Difference betn vfork & fork

quick question: what’s the difference between fork() and vfork() system calls ?

quick answer: vfork() system call creates a process that shares the memory address space of its parent.

details:

fork() is implemented by linux as a clone() system call whose flags parameter specifies both a SIGCHLD signal and all the clone flags cleared and whose child_stack parameter is 0.

vfork() is implemented by linux as a clone() system call whose flags parameter specifies both a SGCHLD signal and flags CLONE_VM and CLONE_VFORK and whose second parameter is 0.

[ discussion: copy on write ]

This is a concept of making the process creation using fork() efficient in that instead of copying the parent’s address space while process creation, it is shared but as soon as either of them write on the page, kernel allocates a new page and assigns it to the writer process.

Most of the time, forking is required just to run a new process in which case it’s a waste to copy the whole parent address space.

source* http://whatilearned2day.wordpress.com/2007/02/05/fork-and-vfork/

Some systems have a system call vfork(), which was originally
designed as a lower-overhead version of fork(). Since
fork() involved copying the entire address space of the process,
and was therefore quite expensive, the vfork() function was
introduced (in 3.0BSD).

However, since vfork() was introduced, the
implementation of fork() has improved drastically, most notably
with the introduction of `copy-on-write', where the copying of the
process address space is transparently faked by allowing both processes
to refer to the same physical memory until either of them modify
it. This largely removes the justification for vfork(); indeed, a
large proportion of systems now lack the original functionality of
vfork() completely. For compatibility, though, there may still be
a vfork() call present, that simply calls fork() without
attempting to emulate all of the vfork() semantics.

As a result, it is very unwise to actually make use of any of the
differences between fork() and vfork(). Indeed, it is
probably unwise to use vfork() at all, unless you know exactly
why you want to.

The basic difference between the two is that when a new process is
created with vfork(), the parent process is temporarily
suspended, and the child process might borrow the parent's address
space. This strange state of affairs continues until the child process
either exits, or calls execve(), at which point the parent
process continues.

This means that the child process of a vfork() must be careful to
avoid unexpectedly modifying variables of the parent process. In
particular, the child process must not return from the function
containing the vfork() call, and it must not call
exit() (if it needs to exit, it should use _exit();
actually, this is also true for the child of a normal fork()).

Source* http://www.unixguide.net/unix/programming/1.1.2.shtml

During the fork() system call the Kernel makes a copy of the parent process�s address space and attaches it to the child process.But the vfork() system call do not makes any copy of the parent�s address space, so it is faster than the fork() system call. The child process as a result of the vfork() system call executes exec() system call. The child process from vfork() system call executes in the parent�s address space (this can overwrite the parent�s data and stack ) which suspends the parent process until the child process exits.

Source* http://www.kyapoocha.com/unix-interview-questions/difference-between-the-fork-and-vfork-system-call/



During the fork() system call the Kernel makes a copy of the parent process’s address space and attaches it to the child process.
But the vfork() system call do not makes any copy of the parent’s address space, so it is faster than the fork() system call. The child process as a result of the vfork() system call executes exec() system call. The child process from vfork() system call executes in the parent’s address space (this can overwrite the parent’s data and stack ) which suspends the parent process until the child process exits.

Source* http://www.technicalinterview.info/difference-between-the-fork-and-vfork-system-call/

Kernel Compiletion (make target)

Informational Targets

Target	Description
kernelrelease	Displays the current kernel version, as determined by the build system.
kernelversion	Displays the current kernel version, as told by the main Makefile. This differs from the kernelrelease target in that it doesn’t use any additional version information based on configuration options or localversion files.

Cleaning Targets

Target	Description
clean	Removes most of the files generated by the kernel build system, but keeps the kernel configuration.
mrproper	Removes all of the generated files by the kernel build system, including the configuration and some various backup files.
distclean	Does everything mrproper does and removes some editor backup and patch leftover files.

Configuration Targets

Target	Description
config	Updates the current kernel configuration by using a line-oriented program.
menuconfig	Updates the current kernel configuration by using a text-based menu program.
xconfig	Updates the current kernel configuration by using a QT-based graphical program.
gconfig	Updates the current kernel configuration by using a GTK+-based graphical program.
oldconfig	Updates the current kernel configuration by using the current .config file and prompting for any new options that have been added to the kernel.
silentoldconfig	Just like oldconfig, but prints nothing to the screen except when a question needs to be answered.
randconfig	Generates a new kernel configuration with random answers to all of the different options.
defconfig	Generates a new kernel configuration with the default answer being used for all options. The default values are taken from a file located in the arch/$ARCH/defconfig file, where $ARCH refers to the specific architecture for which the kernel is being built.
allmodconfig	Generates a new kernel configuration in which modules are enabled whenever possible.

allyesconfig	Generates a new kernel configuration with all options set to yes.
allnoconfig	Generates a new kernel configuration with all options set to no.

