RustPython
diff --git a/‎crates/capi/src/buffer.rs‎
Lines changed: 9 additions & 28 deletions b/‎crates/capi/src/buffer.rs‎
Lines changed: 9 additions & 28 deletions
@@ -331,36 +331,14 @@ pub unsafe extern "C" fn PyBuffer_FillInfo(
 
 #[unsafe(no_mangle)]
 pub unsafe extern "C" fn PyBuffer_SizeFromFormat(format: *const c_char) -> isize {
-    if format.is_null() {
-        return -1;
-    }
     with_vm(|vm| {
         let fmt = unsafe { format.try_as_str(vm) }?;
-        // Basic format string size parsing following struct module rules
-        let mut size: isize = 0;
-        let mut count: isize = 0;
-        for ch in fmt.chars() {
-            match ch {
-                '@' | '=' | '<' | '>' | '!' | ' ' => continue,
-                '0'..='9' => {
-                    let digit = (ch as u8 - b'0') as isize;
-                    count = count * 10 + digit;
-                }
-                _ => {
-                    let item_count = if count == 0 { 1 } else { count };
-                    count = 0;
-                    let elem_size = match ch {
-                        'x' | 'c' | 'b' | 'B' | '?' => 1,
-                        'h' | 'H' | 'e' => 2,
-                        'i' | 'I' | 'l' | 'L' | 'f' => 4,
-                        'q' | 'Q' | 'd' => 8,
-                        'P' => core::mem::size_of::<*mut c_void>() as isize,
-                        _ => return Err(vm.new_value_error(format!("unknown format code: {ch}"))),
-                    };
-                    size += item_count * elem_size;
-                }
-            }
-        }
+        let struct_mod = vm.import("struct", 0)?;
+        let calcsize = struct_mod.get_attr("calcsize", vm)?;
+        let res = calcsize.call((fmt,), vm)?;
+        let size: isize = res.try_index(vm)?.as_bigint().try_into().map_err(|_| {
+            vm.new_overflow_error("struct.calcsize result too large for isize".to_owned())
+        })?;
         Ok(size)
     })
 }
@@ -423,6 +401,9 @@ mod tests {
 
             let size_double = unsafe { PyBuffer_SizeFromFormat(c"d".as_ptr()) };
             assert_eq!(size_double, 8);
+
+            let null_size = unsafe { PyBuffer_SizeFromFormat(core::ptr::null()) };
+            assert_eq!(null_size, -1);
         });
     }