linux-kernel - Re: OT: calling kernel syscall manually

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Message-Id: <200609130003.12789.arnd@arndb.de>
Date:	Wed, 13 Sep 2006 00:03:12 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Phillip Susi <psusi@....rr.com>
Cc:	David Woodhouse <dwmw2@...radead.org>, guest01 <guest01@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: OT: calling kernel syscall manually

On Tuesday 12 September 2006 22:25, Phillip Susi wrote:
> 
> What do you mean you have removed the ability to make system calls 
> directly?  That makes no sense.  Glibc has to be able to make system 
> calls so you can write your own code that does the same thing if you want.

the header file <asm/unistd.h> that used to provide the necessary _syscallX()
macros doesn't do that any more. You can still use your own copy of the
macros though, like every libc does internally.

> For the OP: you might want to study the glibc sources to see how it 
> implements syscall, and mimic that.  IIRC it involves making an int 80 
> call on i386.
> 

char *pathname = "/tmp/dir";
int mode = 0644;
int result;
__asm__ volatile ("push %%ebx ; movl %2,%%ebx ; int $0x80 ; pop %%ebx"
	: "=a" (result) 
	: "0" (__NR_mkdir),"ri" (pathname),"c" (mode)
	: "memory");

Understanding that inline assembly in detail is beyond what most people
do at university, but interesting nonetheless.

	Arnd <><
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