Build Targets

Target	Description
all	Builds all of the different targets needed for this kernel to be able to be used. This includes both the modules and the static portion of the kernel.
vmlinux	Builds just the static portion of the kernel, not any loadable modules.
modules	Builds all of the loadable kernel modules for this configuration.
modules_install	Installs all of the modules into the specified location. If no location is specified with the INSTALL_MODULE_PATH environment variable, they are installed in the default root directory of the machine.
dir/	Builds all of the files in the specified directory and in all subdirectories below it.
dir/file.[o|i|s]	Builds only the specified file.
dir/file.ko	Builds all of the needed files and links them together to form the specified module.
tags	Builds all of the needed tags that most common text editors can use while editing the source code.
TAGS	Builds all of the needed tags that most common text editors can use while editing the source code.
cscope	Builds a cscope image, useful in source tree searches, of the source tree for the architecture specified by the configuration file (not all of the kernel source files).

Environment variables

Variable	Value	Description
V	0	This tells the build system to run in a quiet manner, showing only the file that is currently being built, and not the entire command that is running in order to build that file. This is the default option for the build system.
V	1	This tells the build system to operate in a verbose way, showing the full command that is being used to generate each of the specific files.
O	dir	This tells the build system to locate all output files in the dir directory, including the kernel configuration files. This allows the kernel to be built from a read-only filesystem and have the output placed in another location.
C	1	This checks all C files that are about to be built with the sparse tool, which detects common programming errors in the kernel source files. sparse can be downloaded using git from git://git.kernel.org/pub/scm/devel/sparse/sparse.git. Nightly snapshots can be found at http://www.codemonkey.org.uk/projects/git-snapshots/sparse/. More information on how to use sparse can be found in the Documentation/sparse.txt file in the kernel source tree.
C	2	This forces all C files to be checked with the sparse tool, even if they did not need to be built.

Packaging Targets

Target	Description
rpm	Builds the kernel first and then packages it up as a RPM package that can be installed.
rpm-pkg	Builds a source RPM package containing the base kernel.
binrpm-pkg	Builds a RPM package that contains a compiled kernel and modules.
deb-pkg	Builds a Debian package that contains the compiled kernel and modules.
tar-pkg	Builds a tarball that contains the compiled kernel and modules.
targz-pkg	Builds a gzip-compressed tarball that contains the compiled kernel and modules.
tarbz2-pkg	Builds a bzip2-compressed tarball that contains the compiled kernel and modules.

Documentation Targets

Target	Description
xmldocs	Builds the kernel documentation as XML DocBook files.
psdocs	Builds the kernel documentation as PostScript files.
pdfdocs	Builds the kernel documentation as PDF files.
htmldocs	Builds the kernel documentation as HTML files.
mandocs	Builds the kernel documentation as a set of manpages, which can then be installed with the installmandocs target.

Architecture-Specific Targets

Target	Description
bzImage	Creates a compressed kernel image and places it in the arch/i386/boot/bzImage file. This is the default target for the i386 kernel build.
install	Installs the kernel image using the distribution-specific /sbin/installkernel program. Note that this does not install the kernel modules; that must be done with the modules_install target.
bzdisk	Creates a boot floppy image and writes it to the /dev/fd0 device.
fdimage	Creates a boot floppy image and places it in the file arch/i386/boot/fdimage. The mtools package must be present on your system in order for this to work properly.
isoimage	Creates a CD-ROM boot image and places it in the file arch/i396/boot/image.iso. The syslinux package must be present on your system in order for this to work properly.

Analysis Targets

Target	Description
checkstack	Generate a list of the functions that use the most of the kernel stack space.
namespacecheck	Generate a list of all of the kernel symbols and their namespaces. This will be a large list.

Process States

R ------> running.
S -----> sleeping in an interruptible wait.
D ------> waiting in interruptible disk sleep.
Z ------> zombie.
T ----> traced or stopped (on a signal).
W ----> paging.

