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<H1><A NAME="SECTION00500000000000000000">4 System calls</A></H1>
<P>
A system call is usually a request to the operating system (kernel) to
do a hardware/system-specific or privileged operation. As of
Linux-1.2, 140 system calls have been defined. System calls like
close() are implemented in the Linux libc. This implementation often
involves calling a macro which eventually calls syscall(). Parameters
passed to syscall() are the number of the system call followed by the
needed arguments. The actual system call numbers can be found in
<<I>linux</I>/<I>unistd</I>.<I>h</I>> while <<I>sys</I>/<I>syscall</I>.<I>h</I>> gets updated with a new
libc. If new calls appear that don't have a stub in libc yet, you can
use syscall(). As an example, you can close a file using syscall()
like this (not advised):
<P>
<PRE>#include <syscall.h>
extern int syscall(int, ...);
int my_close(int filedescriptor)
{
return syscall(SYS_close, filedescriptor);
}</PRE>
<P>
On the i386 architecture, system calls are limited to 5 arguments
besides the system call number because of the number of hardware
registers. If you use Linux on another architecture you can check
<<I>asm</I>/<I>unistd</I>.<I>h</I>> for the _syscall macros to see how many arguments
your hardware supports or how many the developers chose to support.
These _syscall macros can be used instead of syscall(), but this is
not recommended since such a macro expands to a full function which
might already exist in a library. Therefore, only kernel hackers
should play with the _syscall macros. To demonstrate, here is the
close() example using a _syscall macro.
<P>
<PRE>#include <linux/unistd.h>
_syscall1(int, close, int, filedescriptor);</PRE>
<P>
The _syscall1 macro expands revealing the close() function. Thus we
have close() twice-once in libc and once in our program. The return
value of syscall() or a _syscall macro is -1 if the system call
failed and 0 or greater on success. Take a look at the global
variable errno to see what happened if a system call failed.
<P>
The following system calls that are available on BSD and SYS V are
not available on Linux: <BR> audit(), auditon(), auditsvc(), fchroot(),
getauid(), getdents(), getmsg(), mincore(), poll(), putmsg(),
setaudit(), setauid().
<P>
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