(PHP 4 >= 4.0.4, PHP 5, PHP 7, PHP 8)

shmop_readRead data from shared memory block


shmop_read(Shmop $shmop, int $offset, int $size): string

shmop_read() will read a string from shared memory block.



The shared memory block identifier created by shmop_open()


Offset from which to start reading; must be greater than or equal to zero and less than or equal to the actual size of the shared memory segment.


The number of bytes to read; must be greater than or equal to zero, and the sum of offset and size must be less than or equal to the actual size of the shared memory segment. 0 reads shmop_size($shmid) - $start bytes.

Return Values

Returns the data or false on failure.


If offset or size are out of range, a ValueError is thrown.


Version Description
8.0.0 shmop expects a Shmop instance now; previously, a resource was expected.
8.0.0 If offset or size are out of range, a ValueError is thrown; previously E_WARNING was emitted, and false was returned,


Example #1 Reading shared memory block

= shmop_read($shm_id, 0, 50);

This example will read 50 bytes from shared memory block and place the data inside $shm_data.

See Also

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User Contributed Notes 5 notes

Craig Manley
18 years ago
shmop_read() reads and returns the whole memory segment's data. This is not useful if you're just working with strings. If you need to read a string from shared memory, call str_from_mem() on the result of shmop_read(). Similarly when writing strings to memory (instead of binary data), null terminate your strings with str_to_nts() before passing the value on to shmop_write().

function str_to_nts($value) {
  return "$value\0";

function str_from_mem(&$value) {
  $i = strpos($value, "\0");
  if ($i === false) {
    return $value;
  $result =  substr($value, 0, $i);
  return $result;
macmaster at pobox dot com
22 years ago
When i need to read the whole string at that shm pointer, setting the count parameter to zero (0) seems work for me.
michael dot wuertz at six dot de
1 year ago
With shmop_read, you probably get a "\0" - padded string.

$zero_padded = shmop_read($shm_seg, 0, 128);

$usable_string = rtrim($zero_padded, "\0");
slavapl at mailandnews dot com
21 years ago
Also you can use the shmop_size() function to determine the block size.
Milan Cvejic
14 years ago
You should always serialize data written in shared memory.
And when you are reading data you should always unserialize.


= 'test';
$shm_bytes_written = shmop_write($shm_id, serialize($data), 0);
$shm_data = unserialize(shmop_read($shm_id, 0, $shm_bytes_written));
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