Linux Signals

Signal	Description	Default Action
SIGABRT	Generated by the abort system call (POSIX)	Process terminates and dumps core
SIGALRM	A timer signal generated by the alarm system call (POSIX)	Process terminates
SIGBUS	Process attempted to use mis-or unaligned memory (4.2 BSD)	Process terminates and dumps core.
SIGCHLD	A child has stopped or terminated (POSIX)	Ignore
SIGCONT	Process continues if it is stopped (POSIX)	Continue (ignore if process is not stopped)
SIGEMT	Bus (hardware) error	Process terminates and dumps core
SIGFPE	Floating-point exception (POSIX)	Process terminates and dumps core
SIGHUP	Process received a hang-up on its controlling terminal or its controlling process died	Process terminates
SIGILL	Illegal instruction (POSIX)	Process terminates and dumps core
SIGINFO	The same as SIGPWR	Ignore
SIGINT	User generated a keyboard interrupt (POSIX)	Process terminates
SIGIO	Asynchronous I/O received (4.2 BSD)	Ignore
SIGIOT	I/O trap. Same as SIGABRT (4.2 BSD)	Process terminates and dumps core
SIGKILL	Kill (POSIX)	Process terminates, cannot catch or ignore
SIGLOST	Process lost a file lock	Process terminates
SIGPIPE	Process attempted to write to a pipe with no readers (POSIX)	Process terminates
SIGPOLL	Pollable event occured (System V)	Process terminates
SIGPROF	Profiling alarm clock set on a segment of code expired (4.2 BSD)	Process terminates
SIGPWR	System detected a power failure (System V)	Ignore
SIGQUIT	User generated a keyboard quit (POSIX)	Process terminates and dumps core
SIGSEGV	Process attempted to reference inaccessible memory (POSIX)	Process terminates and dumps core
SIGSTKFLT	Process generated a stack fault ("blew the stack")	Process terminates
SIGSTOP	Stop signal (POSIX)	Stop, cannot catch or ignore
SIGSYS	Bad argument to routine	Process terminates and dumps core
SIGTERM	Process received a termination signal (POSIX)	Process terminates
SIGTRAP	Process hit a trace or breakpoint trap (usually during debugging) (POSIX)	Process terminates and dumps core
SIGTSTP	User generated a stop from the keyboard (POSIX)	Stop
SIGTTIN	Process attempted to read from stdin while running in the background (POSIX)	Stop
SIGTTOU	Process attempted to write Stop to stdout while running in the background (POSIX)
SIGUNUSED	Unused signal	Process terminates
SIGURG	Urgent condition on socket (4.2 BSD)	Ignore
SIGUSR1	User-defined signal 1 (POSIX)	Process terminates
SIGUSR2	User-defined signal 2 (POSIX)	Process terminates
SIGVTALRM	Interval alarm clock expired (4.2 BSD)	Process terminates
SIGWINCH	The size of a terminal window changed (4.3 BSD, Sun)	Ignore
SIGXCPU	Process exceeded the amount of CPU time it can use (4.2 BSD)	Process terminates and dumps core
SIGXFSZ	Process tried to access or manipulate a file larger than the syste file size limit (4.2 BSD)	Process terminates and dumps core

bits/signum.h

System Call Error Codes

Error No.	Error	Description
1	EPERM	The process lacks sufficient permissions to perform the operation it is attempting to perform.
2	ENOENT	The process is attempting to access a file or directory that does not exist.
3	ESRCH	No such process exists.
4	EINTR	A system call was interrupted.
5	EIO	Some sort of (usually hardware-related) I/O error occurred.
6	ENXIO	The I/O device or address does not exist.
7	E2BIG	The argument list passed to an exec call was too long.
8	ENOEXEC	The format of a binary that a process attempted to execute was incorrect (such as trying to run a SPARC binary on an x86 processor).
9	EBADF	An invalid file number was passed to a function that opens/close/reads/writes a file.
10	ECHILD	The process had no child process on which to wait.
11	EAGAIN	A process attempted to perform non-blocking I/O when no input was available.
12	ENOMEM	Insufficient memory is available for the requested operation.
13	EACCESS	Access to a file or other resource would be denied.
14	EFAULT	A bad pointer (one that points to inaccessible memory) was passed to a system call.
15	ENOTBLK	A process attempted to mount a device that is not a block device.
16	EBUSY	A process attempted to mount a device that is already mounted or attempted to unmount a filesystem currently in use.
17	EEXIST	Returned when you try to create a file that already exists.
18	EXDEV	Returned by the link call if the source and destination files are not on the same filesystem.
19	ENODEV	A process attempted to use a filesystem type that the kernel does not support.
20	ENOTDIR	A directory component in a pathname is not, in fact, a directory.
21	EISDIR	The filename component of a pathname is a directory, not a filename.
22	EINVAL	A process passed an invalid argument to a system call.
23	ENFILE	The system has reached the maximum number of open files it supports.
24	EMFILE	The calling process cannot open any more files because it has already opened the maximum number allowed.
25	ENOTTY	A process attempted to do terminal style I/O on a device or file that is not a terminal. This error is the famous "not a typewriter" message.
26	ETXTBSY	A process attempted to open a binary or library file that is currently in use.
27	EFBIG	The calling process attempted to write a file longer than the system maximum or the process's resource limits permit.
28	ENOSPC	A filesystem or device is full.
29	ESPIPE	A process attempted to lseek on a non-seekable file.
30	EROFS	A process attempted to write on a read-only filesystem.
31	EMLINK	The file being linked has reached the maximum number of links allowed.
32	EPIPE	The read end of a pipe is closed and SIGPIPE is being ignored or trapped.
33	EDOM	Set by math functions when an argument exceeds the function's domain.
34	ERANGE	Set by math functions when the result of the function can't be represented by the function's return type.
36	ENAMETOOLONG	A path or filename is too long.
38	ENOSYS	The system call invoked has not been implemented.
39	ENOTEMPTY	A directory on which rmdir was called is not empty.
40	ELOOP	A path involves too long a chain of symbolic links.

