This document explains how to implement a new system call on linux 2.6. A system call is used by application (user) programs to request service from the operating system.

A system call is used by application (user) programs to request service from the operating system. The following statements illustrate why system calls are needed. An operating system can access a system's hardware directly, but a user program is not given direct access to the hardware. This is done so that the kernel can keep the system safe and secure from malicious user programs. But often, a user program requires some information from the hardware (e.g., from a web camera to show you the picture), but it cannot get the information directly. So, it requests the operating system to supply it the information. This request is made by using an appropriate system call.

A system call executes in the kernel mode. Every system call has a number associated with it. This number is passed to the kernel and that's how the kernel knows which system call was made. When a user program issues a system call, it is actually calling a library routine. The library routine issues a trap to the Linux operating system by executing INT 0x80 assembly instruction. It also passes the system call number to the kernel using the EAX register. The arguments of the system call are also passed to the kernel using other registers (EBX, ECX, etc.). The kernel executes the system call and returns the result to the user program using a register. If the system call needs to supply the user program with large amounts of data, it will use another mechanism (e.g., copy_to_user call).

Assume that your linux source base directory is /usr/src/linux.

Kernel files to be modified are listed below:

1. /usr/src/linux/arch/i386/kernel/syscall_table.S

2. /usr/src/linux/include/asm-i386/unistd.h

3. /usr/src/linux/include/linux/syscalls.h

4. /usr/src/linux/Makefile

New kernel files/directories to be created are listed below:

1. /usr/src/linux/mycall - Directory that will contain the source file, header file and the Makefile for our system call (You can also implement your system call in an existing file).

2. /usr/src/linux/mycall/mycall.c - Source file containing our system call code.

3. /usr/src/linux/mycall/Makefile - Makefile

New user space files, to be created, to test our system call are listed below:

1. testmycall.c - Source file that will call our system call.

2. testmycall.h - Header file.

Full path of the file - /usr/src/linux/arch/i386/kernel/syscall_table.S

This file contains system call names.

Full path of the file - /usr/src/linux/include/asm-i386/unistd.h

This file contains the system call number that is passed to the kernel through the register (EAX) when a system call is invoked.

If the last system call defined here is:

"#define __NR_vmsplice316", then add:

"#define __NR_mycall317" at the end of the list.

"#define NR_syscalls 317", then change it to:

"#define NR_syscalls 318"

Full path of the file - /usr/src/linux/include/linux/syscalls.h

This file contain the declarations for system calls.

"asmlinkage long sys_mycall(int i);"

Full path of the file - /usr/src/linux/Makefile

Full path of the file - /usr/src/linux/mycall

Full path of the file - /usr/src/linux/mycall/mycall.c

/*---Start of mycall.c----*/

#include<linux/linkage.h>

asmlinkage long sys_mycall(int i)

{

return i+10;

}

/*---End of mycall.c------*/

What is asmlinkage?

Asmlinkage is used to look for the arguments on the kernel stack.

Full path of the file - /usr/src/linux/mycall/Makefile

The Makefile in dir "mycall" will have only one line:

#####Makefile Start#####

obj-y := mycall.o

#####Makefile End#######

testmycall.h

#include<linux/unistd.h>

#define __NR_mycall 317

_syscall1(long, mycall, int, i)

So, our user header file looks like:

/*---Start of header file------*/

#include<linux/unistd.h>

#define __NR_mycall 317

_syscall1(long, mycall, int, i)

/*---End of header file--------*/

testmycall.c

/*---Start of C file------*/

#include<stdio.h>

#include "testmycall.h"

int main(void)

{

printf("%d\n", mycall(15));

}

/*---End of C file------*/

When you expand _syscall1(long,mycall,int,i), you get the following code:

long mycall(int i)

{

return syscall(__NR_mycall, i);

}

But the definition of _syscallN macros are different in the kernel. You can look at /usr/src/linux/include/asm-i386/unistd.h for the definition.

