How to Use FFI::memcpy in PHP to Copy an Array of C Structs: A Practical Example

gitbox 2025-06-06

1. Environment Setup

Make sure your PHP installation has the FFI extension enabled. You can do this by modifying the php.ini file and adding the following line:

ffi.enable=1

Next, we need a C struct definition, for example:

typedef struct {  
    int id;  
    float value;  
} Item;

We'll define this struct in PHP using a C declaration and work with an array of these structs.

2. Declaring the Struct in PHP with FFI

$ffi = FFI::cdef("  
    typedef struct {  
        int id;  
        float value;  
    } Item;  
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", "libc.so.6"); // Path to libc on Linux; on Windows, use "msvcrt.dll" instead

3. Creating and Initializing the Struct Array

We create a struct array and initialize several elements.

$length = 3;  
$items1 = $ffi->new("Item[$length]");  
<p>$items1[0]->id = 1;<br>
$items1[0]->value = 3.14;</p>
<p>$items1[1]->id = 2;<br>
$items1[1]->value = 2.71;</p>
<p>$items1[2]->id = 3;<br>
$items1[2]->value = 1.62;<br>

4. Copying the Struct Array Using FFI::memcpy

We first allocate memory for the target array, then use memcpy to perform the memory copy.

$items2 = $ffi->new("Item[$length]");  
<p>// Calculate total bytes: size of one struct times the number of elements<br>
$size = FFI::sizeof($items1);</p>
<p>// Use memcpy to copy memory<br>
$ffi->memcpy(FFI::addr($items2[0]), FFI::addr($items1[0]), $size);<br>

5. Verifying the Copy Result

for ($i = 0; $i < $length; $i++) {  
    echo "Item {$i}:\n";  
    echo "  ID: " . $items2[$i]->id . "\n";  
    echo "  Value: " . $items2[$i]->value . "\n";  
}

The output should be:

Item 0:  
  ID: 1  
  Value: 3.14  
Item 1:  
  ID: 2  
  Value: 2.71  
Item 2:  
  ID: 3  
  Value: 1.62

6. Notes and Considerations

memcpy copies memory byte by byte, so ensure the destination memory is sufficiently allocated.
FFI::sizeof can safely be used to get the byte size of the array.
If you want to run PHP FFI code via script, remember to use php -dffi.enable=1 yourfile.php on the command line.
On some platforms (like macOS or Windows), the C standard library file name differs, so you’ll need to modify the cdef library path accordingly.

FFI::memcpy