Edit report at https://bugs.php.net/bug.php?id=52602&edit=1
ID: 52602
Updated by: kalle@php.net
Reported by: g at gsxm dot net
Summary: fread is blocking even with the use of stream_select
-Status: Verified
+Status: Assigned
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
Previous Comments:
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[2014-07-18 12:18:42] rob at associatedtechs dot com
I just got bit by this exact bug after spending a while trying to figure out what the heck was going
on. PHP version 5.5.10-1, Debian x86_64.
gauthierm's solution prevents fread() from blocking, but I can only get the first 8k chunk.
stream_select() subsequently returns 0 even though there's data left in the stream. I
understand why it does that, but this makes fread() and stream_select() impossible to reliably use
together, which means non-blocking functions have to be used for TLS connections.
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[2013-01-06 09:23:34] gauthierm@php.net
As another workaround, you can use the same chunk size in your read calls as the
PHP stream chunk size. Then there is never buffered data that has not been
consumed. In most cases the chunk size is 8192. In PHP 5.4 I think you can set the
chunk size using stream_set_chunk_size().
This workaround allows you to continue using fread().
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[2013-01-06 09:01:59] gauthierm@php.net
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?
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[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.
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[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.
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