System calls

                                           System calls

A system call is usually a request to the operating system (kernel) to do a hardware/systemspecific
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 < linux/unistd.h > while < sys/syscall.h > 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):
#include <syscall.h>
extern int syscall(int, ...);
int my_close(int filedescriptor)
{
return syscall(SYS_close, filedescriptor);
}
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 < asm/unistd.h > 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.
#include <linux/unistd.h>
_syscall1(int, close, int, filedescriptor);
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.
The following system calls that are available on BSD and SYS V are not available on
Linux:
audit(), auditon(), auditsvc(), fchroot(), getauid(), getdents(), getmsg(), mincore(), poll(),
putmsg(), setaudit(), setauid().
Sven Goldt The Linux Programmer’s Guide