Copyright (c) 2006 by Amit Choudhary.

Permission is granted to copy, distribute and/or modify this document

under the terms of the GNU Free Documentation License, Version 1.2

or any later version published by the Free Software Foundation;

with no Invariant Sections, no Front-Cover Texts, and no Back-Cover

Texts. A copy of the license is included in the section entitled "GNU

Free Documentation License".

GNU Free Documentation License

Version 1.2, November 2002

Copyright (C) 2000,2001,2002 Free Software Foundation, Inc.

51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA

Everyone is permitted to copy and distribute verbatim copies

of this license document, but changing it is not allowed.

0. PREAMBLE

The purpose of this License is to make a manual, textbook, or other

functional and useful document "free" in the sense of freedom: to

assure everyone the effective freedom to copy and redistribute it,

with or without modifying it, either commercially or noncommercially.

Secondarily, this License preserves for the author and publisher a way

to get credit for their work, while not being considered responsible

for modifications made by others.

This License is a kind of "copyleft", which means that derivative

works of the document must themselves be free in the same sense. It

complements the GNU General Public License, which is a copyleft

license designed for free software.

We have designed this License in order to use it for manuals for free

software, because free software needs free documentation: a free

program should come with manuals providing the same freedoms that the

software does. But this License is not limited to software manuals;

it can be used for any textual work, regardless of subject matter or

whether it is published as a printed book. We recommend this License

principally for works whose purpose is instruction or reference.

1. APPLICABILITY AND DEFINITIONS

This License applies to any manual or other work, in any medium, that

contains a notice placed by the copyright holder saying it can be

distributed under the terms of this License. Such a notice grants a

world-wide, royalty-free license, unlimited in duration, to use that

work under the conditions stated herein. The "Document", below,

refers to any such manual or work. Any member of the public is a

licensee, and is addressed as "you". You accept the license if you

copy, modify or distribute the work in a way requiring permission

under copyright law.

A "Modified Version" of the Document means any work containing the

Document or a portion of it, either copied verbatim, or with

modifications and/or translated into another language.

A "Secondary Section" is a named appendix or a front-matter section of

the Document that deals exclusively with the relationship of the

publishers or authors of the Document to the Document's overall subject

(or to related matters) and contains nothing that could fall directly

within that overall subject. (Thus, if the Document is in part a

textbook of mathematics, a Secondary Section may not explain any

mathematics.) The relationship could be a matter of historical

connection with the subject or with related matters, or of legal,

commercial, philosophical, ethical or political position regarding

them.

The "Invariant Sections" are certain Secondary Sections whose titles

are designated, as being those of Invariant Sections, in the notice

that says that the Document is released under this License. If a

section does not fit the above definition of Secondary then it is not

allowed to be designated as Invariant. The Document may contain zero

Invariant Sections. If the Document does not identify any Invariant

Sections then there are none.

The "Cover Texts" are certain short passages of text that are listed,

as Front-Cover Texts or Back-Cover Texts, in the notice that says that

the Document is released under this License. A Front-Cover Text may

be at most 5 words, and a Back-Cover Text may be at most 25 words.

A "Transparent" copy of the Document means a machine-readable copy,

represented in a format whose specification is available to the

general public, that is suitable for revising the document

straightforwardly with generic text editors or (for images composed of

pixels) generic paint programs or (for drawings) some widely available

drawing editor, and that is suitable for input to text formatters or

for automatic translation to a variety of formats suitable for input

to text formatters. A copy made in an otherwise Transparent file

format whose markup, or absence of markup, has been arranged to thwart

or discourage subsequent modification by readers is not Transparent.

An image format is not Transparent if used for any substantial amount

of text. A copy that is not "Transparent" is called "Opaque".

