PHP read shared memory binary data (uint32 x = shmop_read(...))

Question

I have a shared memory region which I to want access with PHP running in a web server. The shared memory is laid out where the first 4 bytes is a 32-bit unsigned integer which contains how many bytes are valid data within the remaining shared memory region (the data can be variable size), i.e.:

Byte Range           Value
------------------   -----------------------------------------------
0 - 3                32-bit unsigned integer - call it numBytes
4 - (numBytes + 4)   char array - The actual data, total of numBytes

How do I read in the first four bytes as an integer? The only thing I can think of is do a shmop_read($key, 0, 4) and convert the returned string value to an array, then convert that array to an integer as described here. This all seems very messy and I was wondering if there is a cleaner way to do this?

Maybe it's just an unpack("V", shmop_read($key, 0, 4));. But could you post a var_dump(shmop_read($key, 0, 4));? — Jens A. Koch
– Jens A. Koch, Commented Jul 7, 2016 at 23:35
After looking at unpack() I think your answer is correct. I will not be able to test it for a day or so; unfortunately. Could you give your comment as an answer because I cannot mark a comment as an answer. — user2205930
– user2205930, Commented Jul 7, 2016 at 23:50

Jens A. Koch · Accepted Answer · 2016-07-08 00:08:57Z

1

I think it's just an unpack with N or V (depending on endianness).

So, assuming your $numBytesString looks like \xff\xff\xff\x7f? Then i would unpack:

$numBytesString = shmop_read($key, 0, 4);

$numBytesInt = unpack("V", $numBytesString); // assuming little-endianess

// 2147483647

answered Jul 8, 2016 at 0:08

Jens A. Koch

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2 Comments

user2205930 Over a year ago

Thank you! This worked perfectly. The only thing I needed to add was $numBytesInt = (integer) $numBytesInt[1]; because unpack returns an array. Although, I'm new to PHP so that last bit may not be needed.

Jens A. Koch Over a year ago

Amazing. It was just a wild guess. Glad it worked out :)

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