As part of tracking down an elusive problem, I wrote a DTrace script to
monitor writes to stdout, which in my case is redirected to a file, so
that file offsets are meaningful.
The script is simple. It catches the first write to descriptor 1 and
sets a global variable to turn on the remaining two clauses. Those
clauses report write results and any other system calls that the target
process makes, Here it is, followed by the first part of its output
(reformatted for compactness).
----------------------------------------------------------------
syscall::write:entry
/pid == $target && arg0 == 1/
{
interesting = 1;
printf("writing 0x%x bytes to stdout at 0x%x\n",
arg2, fds[1].fi_offset);
}
syscall::write:return
/pid == $target && interesting == 1/
{
printf("wrote 0x%x bytes\n", arg0);
}
syscall:::entry
/pid == $target && interesting == 1/
{
}
----------------------------------------------------------------
CPU ID FUNCTION:NAME
0 1408 write:entry writing 0x100000 bytes to stdout at 0x0
0 1408 write:entry
0 1409 write:return wrote 0x100000 bytes
0 1408 write:entry writing 0x100000 bytes to stdout at 0x100000
0 1408 write:entry
0 1409 write:return wrote 0x100000 bytes
0 1408 write:entry writing 0x100000 bytes to stdout at 0x100000
0 1408 write:entry
0 1694 lwp_cond_wait:entry
0 1409 write:return wrote 0x100000 bytes
0 1408 write:entry writing 0x100000 bytes to stdout at 0x100000
0 1408 write:entry
0 1409 write:return wrote 0x100000 bytes
0 1408 write:entry writing 0x100000 bytes to stdout at 0x100000
0 1408 write:entry
0 1409 write:return wrote 0x100000 bytes
----------------------------------------------------------------
The target process does a sequence of writes to stdout, which the
output excerpt confirms. The offset reported for the writes starts at
0 and increases to by the write size to 0x100000. But then it sticks
at 0x100000 and no longer increases by the size of subsequent writes.
This is certainly not what I expected!
The program's output in this case is correct, so the file offsets must
have actually increased as expected. So there must be something wrong
with my script. But what? Can anybody help?
-- Glenn Skinner
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