The Linux FAQ page 13 Software Development

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This is the list of Frequently Asked Questions for Linux, the Free/Open Source UNIX-like operating system kernel that runs on many modern computer systems..

Table of Contents

13. Software Development

Q: How Do I Compile Programs?

Q: How Do I Port XXX to Linux?

Q: Can I Use Code or a Compiler Compiled for a 486 on a 386?

Q: What Does gcc -O6 Do?

Q: What Do I Do About Errors Trying to Compile the Kernel?

Q: How Do I Make a Shared Library?

Q: Why Are My Programs So Large?

Q: How To Prevent Errors when Linking Programs with Math Functions

Q: How To Program XYZ Under Linux

Q: How To Upgrade/Recompile a Kernel

Q: What Is a .gz File? And a .tgz? And .bz2? And... ?

Q: Where Are linux/*.h and asm/*.h?

Q: Configuring Emacs' Default Settings

Q: How Do I Compile Programs?

Q: How Do I Compile Programs?

A: Most Linux software is written in C and compiled with the GNU C compiler. GCC is a part of every Linux

distribution. The latest compiler version, documentation, and patches are on ftp://ftp.gnu.org/pub/gnu/ .

Programs that are written in C++ must be compiled with the GNU G++ compiler, which is also included inLinux distributions and available from the same place as GCC.To build version 2.0.x kernels, you will need GCC version 2.7.2.x, approximately. Trying to build an earlyLinux kernel with a different compiler, like GCC 2.8.x, EGCS, or PGCC, may cause problems because ofGCC related code dependencies. Kernel versions 2.2, 2.4, and the 2.5 development kernels should compilecorrectly with more recent compilers.

Information on the EGCS compiler is at http://www.gnu.org/software/gcc/gcc.html.

Note that at this time, the kernel developers are not answering bug requests for earlier kernels, but instead are concentrating on developing 2.5.x version kernels and maintaining 2.2.x and 2.4.x version kernels.

[J.H.M. Dassen, Axel Boldt]

Q: How Do I Port XXX to Linux?

A: In general, *nix programs need very little porting. Simply follow the installation instructions. If you don'tknow and don't know how to find out the answers to some of the questions asked during the installationprocedure, you can guess, but this tends to produce buggy programs. In this case, you're probably better offasking someone else to do the port.

If you have a BSD-ish program, you should try using -I/usr/include/bsd and -lbsd on the appropriate parts

of the compilation lines.

Q: Can I Use Code or a Compiler Compiled for a 486 on a 386?

A: Yes, unless it's the kernel.

.The -m486 option to GCC, which is used to compile binaries for x486 machines, merely changes certain

optimizations. This makes for slightly larger binaries that run somewhat faster on a 486. They still work fineon a 386, though, with a small performance hit.However, from version 1.3.35 the kernel uses 486 or Pentium-specific instructions if configured for a 486 orPentium, thus making it unusable on a 386.

GCC can be configured for a 386 or 486; the only difference is that configuring it for a 386 makes -m386 the

default and configuring for a 486 makes -m486 the default. In either case, these can be overridden on a

per-compilation basis or by editing /usr/lib/gcc-lib/i*-linux/ n.n.n/specs.

There is an alpha version of GCC that knows how to do optimization well for the 586, but it is quite

unreliable, especially at high optimization settings. The Pentium GCC can be found on

ftp://tsx-11.mit.edu/pub/linux/ALPHA/pentium-gcc/ .

The ordinary 486 GCC supposedly produces better code for the Pentium using the -m386, or at least slightly

smaller.

Q: What Does gcc -O6 Do?

A: Currently, the same as -O2 (GCC 2.5) or -O3 (GCC 2.6, 2.7). Any number greater than that does the same thing. The Makefiles of newer kernels use -O2, and you should probably do the same.

Q: What Do I Do About Errors Trying to Compile the Kernel?

A: See the previous question regarding the header files.

Remember that when you apply a patch to the kernel, you must use the -p0 or -p1 option: otherwise, the

patch may be misapplied. See the patch manual page for details.

ld: unrecognized option -qmagic means that you should get a newer linker, from

ftp://tsx-11.mit.edu/pub/linux/packages/GCC/ , in the file binutils-2.8.1.0.1.bin.tar.gz.

Q: How Do I Make a Shared Library?

A: For ELF,

$ gcc -fPIC -c *.c

$ gcc -shared -Wl,-soname,libfoo.so.1 -o libfoo.so.1.0 *.o

For a.out, get tools-n.nn.tar.gz from tsx-11.mit.edu/pub/linux/packages/GCC/src/.

