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 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:

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.

CONFORMING TO

The dl_iterate_phdr() function is Linux specific and should be avoided in portable applications.

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), dlopen(3), feature_test_macros(7), ld.so(8), and the Executable and Linking Format Specification available at various locations online.