Call Frame Construction with libffi
When PHP code calls a native function through FFI, it relies on the libffi library to bridge the gap between PHP’s runtime and the C function’s calling conventions. libffi provides a portable way to construct call frames—the data structures that hold all arguments, return values, and context needed for a native function call. This is essential because different platforms and architectures have unique requirements for how arguments are passed (in registers or stack), how return values are handled, and how the call stack is managed.
Before making a native call, PHP FFI uses libffi to marshal each argument from PHP’s types (such as integers, floats, or pointers) into the format expected by the native function. This process is called argument marshaling. Each argument is converted based on its declared C type, ensuring correct size, alignment, and representation. The call frame is then constructed: it contains the marshaled arguments in the correct order, a space for the return value, and metadata describing the function signature.
When the call is invoked, libffi assembles the call frame according to the platform’s ABI (Application Binary Interface), executes the function, and then collects the return value, converting it back to a PHP value if needed. This mechanism allows PHP FFI to call arbitrary native functions at runtime without requiring custom extensions or recompilation.
call_c_function.php
123456789101112131415161718<?php // Define a C function in a shared library (assume 'libmath.so' is available): // int add_and_multiply(int a, int b, int c) { return (a + b) * c; } $ffi = FFI::cdef( "int add_and_multiply(int a, int b, int c);", "libmath.so" ); $a = 2; $b = 3; $c = 4; // PHP FFI marshals $a, $b, $c as C ints, constructs the call frame, and invokes the native function. $result = $ffi->add_and_multiply($a, $b, $c); echo "Result: $result\n"; // Output: Result: 20 ?>
A common mistake when using PHP FFI is mismatching argument types or passing them in the wrong order. For instance, if the C function expects an int, float, char*, but you provide a float, int, string, libffi will marshal the arguments incorrectly. This can corrupt the call frame, leading to stack corruption, segmentation faults, or unpredictable crashes. Always ensure your PHP arguments match the exact C types and order declared in the function signature.
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When PHP code calls a native function through FFI, it relies on the libffi library to bridge the gap between PHP’s runtime and the C function’s calling conventions. libffi provides a portable way to construct call frames—the data structures that hold all arguments, return values, and context needed for a native function call. This is essential because different platforms and architectures have unique requirements for how arguments are passed (in registers or stack), how return values are handled, and how the call stack is managed.
Before making a native call, PHP FFI uses libffi to marshal each argument from PHP’s types (such as integers, floats, or pointers) into the format expected by the native function. This process is called argument marshaling. Each argument is converted based on its declared C type, ensuring correct size, alignment, and representation. The call frame is then constructed: it contains the marshaled arguments in the correct order, a space for the return value, and metadata describing the function signature.
When the call is invoked, libffi assembles the call frame according to the platform’s ABI (Application Binary Interface), executes the function, and then collects the return value, converting it back to a PHP value if needed. This mechanism allows PHP FFI to call arbitrary native functions at runtime without requiring custom extensions or recompilation.
call_c_function.php
123456789101112131415161718<?php // Define a C function in a shared library (assume 'libmath.so' is available): // int add_and_multiply(int a, int b, int c) { return (a + b) * c; } $ffi = FFI::cdef( "int add_and_multiply(int a, int b, int c);", "libmath.so" ); $a = 2; $b = 3; $c = 4; // PHP FFI marshals $a, $b, $c as C ints, constructs the call frame, and invokes the native function. $result = $ffi->add_and_multiply($a, $b, $c); echo "Result: $result\n"; // Output: Result: 20 ?>
A common mistake when using PHP FFI is mismatching argument types or passing them in the wrong order. For instance, if the C function expects an int, float, char*, but you provide a float, int, string, libffi will marshal the arguments incorrectly. This can corrupt the call frame, leading to stack corruption, segmentation faults, or unpredictable crashes. Always ensure your PHP arguments match the exact C types and order declared in the function signature.
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