I do agree that the example with memchr is not correct.

About your suggestion, I've done it. With 32. And that works fine.
All 3 values are passed by value.


# cat Pb-3.8.5.py
#!/usr/bin/env python3

from ctypes import *

mine = CDLL('./MemchrArgsHack2.so')

class MemchrArgsHack2(Structure):
    _fields_ = [("s",   c_char_p),
                ("c_n", c_ulong * 2)]

memchr_args_hack2 = MemchrArgsHack2()
memchr_args_hack2.s = b"abcdef"
memchr_args_hack2.c_n[0] = ord('d')
memchr_args_hack2.c_n[1] = 7

print( "sizeof(MemchrArgsHack2): ", sizeof(MemchrArgsHack2) )

print( CFUNCTYPE(c_char_p, MemchrArgsHack2, c_void_p)           (('my_memchr', mine)) (memchr_args_hack2, None) )


# cat MemchrArgsHack2.c
#include <string.h>
#include <stdio.h>

struct MemchrArgsHack2
{
        char *s;
        unsigned long c_n[2];
};

extern char *my_memchr(struct MemchrArgsHack2 args)
{
        printf("s   element  : char pointer: %p %s\n", args.s, args.s);
        printf("c_n element 0: a Long:       %ld\n",           args.c_n[0]);
        printf("c_n element 1: a Long:       %ld\n",           args.c_n[1]);

        return(args.s +3);
}



TONY Modules/_ctypes/stgdict.c: MAX_STRUCT_SIZE=32
sizeof(MemchrArgsHack2):  24
TONY: libffi: src/powerpc/ffi_darwin.c : ffi_prep_cif_machdep()
TONY: libffi: src/powerpc/ffi_darwin.c : aix_adjust_aggregate_sizes() s->size: 8 s->type:14 : FFI_TYPE_POINTER
TONY: libffi: src/powerpc/ffi_darwin.c : aix_adjust_aggregate_sizes() s->size:24 s->type:13 : FFI_TYPE_STRUCT
TONY: libffi: src/powerpc/ffi_darwin.c : aix_adjust_aggregate_sizes() FFI_TYPE_STRUCT Before s->size:24
TONY: libffi: src/powerpc/ffi_darwin.c : aix_adjust_aggregate_sizes() s->size: 8 s->type:14 : FFI_TYPE_POINTER
TONY: libffi: src/powerpc/ffi_darwin.c : aix_adjust_aggregate_sizes() p->size: 8 s->size: 8
TONY: libffi: src/powerpc/ffi_darwin.c : aix_adjust_aggregate_sizes() s->size:16 s->type:13 : FFI_TYPE_STRUCT
TONY: libffi: src/powerpc/ffi_darwin.c : aix_adjust_aggregate_sizes() FFI_TYPE_STRUCT Before s->size:16
TONY: libffi: src/powerpc/ffi_darwin.c : aix_adjust_aggregate_sizes() s->size: 8 s->type:11 : FFI_TYPE_UINT64
TONY: libffi: src/powerpc/ffi_darwin.c : aix_adjust_aggregate_sizes() p->size: 8 s->size: 8
TONY: libffi: src/powerpc/ffi_darwin.c : aix_adjust_aggregate_sizes() s->size: 8 s->type:11 : FFI_TYPE_UINT64
TONY: libffi: src/powerpc/ffi_darwin.c : aix_adjust_aggregate_sizes() p->size: 8 s->size:16
TONY: libffi: src/powerpc/ffi_darwin.c : aix_adjust_aggregate_sizes() After ALIGN s->size:16
TONY: libffi: src/powerpc/ffi_darwin.c : aix_adjust_aggregate_sizes() p->size:16 s->size:24
TONY: libffi: src/powerpc/ffi_darwin.c : aix_adjust_aggregate_sizes() After ALIGN s->size:24
TONY: libffi: src/powerpc/ffi_darwin.c : aix_adjust_aggregate_sizes() s->size: 8 s->type:14 : FFI_TYPE_POINTER
TONY: libffi: src/powerpc/ffi_darwin.c: ffi_call: FFI_AIX
TONY: libffi: cif->abi:  1  -(long)cif->bytes : -144  cif->flags :  8  ecif.rvalue : fffffffffffd210  fn: 9001000a0227760  FFI_FN(ffi_prep_args) : 9001000a050a108

s   element  : char pointer: a00000000154d40 abcdef
c_n element 0: a Long:       100
c_n element 1: a Long:       7        <<<<  Correct value appears.
b'def'

With the regular version (16), I have:

sizeof(MemchrArgsHack2):  24
TONY: libffi: src/powerpc/ffi_darwin.c : ffi_prep_cif_machdep()
TONY: libffi: src/powerpc/ffi_darwin.c : aix_adjust_aggregate_sizes() s->size: 8 s->type:14 : FFI_TYPE_POINTER
TONY: libffi: src/powerpc/ffi_darwin.c : aix_adjust_aggregate_sizes() s->size:24 s->type:13 : FFI_TYPE_STRUCT
TONY: libffi: src/powerpc/ffi_darwin.c : aix_adjust_aggregate_sizes() FFI_TYPE_STRUCT Before s->size:24
TONY: libffi: src/powerpc/ffi_darwin.c : aix_adjust_aggregate_sizes() s->size: 8 s->type:14 : FFI_TYPE_POINTER
TONY: libffi: src/powerpc/ffi_darwin.c : aix_adjust_aggregate_sizes() p->size: 8 s->size: 8
TONY: libffi: src/powerpc/ffi_darwin.c : aix_adjust_aggregate_sizes() s->size: 8 s->type:14 : FFI_TYPE_POINTER
TONY: libffi: src/powerpc/ffi_darwin.c : aix_adjust_aggregate_sizes() p->size: 8 s->size:16
TONY: libffi: src/powerpc/ffi_darwin.c : aix_adjust_aggregate_sizes() After ALIGN s->size:16
TONY: libffi: src/powerpc/ffi_darwin.c : aix_adjust_aggregate_sizes() s->size: 8 s->type:14 : FFI_TYPE_POINTER
TONY: libffi: src/powerpc/ffi_darwin.c: ffi_call: FFI_AIX
TONY: libffi: cif->abi:  1  -(long)cif->bytes : -144  cif->flags :  8  ecif.rvalue : fffffffffffd210  fn: 9001000a0227760  FFI_FN(ffi_prep_args) : 9001000a050a108

s   element  : char pointer: a00000000154d40 abcdef
c_n element 0: a Long:       100
c_n element 1: a Long:       0        <<< Python pushed nothing for this.