[jim_yang]

#include "amxxmodule.h"
#include <windows.h>
#define HOOK_Spawn 0
void *pOriginalFunction = NULL;
void Hook_Spawn()
{
    //save this pointer
    void *pthis;
    __asm mov pthis, ecx;
    //pre hook
    SERVER_PRINT("::Spawn(void) prehook!\n");
    //call original function
    __asm mov ecx, pthis;
    __asm call [pOriginalFunction];
    //post hook
    SERVER_PRINT("::Spawn(void) posthook!\n");
}
void Hook()
{
    //get classname of the entity you want to hook
    const char *classname = CMD_ARGV(1);
    edict_t *pEdict = CREATE_ENTITY();
    CALL_GAME_ENTITY(PLID, classname, &pEdict->v);
    if(pEdict->pvPrivateData == NULL)
    {
        REMOVE_ENTITY(pEdict);
        return;
    }
    //get this pointer
    void *pthis = pEdict->pvPrivateData;
    //get vtable
    void **vtbl = *(void ***)pthis;
    REMOVE_ENTITY(pEdict);
    if(vtbl == NULL)
        return;
    
    int **ivtbl = (int **)vtbl;
    //get the original function
    pOriginalFunction = (void *)ivtbl[HOOK_Spawn];
    
    //patch original function to our function
    DWORD oldflags;
    if(VirtualProtect(&ivtbl[HOOK_Spawn], sizeof(int *), PAGE_READWRITE, &oldflags))
    {
        ivtbl[HOOK_Spawn] = (int *)Hook_Spawn;
    }
}
void OnMetaAttach()
{
    REG_SVR_COMMAND("hook", Hook);
}

ecx store this pointer, you need to do some calculate to get the edict_t *
in windows this calc is

mov eax, [ecx + 4];  get this->pev
mov eax, [eax + 520]; get pev->pContainingEntity
mov pEdict, eax