Examples of suitable formats for Transparent copies include plain

ASCII without markup, Texinfo input format, LaTeX input format, SGML

or XML using a publicly available DTD, and standard-conforming simple

HTML, PostScript or PDF designed for human modification. Examples of

transparent image formats include PNG, XCF and JPG. Opaque formats

include proprietary formats that can be read and edited only by

proprietary word processors, SGML or XML for which the DTD and/or

processing tools are not generally available, and the

machine-generated HTML, PostScript or PDF produced by some word

processors for output purposes only.

The "Title Page" means, for a printed book, the title page itself,

plus such following pages as are needed to hold, legibly, the material

this License requires to appear in the title page. For works in

formats which do not have any title page as such, "Title Page" means

the text near the most prominent appearance of the work's title,

preceding the beginning of the body of the text.

A section "Entitled XYZ" means a named subunit of the Document whose

title either is precisely XYZ or contains XYZ in parentheses following

text that translates XYZ in another language. (Here XYZ stands for a

specific section name mentioned below, such as "Acknowledgements",

"Dedications", "Endorsements", or "History".) To "Preserve the Title"

of such a section when you modify the Document means that it remains a

section "Entitled XYZ" according to this definition.

The Document may include Warranty Disclaimers next to the notice which

states that this License applies to the Document. These Warranty

Disclaimers are considered to be included by reference in this

License, but only as regards disclaiming warranties: any other

implication that these Warranty Disclaimers may have is void and has

no effect on the meaning of this License.

2. VERBATIM COPYING

You may copy and distribute the Document in any medium, either

commercially or noncommercially, provided that this License, the

copyright notices, and the license notice saying this License applies

to the Document are reproduced in all copies, and that you add no other

conditions whatsoever to those of this License. You may not use

technical measures to obstruct or control the reading or further

copying of the copies you make or distribute. However, you may accept

compensation in exchange for copies. If you distribute a large enough

number of copies you must also follow the conditions in section 3.

You may also lend copies, under the same conditions stated above, and

you may publicly display copies.

3. COPYING IN QUANTITY

If you publish printed copies (or copies in media that commonly have

printed covers) of the Document, numbering more than 100, and the

Document's license notice requires Cover Texts, you must enclose the

copies in covers that carry, clearly and legibly, all these Cover

Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on

the back cover. Both covers must also clearly and legibly identify

you as the publisher of these copies. The front cover must present

the full title with all words of the title equally prominent and

visible. You may add other material on the covers in addition.

Copying with changes limited to the covers, as long as they preserve

the title of the Document and satisfy these conditions, can be treated

as verbatim copying in other respects.

If the required texts for either cover are too voluminous to fit

legibly, you should put the first ones listed (as many as fit

reasonably) on the actual cover, and continue the rest onto adjacent

pages.

If you publish or distribute Opaque copies of the Document numbering

more than 100, you must either include a machine-readable Transparent

copy along with each Opaque copy, or state in or with each Opaque copy

a computer-network location from which the general network-using

public has access to download using public-standard network protocols

a complete Transparent copy of the Document, free of added material.

If you use the latter option, you must take reasonably prudent steps,

when you begin distribution of Opaque copies in quantity, to ensure

that this Transparent copy will remain thus accessible at the stated

location until at least one year after the last time you distribute an

Opaque copy (directly or through your agents or retailers) of that

edition to the public.

It is requested, but not required, that you contact the authors of the

Document well before redistributing any large number of copies, to give

them a chance to provide you with an updated version of the Document.

4. MODIFICATIONS

You may copy and distribute a Modified Version of the Document under

the conditions of sections 2 and 3 above, provided that you release

the Modified Version under precisely this License, with the Modified

Version filling the role of the Document, thus licensing distribution

and modification of the Modified Version to whoever possesses a copy

of it. In addition, you must do these things in the Modified Version:

A. Use in the Title Page (and on the covers, if any) a title distinct

from that of the Document, and from those of previous versions

(which should, if there were any, be listed in the History section

of the Document). You may use the same title as a previous version

if the original publisher of that version gives permission.