Manpages

Number	Description
1	Executable programs or shell commands.
2	System calls (functions provided by the kernel)
3	Library calls (functions within program libraries)
4	Special files (usually found in /dev)
5	File formats and conventions (/etc/fstab, /proc)
6	Games
7	Miscellaneous (including macro packages and conventions), for example, man(7), groff(7)
8	System administration commands (usually only for root)
9	Kernel routines (nonstandard)

Applying Patches To The Linux Kernel
------------------------------------

Original by: Jesper Juhl.


A frequently asked question on the Linux Kernel Mailing List is how to apply
a patch to the kernel or, more specifically, what base kernel a patch for
one of the many trees/branches should be applied to. Hopefully this document
will explain this to you.

In addition to explaining how to apply and revert patches, a brief
description of the different kernel trees (and examples of how to apply
their specific patches) is also provided.


What is a patch?
---
A patch is a small text document containing a delta of changes between two
different versions of a source tree. Patches are created with the `diff'
program.
To correctly apply a patch you need to know what base it was generated from
and what new version the patch will change the source tree into. These
should both be present in the patch file metadata or be possible to deduce
from the filename.


How do I apply or revert a patch?
---
You apply a patch with the `patch' program. The patch program reads a diff
(or patch) file and makes the changes to the source tree described in it.

Patches for the Linux kernel are generated relative to the parent directory
holding the kernel source dir.

This means that paths to files inside the patch file contain the name of the
kernel source directories it was generated against (or some other directory
names like "a/" and "b/").
Since this is unlikely to match the name of the kernel source dir on your
local machine (but is often useful info to see what version an otherwise
unlabeled patch was generated against) you should change into your kernel
source directory and then strip the first element of the path from filenames
in the patch file when applying it (the -p1 argument to `patch' does this).

To revert a previously applied patch, use the -R argument to patch.
So, if you applied a patch like this:
patch -p1 < ../patch-x.y.z

You can revert (undo) it like this:
patch -R -p1 < ../patch-x.y.z


How do I feed a patch/diff file to `patch'?
---
This (as usual with Linux and other UNIX like operating systems) can be
done in several different ways.
In all the examples below I feed the file (in uncompressed form) to patch
via stdin using the following syntax:
patch -p1 <>

If you just want to be able to follow the examples below and don't want to
know of more than one way to use patch, then you can stop reading this
section here.

Patch can also get the name of the file to use via the -i argument, like
this:
patch -p1 -i path/to/patch-x.y.z

If your patch file is compressed with gzip or bzip2 and you don't want to
uncompress it before applying it, then you can feed it to patch like this
instead:
zcat path/to/patch-x.y.z.gz | patch -p1
bzcat path/to/patch-x.y.z.bz2 | patch -p1

If you wish to uncompress the patch file by hand first before applying it
(what I assume you've done in the examples below), then you simply run
gunzip or bunzip2 on the file -- like this:
gunzip patch-x.y.z.gz
bunzip2 patch-x.y.z.bz2

Which will leave you with a plain text patch-x.y.z file that you can feed to
patch via stdin or the -i argument, as you prefer.

A few other nice arguments for patch are -s which causes patch to be silent
except for errors which is nice to prevent errors from scrolling out of the
screen too fast, and --dry-run which causes patch to just print a listing of
what would happen, but doesn't actually make any changes. Finally --verbose
tells patch to print more information about the work being done.


Common errors when patching
---
When patch applies a patch file it attempts to verify the sanity of the
file in different ways.
Checking that the file looks like a valid patch file & checking the code
around the bits being modified matches the context provided in the patch are
just two of the basic sanity checks patch does.

If patch encounters something that doesn't look quite right it has two
options. It can either refuse to apply the changes and abort or it can try
to find a way to make the patch apply with a few minor changes.

One example of something that's not 'quite right' that patch will attempt to
fix up is if all the context matches, the lines being changed match, but the
line numbers are different. This can happen, for example, if the patch makes
a change in the middle of the file but for some reasons a few lines have
been added or removed near the beginning of the file. In that case
everything looks good it has just moved up or down a bit, and patch will
usually adjust the line numbers and apply the patch.

Whenever patch applies a patch that it had to modify a bit to make it fit
it'll tell you about it by saying the patch applied with 'fuzz'.
You should be wary of such changes since even though patch probably got it
right it doesn't /always/ get it right, and the result will sometimes be
wrong.

When patch encounters a change that it can't fix up with fuzz it rejects it
outright and leaves a file with a .rej extension (a reject file). You can
read this file to see exactly what change couldn't be applied, so you can
go fix it up by hand if you wish.

If you don't have any third-party patches applied to your kernel source, but
only patches from kernel.org and you apply the patches in the correct order,
and have made no modifications yourself to the source files, then you should
never see a fuzz or reject message from patch. If you do see such messages
anyway, then there's a high risk that either your local source tree or the
patch file is corrupted in some way. In that case you should probably try
re-downloading the patch and if things are still not OK then you'd be advised
to start with a fresh tree downloaded in full from kernel.org.

Let's look a bit more at some of the messages patch can produce.

