On 05/15/2014 06:46 AM, Michael Kerrisk (man-pages) wrote: > > People have a number of times noted that there are problems > with syscall(), but I'm not knowledgeable on the details. > I'd happily take a patch to the man page (which, for historical > reasons, is actually syscall(2)) that explains the the problems > (and ideally notes those platforms where there are no problems). > It has to do with how ABIs deal with doublewidth arguments. There is a reason why Linux syscall ABIs generally have a 1:1 mapping with the user space ABIs, and why the system call argument is passed not in the first argument but in a different place (usually a separately clobbered register, e.g. %eax on x86-64). On some platforms, doublewidth registers have to be aligned in register pairs. On some other platforms, enough arguments mean some will be passed in memory, where they are forced to be aligned, or they are not allowed to straddle the register-memory boundary. All of this means that padding words might be introduced, and they will be introduced in the wrong place because of the additional argument introduced at the beginning of the argument sequence. On the other hand, the old SYSCALL user-space macros just plain didn't handle doubleword arguments. -hpa -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
