk through list of shared objects
SYNOPSIS
#define _GNU_SOURCE /* See feature_test_macros(7) */ #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 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 nonzero 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 using the
formula:
addr == info->dlpi_addr + info->dlpi_phdr[x].p_vaddr;
RETURN VALUE
The dl_iterate_phdr() function returns whatever value was returned by the last call to callback.
VERSIONS
dl_iterate_phdr() has been supported in glibc since version 2.2.4.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7). ┌──────────────────┬───────────────┬─────────┐ │Interface │ Attribute │ Value │ ├──────────────────┼───────────────┼─────────┤ │dl_iterate_phdr() │ Thread safety │ MT-Safe │ └──────────────────┴───────────────┴─────────┘
CONFORMING TO
The dl_iterate_phdr() function is not specified in any standard. Various other systems provide a version
of this function, although details of the returned dl_phdr_info structure differ. On the BSDs and
Solaris, the structure includes the fields dlpi_addr, dlpi_name, dlpi_phdr, and dlpi_phnum in addition to
other implementation-specific fields.
NOTES
Future versions of the C library may add further fields to the dl_phdr_info structure; in that event, the
size argument provides a mechanism for the callback function to discover whether it is running on a
system with added fields.
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);
}
SEE ALSO
ldd(1), objdump(1), readelf(1), dladdr(3), dlopen(3), elf(5), ld.so(8) Executable and Linking Format Specification, available at various locations online.
COLOPHON
This page is part of release 4.04 of the Linux man-pages project. A description of the project,
information about reporting bugs, and the latest version of this page, can be found at
http://www.kernel.org/doc/man-pages/.