If patch stops and presents a "File to patch:" prompt, then patch could not
find a file to be patched. Most likely you forgot to specify -p1 or you are
in the wrong directory. Less often, you'll find patches that need to be
applied with -p0 instead of -p1 (reading the patch file should reveal if
this is the case -- if so, then this is an error by the person who created
the patch but is not fatal).

If you get "Hunk #2 succeeded at 1887 with fuzz 2 (offset 7 lines)." or a
message similar to that, then it means that patch had to adjust the location
of the change (in this example it needed to move 7 lines from where it
expected to make the change to make it fit).
The resulting file may or may not be OK, depending on the reason the file
was different than expected.
This often happens if you try to apply a patch that was generated against a
different kernel version than the one you are trying to patch.

If you get a message like "Hunk #3 FAILED at 2387.", then it means that the
patch could not be applied correctly and the patch program was unable to
fuzz its way through. This will generate a .rej file with the change that
caused the patch to fail and also a .orig file showing you the original
content that couldn't be changed.

If you get "Reversed (or previously applied) patch detected! Assume -R? [n]"
then patch detected that the change contained in the patch seems to have
already been made.
If you actually did apply this patch previously and you just re-applied it
in error, then just say [n]o and abort this patch. If you applied this patch
previously and actually intended to revert it, but forgot to specify -R,
then you can say [y]es here to make patch revert it for you.
This can also happen if the creator of the patch reversed the source and
destination directories when creating the patch, and in that case reverting
the patch will in fact apply it.

A message similar to "patch: **** unexpected end of file in patch" or "patch
unexpectedly ends in middle of line" means that patch could make no sense of
the file you fed to it. Either your download is broken, you tried to feed
patch a compressed patch file without uncompressing it first, or the patch
file that you are using has been mangled by a mail client or mail transfer
agent along the way somewhere, e.g., by splitting a long line into two lines.
Often these warnings can easily be fixed by joining (concatenating) the
two lines that had been split.

As I already mentioned above, these errors should never happen if you apply
a patch from kernel.org to the correct version of an unmodified source tree.
So if you get these errors with kernel.org patches then you should probably
assume that either your patch file or your tree is broken and I'd advise you
to start over with a fresh download of a full kernel tree and the patch you
wish to apply.


Are there any alternatives to `patch'?
---
Yes there are alternatives.

You can use the `interdiff' program (http://cyberelk.net/tim/patchutils/) to
generate a patch representing the differences between two patches and then
apply the result.
This will let you move from something like 2.6.12.2 to 2.6.12.3 in a single
step. The -z flag to interdiff will even let you feed it patches in gzip or
bzip2 compressed form directly without the use of zcat or bzcat or manual
decompression.

Here's how you'd go from 2.6.12.2 to 2.6.12.3 in a single step:
interdiff -z ../patch-2.6.12.2.bz2 ../patch-2.6.12.3.gz | patch -p1

Although interdiff may save you a step or two you are generally advised to
do the additional steps since interdiff can get things wrong in some cases.

Another alternative is `ketchup', which is a python script for automatic
downloading and applying of patches (http://www.selenic.com/ketchup/).

Other nice tools are diffstat, which shows a summary of changes made by a
patch; lsdiff, which displays a short listing of affected files in a patch
file, along with (optionally) the line numbers of the start of each patch;
and grepdiff, which displays a list of the files modified by a patch where
the patch contains a given regular expression.


Where can I download the patches?
---
The patches are available at http://kernel.org/
Most recent patches are linked from the front page, but they also have
specific homes.

The 2.6.x.y (-stable) and 2.6.x patches live at
ftp://ftp.kernel.org/pub/linux/kernel/v2.6/

The -rc patches live at
ftp://ftp.kernel.org/pub/linux/kernel/v2.6/testing/

The -git patches live at
ftp://ftp.kernel.org/pub/linux/kernel/v2.6/snapshots/

The -mm kernels live at
ftp://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/

In place of ftp.kernel.org you can use ftp.cc.kernel.org, where cc is a
country code. This way you'll be downloading from a mirror site that's most
likely geographically closer to you, resulting in faster downloads for you,
less bandwidth used globally and less load on the main kernel.org servers --
these are good things, so do use mirrors when possible.


The 2.6.x kernels
---
These are the base stable releases released by Linus. The highest numbered
release is the most recent.

If regressions or other serious flaws are found, then a -stable fix patch
will be released (see below) on top of this base. Once a new 2.6.x base
kernel is released, a patch is made available that is a delta between the
previous 2.6.x kernel and the new one.

To apply a patch moving from 2.6.11 to 2.6.12, you'd do the following (note
that such patches do *NOT* apply on top of 2.6.x.y kernels but on top of the
base 2.6.x kernel -- if you need to move from 2.6.x.y to 2.6.x+1 you need to
first revert the 2.6.x.y patch).

