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>ioctls: Not writing a new system call</H1
><P
> A system call generally looks like this
</P
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><PRE
CLASS="PROGRAMLISTING"
>asmlinkage int sys_mycall(int arg)
{
return 0;
}
</PRE
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><P
> First, in most cases you don't want to create a new system call.
You create a character device and implement an appropriate ioctl
for it. This is much more flexible than system calls, doesn't have
to be entered in every architecture's
<TT
CLASS="FILENAME"
>include/asm/unistd.h</TT
> and
<TT
CLASS="FILENAME"
>arch/kernel/entry.S</TT
> file, and is much more
likely to be accepted by Linus.
</P
><P
> If all your routine does is read or write some parameter, consider
implementing a <TT
CLASS="FUNCTION"
>sysctl</TT
> interface instead.
</P
><P
> Inside the ioctl you're in user context to a process. When a
error occurs you return a negated errno (see
<TT
CLASS="FILENAME"
>include/linux/errno.h</TT
>),
otherwise you return <SPAN
CLASS="RETURNVALUE"
>0</SPAN
>.
</P
><P
> After you slept you should check if a signal occurred: the
Unix/Linux way of handling signals is to temporarily exit the
system call with the <TT
CLASS="CONSTANT"
>-ERESTARTSYS</TT
> error. The
system call entry code will switch back to user context, process
the signal handler and then your system call will be restarted
(unless the user disabled that). So you should be prepared to
process the restart, e.g. if you're in the middle of manipulating
some data structure.
</P
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>if (signal_pending())
return -ERESTARTSYS;
</PRE
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><P
> If you're doing longer computations: first think userspace. If you
<I
CLASS="EMPHASIS"
>really</I
> want to do it in kernel you should
regularly check if you need to give up the CPU (remember there is
cooperative multitasking per CPU). Idiom:
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>if (current->need_resched)
schedule(); /* Will sleep */
</PRE
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><P
> A short note on interface design: the UNIX system call motto is
"Provide mechanism not policy".
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