[bilbo]

Are you aware of a way to do that without any api ?

Without API and from RING 3?
Here you are!
Platform Windows XP.

// get_parent.c - coded by bilbo, 23jul04 - build with "cl -W3 get_parent.c"
// platform: Windows XP

#include <windows.h>
#include <stdio.h>

#pragma comment(lib, "advapi32")

#define DebugReadMemory 8
#define FCHK(a) if (!(a)) { printf(#a " failed\n"); exit(0); }

typedef DWORD (NTAPI *PZwSystemDebugControl)(DWORD ControlCode,
	PVOID InputBuffer, ULONG InputBufferLength, PVOID OutputBuffer,
	ULONG OutputBufferLength, PULONG ReturnLength);
PZwSystemDebugControl ZwSystemDebugControl;
DWORD pSystemProc;
struct mem {
	PVOID kernel_addr;
	PVOID user_addr;
	DWORD len;
	};

void
EnablePrivilege(LPCSTR lpszName)
{
	HANDLE hToken;
	TOKEN_PRIVILEGES tok;

	FCHK(OpenProcessToken(GetCurrentProcess(),
		TOKEN_ADJUST_PRIVILEGES | TOKEN_QUERY, &hToken));

	tok.PrivilegeCount = 1;
	tok.Privileges[0].Attributes = SE_PRIVILEGE_ENABLED;

	FCHK(LookupPrivilegeValue(NULL, lpszName, &tok.Privileges[0].Luid));
	FCHK(AdjustTokenPrivileges(hToken, FALSE, &tok, sizeof(tok), NULL, NULL));
}

	/*
	 * read one long from kernel space
	 */
DWORD
rd(DWORD addr)
{
	struct mem mem;
	DWORD m;

	mem.kernel_addr = (PVOID)addr;
	mem.user_addr = &m;
	mem.len = 4;

	FCHK(!ZwSystemDebugControl(DebugReadMemory, &mem, sizeof(mem), 0, 0, 0));

	return m;
}

void
rdstr(DWORD addr, LPSTR buf, int buflen)
{
	int i;
	struct mem mem;

	mem.kernel_addr = (PVOID)addr;
	mem.user_addr = buf;
	mem.len = 1;

	for (i=0; i<buflen; i++) {
		FCHK(!ZwSystemDebugControl(DebugReadMemory, &mem, sizeof(mem), 0,0,0));
		if (!*buf++) break;
		addr++;
			// reassign structure mem
		mem.kernel_addr = (PVOID)addr;
		mem.user_addr = buf;
		}
}

void
prepare_me(void)
{
	HMODULE hModule;

		// get the API address
	FCHK(hModule = GetModuleHandle("ntdll.dll"));
	FCHK((ZwSystemDebugControl = (PZwSystemDebugControl)
		GetProcAddress(hModule, "ZwSystemDebugControl")));

	EnablePrivilege(SE_DEBUG_NAME);
}

void
main(void)
{
	DWORD eprocess, mypid, parentpid, tmppid;
	char myname[17]={0}, parentname[17]={0};

	prepare_me();

		// get current EPROCESS from kernel space
	eprocess = rd(0xFFDFF000+0x124);  // KPRCB.KTHREAD
	eprocess = rd(eprocess+0x44);  // KTHREAD.KAPC_STATE.KPROCESS

		// get pids from kernel space
	mypid = rd(eprocess+0x84);  // EPROCESS.UniqueProcessId
	parentpid = rd(eprocess+0x14C);  // EPROCESS.InheritedFromUniqueProcessId

		// get my name from current EPROCESS
	rdstr(eprocess+0x174, myname, 16);  // EPROCESS.ImageFileName

		// scan back process list to find parent EPROCESS
	//printf("%x %d\n", eprocess, mypid);
	eprocess = rd(eprocess+0x8C);  // ActiveProcessLinks
	while (1) {
		tmppid = rd(eprocess+(0x84-0x88));  // UniqueProcessId
		//printf("%x %d\n", eprocess, tmppid);
		if (tmppid == parentpid) break;
		eprocess = rd(eprocess+(0x8c-0x88));  // Blink
		}

		// get parent name from parent EPROCESS
	rdstr(eprocess+(0x174-0x88), parentname, 16);  // EPROCESS.ImageFileName

	printf("mypid: %d(%.16s), parentpid: %d(%.16s)\n",
		mypid, myname, parentpid, parentname);
}

Have fun! bilbo