Here are some examples:

# moving from 2.6.11 to 2.6.12
$ cd ~/linux-2.6.11 # change to kernel source dir
$ patch -p1 < ../patch-2.6.12 # apply the 2.6.12 patch
$ cd ..
$ mv linux-2.6.11 linux-2.6.12 # rename source dir

# moving from 2.6.11.1 to 2.6.12
$ cd ~/linux-2.6.11.1 # change to kernel source dir
$ patch -p1 -R < ../patch-2.6.11.1 # revert the 2.6.11.1 patch
# source dir is now 2.6.11
$ patch -p1 < ../patch-2.6.12 # apply new 2.6.12 patch
$ cd ..
$ mv linux-2.6.11.1 linux-2.6.12 # rename source dir


The 2.6.x.y kernels
---
Kernels with 4-digit versions are -stable kernels. They contain small(ish)
critical fixes for security problems or significant regressions discovered
in a given 2.6.x kernel.

This is the recommended branch for users who want the most recent stable
kernel and are not interested in helping test development/experimental
versions.

If no 2.6.x.y kernel is available, then the highest numbered 2.6.x kernel is
the current stable kernel.

note: the -stable team usually do make incremental patches available as well
as patches against the latest mainline release, but I only cover the
non-incremental ones below. The incremental ones can be found at
ftp://ftp.kernel.org/pub/linux/kernel/v2.6/incr/

These patches are not incremental, meaning that for example the 2.6.12.3
patch does not apply on top of the 2.6.12.2 kernel source, but rather on top
of the base 2.6.12 kernel source .
So, in order to apply the 2.6.12.3 patch to your existing 2.6.12.2 kernel
source you have to first back out the 2.6.12.2 patch (so you are left with a
base 2.6.12 kernel source) and then apply the new 2.6.12.3 patch.

Here's a small example:
# moving from 2.6.12.2 to 2.6.12.3
$ cd ~/linux-2.6.12.2 # change into the kernel source dir
$ patch -p1 -R < ../patch-2.6.12.2 # revert the 2.6.12.2 patch
$ patch -p1 < ../patch-2.6.12.3 # apply the new 2.6.12.3 patch
$ cd ..
$ mv linux-2.6.12.2 linux-2.6.12.3 # rename the kernel source dir


The -rc kernels
---
These are release-candidate kernels. These are development kernels released
by Linus whenever he deems the current git (the kernel's source management
tool) tree to be in a reasonably sane state adequate for testing.

These kernels are not stable and you should expect occasional breakage if
you intend to run them. This is however the most stable of the main
development branches and is also what will eventually turn into the next
stable kernel, so it is important that it be tested by as many people as
possible.

This is a good branch to run for people who want to help out testing
development kernels but do not want to run some of the really experimental
stuff (such people should see the sections about -git and -mm kernels below).

The -rc patches are not incremental, they apply to a base 2.6.x kernel, just
like the 2.6.x.y patches described above. The kernel version before the -rcN
suffix denotes the version of the kernel that this -rc kernel will eventually
turn into.
So, 2.6.13-rc5 means that this is the fifth release candidate for the 2.6.13
kernel and the patch should be applied on top of the 2.6.12 kernel source.

Here are 3 examples of how to apply these patches:

# first an example of moving from 2.6.12 to 2.6.13-rc3
$ cd ~/linux-2.6.12 # change into the 2.6.12 source dir
$ patch -p1 < ../patch-2.6.13-rc3 # apply the 2.6.13-rc3 patch
$ cd ..
$ mv linux-2.6.12 linux-2.6.13-rc3 # rename the source dir

# move from 2.6.13-rc3 to 2.6.13-rc5
$ cd ~/linux-2.6.13-rc3 # change into the 2.6.13-rc3 dir
$ patch -p1 -R < ../patch-2.6.13-rc3 # revert the 2.6.13-rc3 patch
$ patch -p1 < ../patch-2.6.13-rc5 # apply the new 2.6.13-rc5 patch
$ cd ..
$ mv linux-2.6.13-rc3 linux-2.6.13-rc5 # rename the source dir

# move from 2.6.12.3 to 2.6.13-rc5
$ cd ~/linux-2.6.12.3 # change to the kernel source dir
$ patch -p1 -R < ../patch-2.6.12.3 # revert the 2.6.12.3 patch
$ patch -p1 < ../patch-2.6.13-rc5 # apply new 2.6.13-rc5 patch
$ cd ..
$ mv linux-2.6.12.3 linux-2.6.13-rc5 # rename the kernel source dir


The -git kernels
---
These are daily snapshots of Linus' kernel tree (managed in a git
repository, hence the name).

These patches are usually released daily and represent the current state of
Linus's tree. They are more experimental than -rc kernels since they are
generated automatically without even a cursory glance to see if they are
sane.

-git patches are not incremental and apply either to a base 2.6.x kernel or
a base 2.6.x-rc kernel -- you can see which from their name.
A patch named 2.6.12-git1 applies to the 2.6.12 kernel source and a patch
named 2.6.13-rc3-git2 applies to the source of the 2.6.13-rc3 kernel.

