L4Re – L4 Runtime Environment

This example shows how two threads can exchange data using the L4 IPC mechanism. One thread is sending an integer to the other thread which is returning the square of the integer. Both values are printed.

* (c) 2008-2009 Author(s)
* economic rights: Technische Universität Dresden (Germany)
* This file is part of TUD:OS and distributed under the terms of the
* GNU General Public License 2.
* Please see the COPYING-GPL-2 file for details.
#include <l4/sys/ipc.h>
#include <pthread-l4.h>
#include <unistd.h>
#include <stdio.h>
static pthread_t t2;
/* Thread1 is the initiator thread, i.e. it initiates the IPC calls. In
* other words, it takes the client role. It uses L4 IPC mechanisms to send
* an integer value to thread2 and received a calculation result back. */
static void *thread1_fn(void *arg)
int ipc_error;
unsigned long value = 1;
while (1)
printf("Sending: %ld\n", value);
/* Store the value which we want to have squared in the first message
* register of our UTCB. */
l4_utcb_mr()->mr[0] = value;
/* To an L4 IPC call, i.e. send a message to thread2 and wait for a
* reply from thread2. The '1' in the msgtag denotes that we want to
* transfer one word of our message registers (i.e. MR0). No timeout. */
tag = l4_ipc_call(pthread_l4_cap(t2), l4_utcb(),
l4_msgtag(0, 1, 0, 0), L4_IPC_NEVER);
/* Check for IPC error, if yes, print out the IPC error code, if not,
* print the received result. */
ipc_error = l4_ipc_error(tag, l4_utcb());
if (ipc_error)
fprintf(stderr, "thread1: IPC error: %x\n", ipc_error);
printf("Received: %ld\n", l4_utcb_mr()->mr[0]);
/* Wait some time and increment our value. */
return NULL;
/* Thread2 is in the server role, i.e. it waits for requests from others and
* sends back the calculation results. */
static void *thread2_fn(void *arg)
l4_umword_t label;
int ipc_error;
/* Wait for requests from any thread. No timeout, i.e. wait forever. */
tag = l4_ipc_wait(l4_utcb(), &label, L4_IPC_NEVER);
while (1)
/* Check if we had any IPC failure, if yes, print the error code
* and just wait again. */
ipc_error = l4_ipc_error(tag, l4_utcb());
if (ipc_error)
fprintf(stderr, "thread2: IPC error: %x\n", ipc_error);
tag = l4_ipc_wait(l4_utcb(), &label, L4_IPC_NEVER);
/* So, the IPC was ok, now take the value out of message register 0
* of the UTCB and store the square of it back to it. */
l4_utcb_mr()->mr[0] = l4_utcb_mr()->mr[0] * l4_utcb_mr()->mr[0];
/* Send the reply and wait again for new messages.
* The '1' in the msgtag indicated that we want to transfer 1 word in
* the message registers (i.e. MR0) */
tag = l4_ipc_reply_and_wait(l4_utcb(), l4_msgtag(0, 1, 0, 0),
&label, L4_IPC_NEVER);
return NULL;
int main(void)
// We will have two threads, one is already running the main function, the
// other (thread2) will be created using pthread_create.
if (pthread_create(&t2, NULL, thread2_fn, NULL))
fprintf(stderr, "Thread creation failed\n");
return 1;
// Just run thread1 in the main thread
return 0;
unsigned long l4_umword_t
Unsigned machine word.
Definition: l4int.h:51
l4_msgtag_t l4_ipc_reply_and_wait(l4_utcb_t *utcb, l4_msgtag_t tag, l4_umword_t *label, l4_timeout_t timeout) L4_NOTHROW
Reply and wait operation (uses the reply capability).
Definition: ipc.h:471
l4_msgtag_t l4_ipc_call(l4_cap_idx_t object, l4_utcb_t *utcb, l4_msgtag_t tag, l4_timeout_t timeout) L4_NOTHROW
Object call (usual invocation).
Definition: ipc.h:463
l4_msgtag_t l4_ipc_wait(l4_utcb_t *utcb, l4_umword_t *label, l4_timeout_t timeout) L4_NOTHROW
Wait for an incoming message from any possible sender.
Definition: ipc.h:496
l4_umword_t l4_ipc_error(l4_msgtag_t tag, l4_utcb_t *utcb) L4_NOTHROW
Get the error code for an object invocation.
Definition: ipc.h:518
l4_msgtag_t l4_msgtag(long label, unsigned words, unsigned items, unsigned flags) L4_NOTHROW
Create a message tag from the specified values.
Definition: types.h:408
l4_msg_regs_t * l4_utcb_mr(void) L4_NOTHROW L4_PURE
Get the message-register block of a UTCB.
Definition: utcb.h:352
l4_utcb_t * l4_utcb(void) L4_NOTHROW L4_PURE
Get the UTCB address.
Definition: utcb.h:340
Message tag data structure.
Definition: types.h:160
l4_umword_t mr[L4_UTCB_GENERIC_DATA_SIZE]
Message registers.
Definition: utcb.h:80