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.\"
.\" 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.
.\"
.\" Tue Jul 6 12:42:46 MDT 1993 <dminer@nyx.cs.du.edu>
.\" Added "Calling Directly" and supporting paragraphs
.\"
.\" Modified Sat Jul 24 15:19:12 1993 by Rik Faith <faith@cs.unc.edu>
.\"
.\" Modified 21 Aug 1994 by Michael Chastain <mec@shell.portal.com>:
.\" Added explanation of arg stacking when 6 or more args.
.\"
.\" Modified 10 June 1995 by Andries Brouwer <aeb@cwi.nl>
.\"
.TH INTRO 2 "22 May 1996" "Linux 1.2.13" "Linux Programmer's Manual"
.SH NAME
intro \- Introduction to system calls
.SH DESCRIPTION
This chapter describes the Linux system calls.
For a list of the 164 syscalls present in Linux 2.0, see syscalls(2).
.SS "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.
.SS "Synopsis"
.B #include <linux/unistd.h>
A _syscall macro
desired system call
.SS 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:
.sp
.RS
.RI _syscall X ( type , name , type1 , arg1 , type2 , arg2 ,...)
.RS
.HP
where \fIX\fP is 0\(en5, which are the number of arguments taken by the
system call
.HP
\fItype\fP is the return type of the system call
.HP
\fIname\fP is the name of the system call
.HP
\fItypeN\fP is the Nth argument's type
.HP
\fIargN\fP is the name of the Nth argument
.RE
.RE
.sp
These macros create a function called \fIname\fP with the arguments you
specify. Once you include the _syscall() in your source file,
you call the system call by \fIname\fP.
.SH EXAMPLE
.nf
.sp
#include <stdio.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 = %ds\\nLoad: 1 min %d / 5 min %d / 15 min %d\\n"
"RAM: total %d / free %d / shared %d\\n"
"Memory in buffers = %d\\nSwap: total %d / free %d\\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);
return(0);
}
.fi
.SH "Sample Output"
.nf
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
.fi
.SH NOTES
The _syscall() macros DO NOT produce a prototype. You may have to
create one, especially for C++ users.
.sp
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., \-\fBEPERM\fP.
The _syscall() macros will return the result \fIr\fP of the system call
when \fIr\fP is nonnegative, but will return \-1 and set the variable
.I errno
to \-\fIr\fP when \fIr\fP is negative.
For the error codes, see
.BR errno (3).
.sp
Some system calls, such as
.BR mmap ,
require more than five arguments. These are handled by pushing the
arguments on the stack and passing a pointer to the block of arguments.
.sp
When defining a system call, the argument types MUST be passed by-value
or by-pointer (for aggregates like structs).
.SH CONFORMING TO
Certain codes are used to indicate Unix variants and standards to
which calls in the section conform. These are:
.TP
SVr4
System V Release 4 Unix, as described in the "Programmer's Reference
Manual: Operating System API (Intel processors)" (Prentice-Hall
1992, ISBN 0-13-951294-2)
.TP
SVID
System V Interface Definition, as described in "The System V Interface
Definition, Fourth Edition", available at
ftp://ftp.fpk.novell.com/pub/unix\-standards/svid in Postscript files.
.TP
POSIX.1
IEEE 1003.1-1990 part 1, aka ISO/IEC 9945-1:1990s, aka "IEEE Portable
Operating System Interface for Computing Environments", as elucidated
in Donald Lewine's "POSIX Programmer's Guide" (O'Reilly & Associates,
Inc., 1991, ISBN 0-937175-73-0.
.TP
POSIX.1b
IEEE Std 1003.1b-1993 (POSIX.1b standard) describing real-time facilities
for portable operating systems, aka ISO/IEC 9945-1:1996, as elucidated in
"Programming for the real world \- POSIX.4"
by Bill O. Gallmeister (O'Reilly & Associates, Inc. ISBN 1\-56592\-074\-0).
.TP
SUS, SUSv2
Single Unix Specification.
(Developed by X/Open and The Open Group. See also
http://www.UNIX-systems.org/version2/ .)
.TP
4.3BSD/4.4BSD
The 4.3 and 4.4 distributions of Berkeley Unix. 4.4BSD was
upward-compatible from 4.3.
.TP
V7
Version 7, the ancestral Unix from Bell Labs.
.SH FILES
.I /usr/include/linux/unistd.h
.SH "SEE ALSO"
.BR errno (3)
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