Here are some examples of how to apply these patches:

# moving from 2.6.12 to 2.6.12-git1
$ cd ~/linux-2.6.12 # change to the kernel source dir
$ patch -p1 < ../patch-2.6.12-git1 # apply the 2.6.12-git1 patch
$ cd ..
$ mv linux-2.6.12 linux-2.6.12-git1 # rename the kernel source dir

# moving from 2.6.12-git1 to 2.6.13-rc2-git3
$ cd ~/linux-2.6.12-git1 # change to the kernel source dir
$ patch -p1 -R < ../patch-2.6.12-git1 # revert the 2.6.12-git1 patch
# we now have a 2.6.12 kernel
$ patch -p1 < ../patch-2.6.13-rc2 # apply the 2.6.13-rc2 patch
# the kernel is now 2.6.13-rc2
$ patch -p1 < ../patch-2.6.13-rc2-git3 # apply the 2.6.13-rc2-git3 patch
# the kernel is now 2.6.13-rc2-git3
$ cd ..
$ mv linux-2.6.12-git1 linux-2.6.13-rc2-git3 # rename source dir


The -mm kernels
---
These are experimental kernels released by Andrew Morton.

The -mm tree serves as a sort of proving ground for new features and other
experimental patches.
Once a patch has proved its worth in -mm for a while Andrew pushes it on to
Linus for inclusion in mainline.

Although it's encouraged that patches flow to Linus via the -mm tree, this
is not always enforced.
Subsystem maintainers (or individuals) sometimes push their patches directly
to Linus, even though (or after) they have been merged and tested in -mm (or
sometimes even without prior testing in -mm).

You should generally strive to get your patches into mainline via -mm to
ensure maximum testing.

This branch is in constant flux and contains many experimental features, a
lot of debugging patches not appropriate for mainline etc., and is the most
experimental of the branches described in this document.

These kernels are not appropriate for use on systems that are supposed to be
stable and they are more risky to run than any of the other branches (make
sure you have up-to-date backups -- that goes for any experimental kernel but
even more so for -mm kernels).

These kernels in addition to all the other experimental patches they contain
usually also contain any changes in the mainline -git kernels available at
the time of release.

Testing of -mm kernels is greatly appreciated since the whole point of the
tree is to weed out regressions, crashes, data corruption bugs, build
breakage (and any other bug in general) before changes are merged into the
more stable mainline Linus tree.
But testers of -mm should be aware that breakage in this tree is more common
than in any other tree.

The -mm kernels are not released on a fixed schedule, but usually a few -mm
kernels are released in between each -rc kernel (1 to 3 is common).
The -mm kernels apply to either a base 2.6.x kernel (when no -rc kernels
have been released yet) or to a Linus -rc kernel.

Here are some examples of applying the -mm patches:

# moving from 2.6.12 to 2.6.12-mm1
$ cd ~/linux-2.6.12 # change to the 2.6.12 source dir
$ patch -p1 < ../2.6.12-mm1 # apply the 2.6.12-mm1 patch
$ cd ..
$ mv linux-2.6.12 linux-2.6.12-mm1 # rename the source appropriately

# moving from 2.6.12-mm1 to 2.6.13-rc3-mm3
$ cd ~/linux-2.6.12-mm1
$ patch -p1 -R < ../2.6.12-mm1 # revert the 2.6.12-mm1 patch
# we now have a 2.6.12 source
$ patch -p1 < ../patch-2.6.13-rc3 # apply the 2.6.13-rc3 patch
# we now have a 2.6.13-rc3 source
$ patch -p1 < ../2.6.13-rc3-mm3 # apply the 2.6.13-rc3-mm3 patch
$ cd ..
$ mv linux-2.6.12-mm1 linux-2.6.13-rc3-mm3 # rename the source dir


This concludes this list of explanations of the various kernel trees.
I hope you are now clear on how to apply the various patches and help testing
the kernel.

Thank you's to Randy Dunlap, Rolf Eike Beer, Linus Torvalds, Bodo Eggert,
Johannes Stezenbach, Grant Coady, Pavel Machek and others that I may have
forgotten for their reviews and contributions to this document.


Ref :- kernel Documentations/applying-patches.txt

dialects of C

1.5. Who defines Valid C?

For you, the programmer, "valid C" is defined by the compiler. There are many dialects of C in existence, thankfully they are all very similar. There are also other languages that are based on C such as Objective C and C++. These languages are very like C in there appearance but there usages is quite different. GCC understands many dialects of C as well as many other languages (including Objective C and C++).

1.5.1. "K&R" C

C was created by Dennis Ritchie between 1969 and 1973. In 1978 Dennis Ritchie along with Brian Kernighan published an excellent C tutorial "The C programming language". This was the first formal definition of the language. Being the original C dialect it is sometimes called Traditional C. Unfortunately the book left many aspects of the language undefined, this meant that people writing compilers had to make decisions as to how to handle these aspects. The result was that a piece of code would behave differently depending on what compiler was used. This dialect is no longer used, GCC supports it only for compiling very old programs. We mention it here purely for historical purposes.

