Hello, I think I have misunderstood StringItem IPC but I don't know where is my mistake. My roottask spawns a thread that sends a stringitem to roottask. I have checked with debugger that this stringitem contains my string : si = L4_StringItem(str.length_trim, (void *) str.c); L4_Clear(&msg); L4_Append(&msg, si); L4_Set_Label(&msg, CALL$PRINT); L4_Load(&msg); L4_Call(L4_Pager()); str is a struct vms$string : struct vms$string { vms$pointer length; vms$pointer length_trim; unsigned char *c; }; My roottask receives this IPC in following function: void sys$loop() { int running; L4_ThreadId_t partner; L4_MsgBuffer_t buffer; L4_MsgTag_t tag; L4_Msg_t msg; running = 1; L4_Accept(L4_MapGrantItems(L4_CompleteAddressSpace) + L4_StringItemsAcceptor); tag = L4_Wait(&partner); while(running) { L4_Clear(&msg); L4_Store(tag, &msg); if ((tag.raw & L4_REQUEST_MASK) == L4_PAGEFAULT) { sys$pagefault(partner, L4_Get(&msg, 0), L4_Get(&msg, 1), tag.raw); } else { switch(L4_Label(tag)) { case CALL$PRINT: notice("Here!\n"); break; default: PANIC(1, notice(IPC_F_UNKNOWN "unknown IPC from $%lX " "with label $%lX\n", L4_ThreadNo(partner), L4_Label(tag))); } } tag = L4_ReplyWait(partner, &partner); } return; } When system boots, I can intercept on serial line: ... %SYS-F-PAGEFLT, pagefault request from $3C at $0000000000800110 %SYS-F-PAGEFLT, pagefault request from $3C at $000000000010FFF8 %SYS-F-PAGEFLT, pagefault request from $3C at $00000000008015C0 %SYS-F-PAGEFLT, pagefault request from $3C at $0000000000801C10 Here! %IPC-F-UNKNOWN, unknown IPC from $0 with label $0 Panic at sys$loop, sys/sys_loop.c line 73 Have a nice day ! Backtrace: <00> [$000000000104F5B8] -> $0000000001004ED5 (sys$loop) <01> [$000000000104F838] -> $0000000001005813 (main) Pagefault messages are written by sys$pagefault() subroutine. As sys$loop() function writes 'Here!', it receives an IPC with CALL$PRINT label. First question: I don't know how read this stringitem. I have tried with msgbuff without any result. I suppose I have misunderstood how StringItem IPC works. Second question: I don't understand last line: "%IPC-F-UNKNOWN, unknown IPC from $0 with label $0". I suppose this error comes from a bad stringitem IPC. I haven't found any stringitem IPC example. Help is welcome. Regards, JB