It comes with documentation that will tell you what to do. Note that a.out shared libraries are a very tricky

business. Consider upgrading your libraries to ELF shared libraries. See the ELF HOWTO, at

ftp://metalab.unc.edu/pub/Linux/docs/HOWTO/ .

Q: Why Are My Programs So Large?

A: With an ELF compiler (What's All This about ELF? glibc?), the most common cause of large executables

is the lack of an appropriate .so library link for one of the libraries you're using. There should be a link like

libc.so for every library like libc.so.5.2.18.

With an a.out compiler the most common cause of large executables is the -g linker (compiler) flag. This

produces (as well as debugging information in the output file) a program which is statically linkedone whichincludes a copy of the C library instead of a dynamically linked copy.

Other things worth investigating are -O and -O2, which enable optimization (check the GCC

documentation), and -s (or the strip command) which strip the symbol information from the resulting binary(making debugging totally impossible).You may wish to use -N on very small executables (less than 8K with the -N), but you shouldn't do this

unless you understand its performance implications, and definitely never with daemons.

Q: How To Prevent Errors when Linking Programs with Math Functions

A: Older run-time libraries included the math library in the C run-time library. It was not necessary tospecify the math library separately when compiling. If the compiler generates a message like this when

linking a program that uses math functions:

/tmp/ccDUQM4J.o: In function "main":/tmp/ccDUQM4J.o(.text+0x19): undefined reference to "sqrt"

collect2: ld returned 1 exit status

You need use the -lm option with GCC to link with the math libraries:

# gcc -o program program.c -lm

Make sure also to use the statement #include <math.h> in the source file.

[Florian Schmidt]

Q: How To Program XYZ Under Linux

A: Read the manuals, or a good book on Unix and the manual pages (type man man). There is a lot of GNU

Info documentation, which is often more useful as a tutorial. Run Emacs and type F1-i, or type info info if

you don't have or don't like Emacs. Note that the Emacs libc node may not exactly describe the latest Linuxlibc, or GNU glibc2. But the GNU project and LDP are always looking for volunteers to upgrade their librarydocumentation.Anyway, between the existing Texinfo documentation, and the manual pages in sections 2 and 3, shouldprovide enough information to get started.As with all free software, the best tutorial is the source code itself.The latest release of the Linux manual pages, a collection of useful GNU Info documentation, and various

other information related to programming Linux, can be found on

ftp.metalab.unc.edu/pub/Linux/docs/man-pages/ .

Q: How To Upgrade/Recompile a Kernel

A: See the Kernel HOWTO or the README files which come with the kernel release on

ftp://ftp.cs.helsinki.fi/pub/Software/Linux/Kernel/  and mirrors. See Where Are the Linux FTP Archives?. You

may already have a version of the kernel source code installed on your system, but if it is part of a standard

.distribution it is likely to be somewhat out of date (this is not a problem if you only want a custom configured

kernel, but it probably is if you need to upgrade.)With newer kernels you can (and should) make all of the following targets. Don't forget that you can specify

multiple targets with one command.

$ make clean dep install modules modules_install

Also remember to update the module dependencies.

$ depmod -a

This command can be run automatically at boot time. On Debian/GNU Linux systems, the command is part of

the /etc/init.d/modutils script, and can be linked appropriately in the /etc/rcx.d/ directories.

For more information on depmod, see the manual page.

Make sure you are using the most recent version of the modutils utilities, as well as all other supporting

packages. Refer to the file Documentation/Changes in the kernel source tree for specifics, and be sure

to consult the README file in the modutils package.

Remember that to make the new kernel boot you must run lilo after copying the kernel into your root

partition. The Makefile in some kernels have a special zlilo target for this; try:

$ make zlilo

On current systems, however, you can simply copy the zImage or bzImage file (in arch/i386/boot/

to the /boot/ directory on the root file system, or to a floppy using the dd command. Refer also to the

question, How do I get LILO to boot the kernel image?Kernel version numbers with an odd minor version (ie, 1.1.x, 1.3.x) are the testing releases; stable productionkernels have even minor versions (1.0.x, 1.2.x). If you want to try the testing kernels you should probably

subscribe to the linux-kernel mailing list. See What Mailing Lists Are There?.

The Web site http://www.kernelnotes.org/  has lots of information and links to other sites that provide

information about Linux kernel updates.

