Edit report at https://bugs.php.net/bug.php?id=52602&edit=1
ID: 52602
Comment by: gauthierm@php.net
Reported by: g at gsxm dot net
Summary: fread is blocking even with the use of stream_select
Status: Verified
Type: Bug
Package: Sockets related
Operating System: Gentoo x86_64
PHP Version: 5.2.14
Assigned To: cataphract
Block user comment: N
Private report: N
New Comment:
This problem affected me while writing a WebSocket server in PHP.
stream_select() returns streams ready for reading and fread() blocks when I try
to read them. What is the point of stream_select() if it behaves like this? How
are you supposed to write a blocking I/O loop?
My solution was to use stream_socket_recvfrom() and give up on implementing
SSL/TLS for my server (making me a bit sad inside).
Since stream_select() returns the stream ready for reading, I would expect
fread() to return the available bytes from the stream buffer in this case rather
than blocking. See Bug 51056 for a long discussion on fread() behaviour with no
conclusion.
Can some conclusion be reached for PHP? Is there any actual evidence of people
writing their servers in a way where fixing fread() will break?
Previous Comments:
------------------------------------------------------------------------
[2010-09-27 04:16:43] cataphract@php.net
To sum up, the emulation behavior of stream_select is worse than useless:
* If using stream_socket_recvfrom(), the buffer will be bypassed anyway, so the emulation behavior
won't kick in.
* If using fread() and there's something in the buffer, it will return the stream in the readfs
set, which is documented "to see if characters become available for reading (more precisely, to
see if a read will not block [...])", but a subsequent fread call will nevertheless eventually
block when trying to fill the buffer.
------------------------------------------------------------------------
[2010-09-27 04:09:08] cataphract@php.net
fread is not really adequate for this because of its buffering (see stream_socket_recvfrom()).
Since you are specifying a length of 128 to fread, if you receive say, a 138 bytes long payload, the
first call to fread will to read into the stream buffer the whole data, but will only return 128
bytes. The stream buffer remains with 10 bytes, but the socket buffer will be empty. On the second
call to fread, it will try to read into the stream buffer again, but this time the socket buffer is
empty, so it will block until it can read another packet or until the timeout.
This appears, however, to be the desired behavior (perhaps fread could just return what it has in
the buffer if it cannot read anything immediately, i.e., if it would otherwise block). So
stream_select implements an emulation behavior that says the stream is readable without calling
select if the there's data in the stream buffer. This is indeed odd, because the usual
semantics of select are that it a read on a socket returned in the readfs set will not block.
------------------------------------------------------------------------
[2010-08-14 05:35:51] g at gsxm dot net
Not sure why I did not think of this before, but perhaps only the stream-* functions works with
stream_select
------------------------------------------------------------------------
[2010-08-14 03:07:41] g at gsxm dot net
Description:
------------
Using PHP CLI
Gentoo 64 bit box
PHP version 5.2.14
clean compile
Dual AMD Opteron 2212 CPU
Something is blocking in the code, it seems to be fread. I am sure this is not the expected result.
I greatly simplified the sample I included.
I can just put the socket/stream in non-blocking mode, but I would much prefer to use the code as I
designed it.
I have considered just using the socket functions in php or just writing the code in C.
Once in a great while it will resume after about thirty to sixty seconds. In non-blocking mode I
have no issues. This is a php cli script that connects to another daemon running on the same
machine, I am using this php script to parse data to place in a sql database.
Test script:
---------------
$sock=@fsockopen("localhost","7777",$errno,$errstr,3);
if (!$sock)
{
echo "sock error".$crlf;
}
while(true)
{
$read=array($sock);
$write=NULL;
$except=NULL;
$status=@stream_select($read,$write,$except,1);
if ($status===false)
{
echo "select error".$crlf;
exit();
}
if ($status>0)
{
$data = @fread($sock,128);
echo $data;
$out .= $data;
}
usleep(10000);
}
Expected result:
----------------
The loop should run for eternity as expected.
Actual result:
--------------
The loop is blocked, I am presuming by fread due to a failure of the expected behavior of
stream_select.
------------------------------------------------------------------------
--
Edit this bug report at https://bugs.php.net/bug.php?id=52602&edit=1