DT_USED entry in .dynamic section of ELF file - c

Question

I am curious about DT_USED entry in .dynamic section. However, I could only find two code examples that describe this entry.
1.
#define DT_USED 0x7ffffffe /* ignored - same as needed */
in https://github.com/switchbrew/switch-tools/blob/master/src/elf_common.h
2.
case DT_USED:
case DT_INIT_ARRAY:
case DT_FINI_ARRAY:
if (do_dynamic)
{
if (entry->d_tag == DT_USED
&& VALID_DYNAMIC_NAME (entry->d_un.d_val))
{
char *name = GET_DYNAMIC_NAME (entry->d_un.d_val);
if (*name)
{
printf (_("Not needed object: [%s]\n"), name);
break;
}
}
print_vma (entry->d_un.d_val, PREFIX_HEX);
putchar ('\n');
}
break;
in http://web.mit.edu/freebsd/head/contrib/binutils/binutils/readelf.c
I want to know, what's the meaning of "Not needed object"? Does it mean that file names listed here are not needed?

Answer 1

In general, when looking at Solaris dynamic linker features, it is possible to find more information in the public Illumos sources (which were once derived from OpenSolaris). In this case, it seems that DT_USED is always treated like DT_NEEDED, so they are the essentially same thing. One of the header files, usr/src/uts/common/sys/link.h also contains this:
/*
* DT_* entries between DT_HIPROC and DT_LOPROC are reserved for processor
* specific semantics.
*
* DT_* encoding rules apply to all tag values larger than DT_LOPROC.
*/
#define DT_LOPROC 0x70000000 /* processor specific range */
#define DT_AUXILIARY 0x7ffffffd /* shared library auxiliary name */
#define DT_USED 0x7ffffffe /* ignored - same as needed */
#define DT_FILTER 0x7fffffff /* shared library filter name */
#define DT_HIPROC 0x7fffffff
There may have been planned something here, but it doesn't seem to be implemented (or it used to be and no longer is).