Also refer to the answers for, Why Doesn't My PCMCIA Card Work after Upgrading the Kernel? and How Do

I Get LILO to Boot the Kernel Image?.

 

A: Alternatively, on Debian GNU/Linux systems, get a kernel source package from the Debian archive or

from a Debian GNU/Linux CD. Then, follow the directions in the README file that is located in the

kernel-package subdirectory.

Q: What Is a .gz File? And a .tgz? And .bz2? And... ?

A: .gz (and .z) files are compressed using GNU gzip. You need to use gunzip (which is a symlink to the

gzip command that comes with most Linux installations) to unpack the file.

.taz, .tar.Z, and .tz are tar files (made with tar) and compressed using compress. The standard *nix

compress is proprietary software, but free equivalents like ncompress exist.

..tgz (or .tpz) is a tar file compressed with gzip.

.bz2 is a file compressed by the more recently introduced (and efficient) bzip2.

.lsm is a Linux Software Map entry, in the form of a short text file. Details about the LSM project and the

LSM itself are available in the subdirectory on ftp://metalab.unc.edu/pub/Linux/docs/ .

.deb is a Debian Binary Package - the binary package format used by the Debian GNU/Linux distribution. It

is manipulated using dpkg and dpkg-deb (available on Debian systems and from:

http://ftp.debian.org/pool/main/d/dpkg/ . If you use anonymous FTP, connect to:

ftp://ftp.debian.org/debian/pool/main/d/dpkg/ ).

.rpm is a Red Hat RPM package, which is used in the Red Hat and similar distributions.

.sit is a compressed Macintosh archive made with StuffIt, a commercial program. Aladdin Systems Inc.,

the manufacturer of StuffIt, has a free expander utility that will uncompress these archives. You can download

it at http://www.aladdinsys.com/expander/.

The file command can often tell you what a file is.

If you find that gzip complains when you try to uncompress a file, you probably downloaded it in ASCII

mode by mistake. You must download most things in binary mode: get, to download the file.

Q: Where Are linux/*.h and asm/*.h?

A: The files /usr/include/linux/ and /usr/include/asm/ are often soft links to the directories

where the kernel headers are. They are usually under /usr/src/kernel*/.

If you don't have the kernel sources, download them. Refer to the answer for How To Upgrade/Recompile a

Kernel.

Then, use rm to remove any garbage, and ln to create the links:

$ rm -rf /usr/include/linux /usr/include/asm$ ln -sf /usr/src/linux/include/linux /usr/include/linux

$ ln -sf /usr/src/linux/include/asm-<architecture> /usr/include/asm

The assembly language files reside in architecture-specific directories, so you need to link

/usr/src/include/asm to /usr/src/linux/include/asm-i386 on PC compatible systems, to

/usr/src/linux/include/asm-sparc on Sun Sparc systems, to

/usr/src/linux/include/asm-ppc on PPC systems, and so on.

You'll also find that you may need to do "make config" as in a newly-unpacked kernel source tree, to create

linux/autoconf.h.

Q: Configuring Emacs' Default Settings

A: Create a file in your home directory named .emacs with the Emacs Lisp commands that you want to run

every time Emacs starts up. You won't see the file in the directory listing. (The leading '.' tells ls not to display

it, unless you use the -a command line switch with ls.)

.Any kind of Emacs Lisp statement will work in the .emacs file, including entire defuns. Emacs uses lisp

variables and statements extensively, and many of the editing functions are written in Emacs Lisp. For

example, to enable word wrapping whenever you edit a file that ends with .txt, add the following statement.

This is from the Emacs Texinfo help document ( F1-i, then m Emacs Return):

(add-hook text-mode-hook

'(lambda () (auto-fill-mode1)))

This adds a statement that calls a hook function whenever a text editing mode is entered for that buffer. The

value of text-mode-hook, which is a variable, to auto-fill-mode, which is a function.

If you want to turn off the menu bar at the top of each Emacs frame, add this statement:

(menu-bar-mode -1)

And if you want to include an Emacs Lisp program that someone has written, like msb.el (an enhanced,

pop-up buffer menu), make sure the lisp file is in a directory where Emacs can find it (usually it will be

named Site-lisp), and add these statements in the .emacs file:

(require 'msb)

(msb-mode 1)

Most tasks have several possible solutions in Emacs Lisp. Any task that can be programmed in Emacs Lisp is

valid in the .emacs file. For more information, consult the Texinfo documentation. There is also a FAQ list

for Emacs (refer to: What other FAQ's are there for Linux? ).

The Linux FAQ page 13 Software Development

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