B. List on the Title Page, as authors, one or more persons or entities

responsible for authorship of the modifications in the Modified

Version, together with at least five of the principal authors of the

Document (all of its principal authors, if it has fewer than five),

unless they release you from this requirement.

C. State on the Title page the name of the publisher of the

Modified Version, as the publisher.

D. Preserve all the copyright notices of the Document.

E. Add an appropriate copyright notice for your modifications

adjacent to the other copyright notices.

F. Include, immediately after the copyright notices, a license notice

giving the public permission to use the Modified Version under the

terms of this License, in the form shown in the Addendum below.

G. Preserve in that license notice the full lists of Invariant Sections

and required Cover Texts given in the Document's license notice.

H. Include an unaltered copy of this License.

I. Preserve the section Entitled "History", Preserve its Title, and add

to it an item stating at least the title, year, new authors, and

publisher of the Modified Version as given on the Title Page. If

there is no section Entitled "History" in the Document, create one

stating the title, year, authors, and publisher of the Document as

given on its Title Page, then add an item describing the Modified

Version as stated in the previous sentence.

J. Preserve the network location, if any, given in the Document for

public access to a Transparent copy of the Document, and likewise

the network locations given in the Document for previous versions

it was based on. These may be placed in the "History" section.

You may omit a network location for a work that was published at

least four years before the Document itself, or if the original

publisher of the version it refers to gives permission.

K. For any section Entitled "Acknowledgements" or "Dedications",

Preserve the Title of the section, and preserve in the section all

the substance and tone of each of the contributor acknowledgements

and/or dedications given therein.

L. Preserve all the Invariant Sections of the Document,

unaltered in their text and in their titles. Section numbers

or the equivalent are not considered part of the section titles.

M. Delete any section Entitled "Endorsements". Such a section

may not be included in the Modified Version.

N. Do not retitle any existing section to be Entitled "Endorsements"

or to conflict in title with any Invariant Section.

O. Preserve any Warranty Disclaimers.

If the Modified Version includes new front-matter sections or

appendices that qualify as Secondary Sections and contain no material

copied from the Document, you may at your option designate some or all

of these sections as invariant. To do this, add their titles to the

list of Invariant Sections in the Modified Version's license notice.

These titles must be distinct from any other section titles.

You may add a section Entitled "Endorsements", provided it contains

nothing but endorsements of your Modified Version by various

parties--for example, statements of peer review or that the text has

been approved by an organization as the authoritative definition of a

standard.

You may add a passage of up to five words as a Front-Cover Text, and a

passage of up to 25 words as a Back-Cover Text, to the end of the list

of Cover Texts in the Modified Version. Only one passage of

Front-Cover Text and one of Back-Cover Text may be added by (or

through arrangements made by) any one entity. If the Document already

includes a cover text for the same cover, previously added by you or

by arrangement made by the same entity you are acting on behalf of,

you may not add another; but you may replace the old one, on explicit

permission from the previous publisher that added the old one.

The author(s) and publisher(s) of the Document do not by this License

give permission to use their names for publicity for or to assert or

imply endorsement of any Modified Version.

5. COMBINING DOCUMENTS

You may combine the Document with other documents released under this

License, under the terms defined in section 4 above for modified

versions, provided that you include in the combination all of the

Invariant Sections of all of the original documents, unmodified, and

list them all as Invariant Sections of your combined work in its

license notice, and that you preserve all their Warranty Disclaimers.

The combined work need only contain one copy of this License, and

multiple identical Invariant Sections may be replaced with a single

copy. If there are multiple Invariant Sections with the same name but

different contents, make the title of each such section unique by

adding at the end of it, in parentheses, the name of the original

author or publisher of that section if known, or else a unique number.

Make the same adjustment to the section titles in the list of

Invariant Sections in the license notice of the combined work.

