On Tue, Oct 28, 2025 at 01:26:05PM -0700, Andrew Morton wrote:
On Tue, 28 Oct 2025 22:28:10 +0530 Ankit Khushwaha ankitkhushwaha.linux@gmail.com wrote:
@@ -236,7 +237,8 @@ TEST_F(user, perf_empty_events) { ASSERT_EQ(1 << reg.enable_bit, self->check); /* Ensure write shows up at correct offset */
- ASSERT_NE(-1, write(self->data_fd, ®.write_index,
- memcpy(&write_index, ®.write_index, sizeof(reg.write_index));
- ASSERT_NE(-1, write(self->data_fd, &write_index, sizeof(reg.write_index)));
Simply casting &write_index to void* would fix this?
yes, this hides the type mismatch from the compiler. But i think casting to void * will not fix the alignment mismatch for packed struct. It works on x86, but might break on other platform.
It's the second argument to write(2)! write(2) expects a const char *, but void* will work.
Hi Andrew, Indeed `ASSERT_NE(-1, write(self->data_fd, (void *)®.write_index, sizeof(reg.write_index)));`
would work. However since `reg` is packed struct, directly taking the address of its member `®.write_index` may lead to unaligned access on some architectures. as indicated by the compiler warning
perf_test.c:239:38: warning: taking address of packed member 'write_index' of class or structure 'user_reg' may result in an unaligned pointer value [-Waddress-of-packed-member] 239 | ASSERT_NE(-1, write(self->data_fd, ®.write_index, | ^~~~~~~~~~~~~~~
Using `memcpy` avoids this by performing a byte-wise copy, which is safe to use for packed structures.
Thanks -- Ankit