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From: Terry Lambert <tlambert@primenet.com>
Message-Id: <200011082005.NAA24535@usr08.primenet.com>
Subject: Re: system call and SYSCALL macro
To: rlaw@vt.edu (Raymond Law)
Date: Wed, 8 Nov 2000 20:05:12 +0000 (GMT)
Cc: freebsd-hackers@FreeBSD.ORG
In-Reply-To: <4.3.0.20001106131527.00df0cc0@mail.vt.edu> from "Raymond Law" at Nov 06, 2000 01:16:51 PM
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> I just try to add a simple system call for testing:
> 
> int my_call(int x, int y) {
>    return (x + y);
> }
> 
> In my user program:
> 
> int main(int argc, char ** argv) {
>    int x = 3;
>    int y = 8;
>    int z = 0;
>    z = syscall(SYS_my_call, x, y);
>    printf("%i + %i = %i\n", x, y, z);
>    return 0;
> }
> 
> But it prints:
> 
> 3 + 8 = -1
> 
> Can you give me some idea what I did wrong and what I should do? Also, can 
> you tell me how to use the SYSCALL macro because I am not familiar with 
> macros at all? Thanks in advance.

Change this to:

int
my_call(int x, int y, int *result)
{
	*result = x + y;
	return( 0);
}

int
main( int ac, char **av)
{
	int x = 3;
	int y = 8;
	int z = 0;
	if( syscall(SYS_my_call, x, y, &z) != 0) {
		perror("system call failed!");
		exit( 1);
	}
	printf("%i + %i = %i\n", x, y, z);
	return 0;
}

---

Since you aren't checking for failure of your system call, you
are confusing a failure of the call with an incorrect result.

I suspect you will see the error "ENOENT", meaning that your
system call has not been loaded into the kernel, and had its
function pointer inserted into sysent[ SYS_my_call].

Look at the KLD documentation and examples on how to load
system calls into the kernel.  see also "kldstat", which
should tell you where your system call was loaded (i.e. it
will give you the value you need to pass as "SYS_my_call"
to syscall(2) for things to work).


					Terry Lambert
					terry@lambert.org
---
Any opinions in this posting are my own and not those of my present
or previous employers.


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