On Aug 26, 2007, at 10:01 PM, Darren Reed wrote:

> Jason Thorpe wrote:
>> On Aug 26, 2007, at 8:29 PM, Thor Lancelot Simon wrote:
>>> Because that's the currently configured limit in syscalls.master.
>>
>> The only reason that's set like that was to make the number of  
>> system call slots a power of two (512 happened to be the closest  
>> power of two).  Making it a power of two enables some optimizations  
>> in the low-level system call handler.
>>
>> There is no harm in simply making it 1024 (aside from some wasted  
>> space).
>
> I looked quickly to see if there was any such beastie but obviously  
> i didn't
> look hard enough (or in the right place.)  Where should I look to  
> see where
> this table is built and used?

makesyscalls.sh builds the system call table from syscalls.master.

It's basically a wrapper around a big awk script, and nsysent is  
processed in the END clause:

         if (nsysent) {
                 if (syscall > nsysent) {
                         printf("%s: line %d: too many syscalls [%d >  
%d]\n", infile, NR, syscall, nsysent)
                         exit 1
                 }
                 while (syscall < nsysent) {
                         printf("\t{ 0, 0, 0,\n\t    %s },\t\t\t/* %d  
= filler */\n", \
                             sys_nosys, syscall) > sysent
                         syscall++
                 }
         }

As you can see, all it does is pad the table out with entries that  
call sys_nosys().  It also defines a constant SYS_NSYSENT, which is  
when used like so in machine-dependent code:

         code &= (SYS_NSYSENT - 1);
         callp += code;

(I.e. no test-and-branch for whether or not the system call number is  
beyond the end of the table.)

-- thorpej