1.5.2. ISO C

In 1983 the American National Standards Institute (ANSI) set up a committee to draw up a more exact standard and fix a few shortcomings they saw in the language. In 1989 they finalised this standard which was accepted by the International Standards Organisation (ISO). This new dialect became known as "C89". It is also called "ISO C" or "ANSI C". GCC is one of the most conforming compilers available.

1.5.3. C99

The ANSI C committee meets infrequently to update the standard. The latest updated standard was released in 1999 and is known as "C99". Few compilers fully support C99 yet; making changes to one of the most important pieces of software to an operating system takes time. GCC's C99 support is mostly complete (at the time of this writing) but the developers are working on it.

1.5.4. GNU C

GNU C is most similar to C89 but has a lot of the new features of C99 added and a few other extensions. These extensions have been added conservatively by the developers as problems are found that C99 doesn't provide good solutions to. GNU C is the default dialect of GCC and is the dialect we will use in this book. We will try our best to point out GNU extensions when we use them but in general, it is better to make full use GNU C. Use of ISO C is limiting your programs to the lowest common denominator and should only be used in special cases.

1.5.5. Choosing a Dialect

If you would like to use a dialect other than the default, you can specify your choice with the -std= switch followed by name of the dialect. The names are: c89, c99, gnu89 and, gnu99. "gnu89" is the current default but "gnu99" will become the default when C99 support is complete. The change will not be very noticeable.

1.5.6. Future Standards

Extensions such as those added by GCC are the main source of inspiration for new ISO C standards. When the ANSI C group see a lot of compilers implementing an extension they review the necessity of that feature and if they decide it would be of benefit they work out a standard way to implement it. Some of GCC's extensions may make it into the next standard, some will not.

Ref : http://www.faqs.org/docs/learnc/x134.html

Files generated by configure

3.2 Files generated by configure
After you have invoked `configure', you will discover a number of generated files in your build tree. The build directory structure created by `configure' and the number of files will vary from package to package. Each of the generated files are described below and their relationships are shown in C. Generated File Dependencies:
`config.cache'
`configure' can cache the results of system tests that have been performed to speed up subsequent tests. This file contains the cache data and is a plain text file that can be hand-modified or removed if desired.
`config.log'
As `configure' runs, it outputs a message describing each test it performs and the result of each test. There is substantially more output produced by the shell and utilities that `configure' invokes, but it is hidden from the user to keep the output understandable. The output is instead redirected to `config.log'. This file is the first place to look when `configure' goes hay-wire or a test produces a nonsense result. A common scenario is that `configure', when run on a Solaris system, will tell you that it was unable to find a working C compiler. An examination of `config.log' will show that Solaris' default `/usr/ucb/cc' is a program that informs the user that the optional C compiler is not installed.
`config.status'
`configure' generates a shell script called `config.status' that may be used to recreate the current configuration. That is, all generated files will be regenerated. This script can also be used to re-run `configure' if the `--recheck' option is given.
`config.h'
Many packages that use `configure' are written in C or C++. Some of the tests that `configure' runs involve examining variability in the C and C++ programming languages and implementations thereof. So that source code can programmatically deal with these differences, #define preprocessor directives can be optionally placed in a config header, usually called `config.h', as `configure' runs



Ref: GNU Autoconf, Automake and Libtool
Authors: G. V. Vaughan, B. Elliston, T. Tromey, and I. L. Taylor

Files generated by configure

3.2 Files generated by configure
After you have invoked `configure', you will discover a number of generated files in your build tree. The build directory structure created by `configure' and the number of files will vary from package to package. Each of the generated files are described below and their relationships are shown in C. Generated File Dependencies:
`config.cache'
`configure' can cache the results of system tests that have been performed to speed up subsequent tests. This file contains the cache data and is a plain text file that can be hand-modified or removed if desired.
`config.log'
As `configure' runs, it outputs a message describing each test it performs and the result of each test. There is substantially more output produced by the shell and utilities that `configure' invokes, but it is hidden from the user to keep the output understandable. The output is instead redirected to `config.log'. This file is the first place to look when `configure' goes hay-wire or a test produces a nonsense result. A common scenario is that `configure', when run on a Solaris system, will tell you that it was unable to find a working C compiler. An examination of `config.log' will show that Solaris' default `/usr/ucb/cc' is a program that informs the user that the optional C compiler is not installed.
`config.status'
`configure' generates a shell script called `config.status' that may be used to recreate the current configuration. That is, all generated files will be regenerated. This script can also be used to re-run `configure' if the `--recheck' option is given.
`config.h'
Many packages that use `configure' are written in C or C++. Some of the tests that `configure' runs involve examining variability in the C and C++ programming languages and implementations thereof. So that source code can programmatically deal with these differences, #define preprocessor directives can be optionally placed in a config header, usually called `config.h', as `configure' runs.




Ref: GNU Autoconf, Automake and Libtool
Aut: G. V. Vaughan, B. Elliston, T. Tromey, and I. L. Taylor