On Tue, May 5, 2015 at 4:22 PM, Michael Jones
In my case, the FFI calls ioctl, which calls i2cdev_ioctl, which calls i2cdev_ioctl_rdwr.
The only function that can assume anything about the structure is i2cdev_ioctl_rdwr, which as you can see below copies the data, but does not free it.
This is a useful bit of context: generally the kernel does not use passed-in user pointers after a system call returns, so you might not need malloc/free at all. Consider this C code sample (don't know anything about this particular API, so the example can be horribly wrong): int some_i2c_bus_transaction(int i2c_dev_fd) { struct i2c_rdwr_ioctl_data req; struct i2c_msg msgs[2]; __u8 read_buf[64], write_buf[8]; write_buf[0] = 0x12; write_buf[1] = 0x34; write_buf[2] = 0x56; msgs[0].flags = I2C_M_RD; msgs[0].buf = write_buf; msgs[0].len = 3; msgs[1].buf = read_buf; msgs[1].len = sizeof(read_buf); req.nmsgs = 2; req.msgs = msgs; return ioctl(i2c_dev_fd, I2C_RDWR, &req); } If your usage is similar to this, you might consider using alloca* functions which are closer to C's automatic stack allocations than heavy-weight "low-level" heap allocations. See the first example in https://wiki.haskell.org/Foreign_Function_Interface#Memory_allocation -- Tolik