#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <assert.h>
#include <linux/if_ether.h>
#include <linux/if_packet.h>
#include <sys/mman.h>

int main()
{
    int sock;
    int ret;
    void *mem;
    size_t mapsize;
    struct tpacket_req req;
  
    sock = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_IP));
    assert(sock != - 1);

    memset(&req, 0, sizeof(req));
    
    req.tp_block_size = 4096;
    req.tp_frame_size = 2048;
    req.tp_block_nr = 4;
    req.tp_frame_nr = 8;

    ret = setsockopt(sock, SOL_SOCKET, PACKET_RX_RING, (void *) &req, sizeof(req));
    assert(ret == 0);

    mapsize = req.tp_block_nr * req.tp_block_size;
    mem = mmap(0, mapsize, PROT_READ | PROT_WRITE, MAP_SHARED, sock, 0 );
    assert(mem != MAP_FAILED);
    
    //...
    //poll()
    //...
    
    close(sock);
    return 0;
}

/*

.config:
[...]
CONFIG_MMU=y
CONFIG_PACKET=y
CONFIG_PACKET_MMAP=y
[...]

output:

$ uname -a
Linux darkstar-laptop 2.6.30.3 #1 SMP Thu Jul 30 23:00:10 CEST 2009 i686 GNU/Linux

$ gcc -Wall ./test_mmap.c
$ sudo ./a.out 
a.out: ./test_mmap.c:34: main: Assertion `mem != ((void *) -1)' failed.
Aborted

*/

------snip-------------------------------------->
strace says:

execve("./a.out", ["./a.out"], [/* 38 vars */]) = 0
brk(0)                                  = 0x87d9000
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
mmap2(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb8012000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY)      = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=80440, ...}) = 0
mmap2(NULL, 80440, PROT_READ, MAP_PRIVATE, 3, 0) = 0xb7ffe000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/tls/i686/cmov/libc.so.6", O_RDONLY) = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0\260e\1"..., 512) = 512
fstat64(3, {st_mode=S_IFREG|0755, st_size=1364388, ...}) = 0
mmap2(NULL, 1369712, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0xb7eaf000
mmap2(0xb7ff8000, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x149) = 0xb7ff8000
mmap2(0xb7ffb000, 9840, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0xb7ffb000
close(3)                                = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7eae000
set_thread_area({entry_number:-1 -> 6, base_addr:0xb7eae6b0, limit:1048575, seg_32bit:1, contents:0, read_exec_only:0, limit_in_pages:1, seg_not_present:0, useable:1}) = 0
mprotect(0xb7ff8000, 4096, PROT_READ)   = 0
munmap(0xb7ffe000, 80440)               = 0
socket(PF_PACKET, SOCK_RAW, 8)          = 3
setsockopt(3, SOL_SOCKET, SO_DONTROUTE, "\0\20\0\0\4\0\0\0\0\10\0\0\10\0\0\0", 16) = 0
mmap2(NULL, 16384, PROT_READ|PROT_WRITE, MAP_SHARED, 3, 0) = -1 EINVAL (Invalid argument)
brk(0)                                  = 0x87d9000
brk(0x87fa000)                          = 0x87fa000
write(2, "a.out: ./test_mmap.c:34: main: A"..., 72a.out: ./test_mmap.c:34: main: Assertion `mem != ((void *) -1)' failed.
) = 72
rt_sigprocmask(SIG_UNBLOCK, [ABRT], NULL, 8) = 0
gettid()                                = 6846
tgkill(6846, 6846, SIGABRT)             = 0
--- SIGABRT (Aborted) @ 0 (0) ---
+++ killed by SIGABRT +++