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16 years ago
<?php



//how to calculate the  minimum $memsize required to store the variable $foo where $foo='foobar'.



// when shm_attach() is called for the first time, PHP writes a header to the beginning of the shared memory. 

$shmHeaderSize = (PHP_INT_SIZE * 4) + 8;



// when shm_put_var() is called, the variable is serialized and a small header is placed in front of it before it is written to shared memory.

$shmVarSize = (((strlen(serialize($foo))+ (4 * PHP_INT_SIZE)) /4 ) * 4 ) + 4;



// now add the two together to get the total memory required. Of course, if you are storing more than one variable then you dont need to add $shmHeaderSize for each variable, only add it once.

$memsize = $shmHeaderSize + $shmVarSize;



//this will give you just enough memory to store the one variable using shm_put_var().

$shm_id = shm_attach ( $key, $memsize, 0666 ) ;

shm_put_var  ( $shm_id  , $variable_key  , $foo  );



any attempt to store another variable will result in a 'not enough memory' error.

Be aware that if you change the contents of $foo to a larger value and then you try to write it to shared memory again using shm_put_var(), then you will get a 'not enough memory' error. In this case, you will have to resize your shared memory segment and then write the new value.



If you are only storing variables that contain a single integer value, then you can avoid having to resize by always allocating the largest amount of memory that is required to store an int, which should be:

$shmIntVarSize = (((strlen(serialize(PHP_INT_MAX))+ (4 * PHP_INT_SIZE)) /4 ) * 4 ) + 4;



?>
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