Get the DIE for function __copy_grp():

$ dwarfdump ./libc_x86_64.so.6 --name=__copy_grp

There will be multiple DIEs probably. Choose the one for 'DW_TAG_subpgrogram' (DWARF term for function):

0x002a24e2: DW_TAG_subprogram
              DW_AT_external    (true)
              DW_AT_name    ("__copy_grp")
              DW_AT_decl_file   ("./grp/grp-merge.c")
              DW_AT_decl_line   (39)
              DW_AT_decl_column (0x01)
              DW_AT_linkage_name    ("__GI___copy_grp")
              DW_AT_prototyped  (true)
              DW_AT_type    (0x002a1e63 "int")
              DW_AT_low_pc  (0x00000000000c9440)
              DW_AT_high_pc (0x00000000000c9642)
              DW_AT_frame_base  (DW_OP_call_frame_cfa)
              DW_AT_GNU_all_call_sites  (true)
              DW_AT_sibling (0x002a2710)

The DW_AT_type gives the return type "int". Note the address 0x002a24e2. Now show this DIE with all its children, looking for formal parameters.

$ dwarfdump ./libc_x86_64.so.6 --debug-info=0x002a24e2 --show-children

Pruned output:

0x002a2508:   DW_TAG_formal_parameter
                DW_AT_name  ("srcgrp")
                DW_AT_type  (0x002a2290 "const group")

0x002a251c:   DW_TAG_formal_parameter
                DW_AT_name  ("buflen")
                DW_AT_type  (0x002a1e7d "const size_t")

0x002a2530:   DW_TAG_formal_parameter
                DW_AT_name  ("destgrp")
                DW_AT_type  (0x002a24dc "group*")

0x002a2544:   DW_TAG_formal_parameter
                DW_AT_name  ("destbuf")
                DW_AT_type  (0x002a1e56 "char*")

0x002a2558:   DW_TAG_formal_parameter
                DW_AT_name  ("endptr")
                DW_AT_type  (0x002a2295 "char**")

Resulting in prototype int __copy_grp(const group srcgrp, const size_t buflen, group* destgrp, char* destbuf, char** endptr);

generated from: 2022-01-20-dwarfdump-find-prototype-function.md