shm and MacOs

Consider me puzzled.
The attached test program runs just fine on MacOS Mavericks; testRock
fails, yet the code looks really the same as in src/ipc/mem/Segment.cc
to me (after removing the O_TRUNC flag to shm_open).. can anyone spot
a difference I can't see?

output:
--------------
mini:shm_test kinkie$ ./shm_test 66584
start
fd: 3
sz: 66584
mmap
mem at 0x10dd9b000
ok!
--------------
testRock:
mini:squid kinkie$ gdb-apple ./src/tests/testRock
[...]

Program received signal EXC_BAD_ACCESS, Could not access memory.
Reason: KERN_INVALID_ADDRESS at address: 0x0000000000000068
0x00007fff92ff8eca in flockfile ()

(gdb) bt
#0 0x00007fff92ff8eca in flockfile ()
#1 0x00007fff93001b10 in vfprintf_l ()
#2 0x00007fff92ffa68e in fprintf ()
#3 0x000000010000b4fb in fatal_common (message=0x1000fb8b0
"Ipc::Mem::Segment::create failed to
ftruncate(/squid-testRock_Store_i.shm, 66584): (22) Invalid
argument\n") at fatal.cc:44
#4 0x000000010000b4ab in fatal (message=0x1000fb8b0
"Ipc::Mem::Segment::create failed to
ftruncate(/squid-testRock_Store_i.shm, 66584): (22) Invalid
argument\n") at fatal.cc:88
#5 0x000000010000b600 in fatalf (fmt=<value temporarily unavailable,
due to optimizations>) at fatal.cc:99
#6 0x00000001000b257b in Ipc::Mem::Segment::create (this=0x100306810,
aSize=66584) at mem/Segment.cc:78
[...]

-- 
    Francesco