In the combination, you must combine any sections Entitled "History"

in the various original documents, forming one section Entitled

"History"; likewise combine any sections Entitled "Acknowledgements",

and any sections Entitled "Dedications". You must delete all sections

Entitled "Endorsements".

6. COLLECTIONS OF DOCUMENTS

You may make a collection consisting of the Document and other documents

released under this License, and replace the individual copies of this

License in the various documents with a single copy that is included in

the collection, provided that you follow the rules of this License for

verbatim copying of each of the documents in all other respects.

You may extract a single document from such a collection, and distribute

it individually under this License, provided you insert a copy of this

License into the extracted document, and follow this License in all

other respects regarding verbatim copying of that document.

7. AGGREGATION WITH INDEPENDENT WORKS

A compilation of the Document or its derivatives with other separate

and independent documents or works, in or on a volume of a storage or

distribution medium, is called an "aggregate" if the copyright

resulting from the compilation is not used to limit the legal rights

of the compilation's users beyond what the individual works permit.

When the Document is included in an aggregate, this License does not

apply to the other works in the aggregate which are not themselves

derivative works of the Document.

If the Cover Text requirement of section 3 is applicable to these

copies of the Document, then if the Document is less than one half of

the entire aggregate, the Document's Cover Texts may be placed on

covers that bracket the Document within the aggregate, or the

electronic equivalent of covers if the Document is in electronic form.

Otherwise they must appear on printed covers that bracket the whole

aggregate.

8. TRANSLATION

Translation is considered a kind of modification, so you may

distribute translations of the Document under the terms of section 4.

Replacing Invariant Sections with translations requires special

permission from their copyright holders, but you may include

translations of some or all Invariant Sections in addition to the

original versions of these Invariant Sections. You may include a

translation of this License, and all the license notices in the

Document, and any Warranty Disclaimers, provided that you also include

the original English version of this License and the original versions

of those notices and disclaimers. In case of a disagreement between

the translation and the original version of this License or a notice

or disclaimer, the original version will prevail.

If a section in the Document is Entitled "Acknowledgements",

"Dedications", or "History", the requirement (section 4) to Preserve

its Title (section 1) will typically require changing the actual

title.

9. TERMINATION

You may not copy, modify, sublicense, or distribute the Document except

as expressly provided for under this License. Any other attempt to

copy, modify, sublicense or distribute the Document is void, and will

automatically terminate your rights under this License. However,

parties who have received copies, or rights, from you under this

License will not have their licenses terminated so long as such

parties remain in full compliance.

10. FUTURE REVISIONS OF THIS LICENSE

The Free Software Foundation may publish new, revised versions

of the GNU Free Documentation License from time to time. Such new

versions will be similar in spirit to the present version, but may

differ in detail to address new problems or concerns. See

http://www.gnu.org/copyleft/.

Each version of the License is given a distinguishing version number.

If the Document specifies that a particular numbered version of this

License "or any later version" applies to it, you have the option of

following the terms and conditions either of that specified version or

of any later version that has been published (not as a draft) by the

Free Software Foundation. If the Document does not specify a version

number of this License, you may choose any version ever published (not

as a draft) by the Free Software Foundation.

ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of

the License in the document and put the following copyright and

license notices just after the title page:

Copyright (c) YEAR YOUR NAME.

Permission is granted to copy, distribute and/or modify this document

under the terms of the GNU Free Documentation License, Version 1.2

or any later version published by the Free Software Foundation;

with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.

A copy of the license is included in the section entitled "GNU

Free Documentation License".

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts,

replace the "with...Texts." line with this:

with the Invariant Sections being LIST THEIR TITLES, with the

Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST.

If you have Invariant Sections without Cover Texts, or some other

combination of the three, merge those two alternatives to suit the

situation.

If your document contains nontrivial examples of program code, we

recommend releasing these examples in parallel under your choice of

free software license, such as the GNU General Public License,

to permit their use in free software.