This chapter describes the Linux system calls. For a
list of the 164 syscalls present in Linux 2.0, see
syscalls(2).
Calling
Directly
In most cases, it is unnecessary to invoke a system
call directly, but there are times when the Standard C
library does not implement a nice function call for you.
In this case, the programmer must manually invoke the
system call using either one of the _syscall macros, or
syscall(). The latter
technique is described in syscall(2). This page
describes the _syscall macros, and includes some notes on
when to use one or other mechanism.
Synopsis
#include
<linux/unistd.h>
A _syscall macro
desired system call
Setup
The important thing to know about a system call is its
prototype. You need to know how many arguments, their
types, and the function return type. There are six macros
that make the actual call into the system easier. They
have the form:
_syscallX(type,name,type1,arg1,type2,arg2,...)
where X is
0–5, which are the number of arguments
taken by the system call
type
is the return type of the system call
name
is the name of the system call
typeN is the Nth
argument's type
argN
is the name of the Nth argument
These macros create a function called name with the arguments
you specify. Once you include the _syscall() in your
source file, you call the system call by name.
FILES
/usr/include/linux/unistd.h
CONFORMING TO
Certain codes are used to indicate Unix variants and
standards to which calls in the section conform. See
standards(7).
NOTES
The _syscall() macros DO NOT produce a prototype. You
may have to create one, especially for C++ users.
System calls are not required to return only positive
or negative error codes. You need to read the source to
be sure how it will return errors. Usually, it is the
negative of a standard error code, e.g.,
−EPERM. The
_syscall() macros will return the result r of the system call when
r is
non-negative, but will return −1 and set the
variable errno to
−r when
r is negative.
For the error codes, see errno(3).
Some system calls, such as mmap(2), require more
than five arguments. These are handled by pushing the
arguments on the stack and passing a pointer to the block
of arguments.
When defining a system call, the argument types MUST
be passed by-value or by-pointer (for aggregates like
structs).
The preferred way to invoke system calls that glibc
does not know about yet is via syscall(2). However,
this mechanism can only be used if using a libc (such as
glibc) that supports syscall(2), and if the
<sys/syscall.h>
header file contains the required SYS_foo definition.
Otherwise, the use of a _syscall macro is required.
Some architectures, notably ia64, do not provide the
_syscall macros. On these architectures, syscall(2) must be
used.
EXAMPLE
#include <stdio.h>
#include <errno.h>
#include <linux/unistd.h> /* for _syscallX macros/related stuff */
#include <linux/kernel.h> /* for struct sysinfo */
_syscall1(int, sysinfo, struct sysinfo *, info);
/* Note: if you copy directly from the nroff source, remember to
REMOVE the extra backslashes in the printf statement. */
int
main(void)
{
struct sysinfo s_info;
int error;
error = sysinfo(&s_info);
printf("code error = %d\n", error);
printf("Uptime = %lds\nLoad: 1 min %lu / 5 min %lu / 15 min %lu\n"
"RAM: total %lu / free %lu / shared %lu\n"
"Memory in buffers = %lu\nSwap: total %lu / free %lu\n"
"Number of processes = %d\n",
s_info.uptime, s_info.loads[0],
s_info.loads[1], s_info.loads[2],
s_info.totalram, s_info.freeram,
s_info.sharedram, s_info.bufferram,
s_info.totalswap, s_info.freeswap,
s_info.procs);
exit(EXIT_SUCCESS);
}
Sample
Output
code error = 0
uptime = 502034s
Load: 1 min 13376 / 5 min 5504 / 15 min 1152
RAM: total 15343616 / free 827392 / shared 8237056
Memory in buffers = 5066752
Swap: total 27881472 / free 24698880
Number of processes = 40
Copyright (c) 1993 Michael Haardt (michael@moria.de),
Fri Apr 2 11:32:09 MET DST 1993
This is free documentation; you can redistribute it and/or
modify it under the terms of the GNU General Public License as
published by the Free Software Foundation; either version 2 of
the License, or (at your option) any later version.
The GNU General Public License's references to "object code"
and "executables" are to be interpreted as the output of any
document formatting or typesetting system, including
intermediate and printed output.
This manual is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public
License along with this manual; if not, write to the Free
Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111,
USA.
