I'm building a .a from C++ code. It only depends on the standard library (libc++/libstdc++). From general reading, it seems that portability of binaries depends on
compiler version (because it can affect the ABI). For gcc, the ABI is linked to the major version number.
libc++/libstdc++ versions (because they could pass a vector<T> into the .a and its representation could change).
I.e. someone using the .a needs to use the same (major version of) the compiler + same standard library.
As far as I can see, if compiler and standard library match, a .a should work across multiple distros. Is this right? Or is there gubbins relating to system calls, etc., meaning a .a for Ubuntu should be built on Ubuntu, .a for CentOS should be built on CentOS, and so on?
Edit: see If clang++ and g++ are ABI incompatible, what is used for shared libraries in binary? (though it doens't answer this q.)
Edit 2: I am not accessing any OS features explicitly (e.g. via system calls). My only interaction with the system is to open files and read from them.
It only depends on the standard library
It could also depend implicitly upon other things (think of resources like fonts, configuration files under /etc/, header files under /usr/include/, availability of /proc/, of /sys/, external programs run by system(3) or execvp(3), specific file systems or devices, particular ioctl-s, available or required plugins, etc...)
These are kind of details which might make the porting difficult. For example look into nsswitch.conf(5).
The evil is in the details.
(in other words, without a lot more details, your question don't have much sense)
Linux is perceived as a free software ecosystem. The usual way of porting something is to recompile it on -or at least for- the target Linux distribution. When you do that several times (for different and many Linux distros), you'll understand what details are significant in your particular software (and distributions).
Most of the time, recompiling and porting a library on a different distribution is really easy. Sometimes, it might be hard.
For shared libraries, reading Program Library HowTo, C++ dlopen miniHowTo, elf(5), your ABI specification (see here for some incomplete list), Drepper's How To Write Shared Libraries could be useful.
My recommendation is to prepare binary packages for various common Linux distributions. For example, a .deb for Debian & Ubuntu (some particular versions of them).
Of course a .deb for Debian might not work on Ubuntu (sometimes it does).
Look also into things like autoconf (or cmake). You may want at least to have some externally provided #define-d preprocessor strings (often passed by -D to gcc or g++) which would vary from one distribution to the next (e.g. on some distributions, you print by popen-ing lp, on others, by popen-ing lpr, on others by interacting with some CUPS server etc...). Details matter.
My only interaction with the system is to open files
But even these vary a lot from one distribution to another one.
It is probable that you won't be able to provide a single -and the same one- lib*.a for several distributions.
NB: you probably need to budget more work than what you believe.
I was reading "Linux Kernel Development" by Robert Love. I came across a line which I am unable to understand fully:-
The kernel source is installed in /usr/src/linux. You should not use this source tree for development because the kernel version against which your C library is compiled is often linked to this tree.
It looks like I am unable to relate it with some very basic concept.
The /usr/src/linux area has a (usually incomplete) set of kernel headers that are used by the library header files. They should match the library, and not get messed up. Headers in /usr/include/linux are "private" but these should be those headers which were used in a compilation of your libraries (notably glibc) and hacking around the with a link in /usr/src is a mistake as Linus tried to explain many times - sometimes quite forcibly. Headers used in a kernel compilation are NOT searched for in subdirectories of /usr/src/linux but are specific to a kernel version and can be drastically different between different versions, or at least you do not have any guarantees that they are not.
I am currently working on updating the build system for a large pile of code, which happens to include a Linux C++ project. It would be nice if all of the developers here could run a build when hacking around with their own ideas, so I was examining if it would be possible to build this on vaguely modern Linux systems despite the target system being 2.6.18.
By 'vaguely modern' I am estimating something like GCC 4.5+, something that a distribution in the past year or two might come with. Currently I solve the libstdc++ issue by compiling that in statically, and any glibc issues are neatly worked around by remapping to old versions of the memcpy symbols (and so on) with a quick bit of wrapper code. So far so good.
The one problem I can't seem to completely figure out is that certain symbols built into the executable from the .o files are of type 'u', which is a GNU unique object, an extension to the ELF standard that 2.6.18 doesn't seem to recognise at all. This means the executable won't run because it can't find the symbols, though they are in fact present (just of type '?' on the target, from 'nm').
One can disable the use of GNU unique objects when compiling G++ but it's not exactly the most convenient solution. I can't see any way to just disable it when compiling code (distro gcc/g++ invariably has this option on), and I imagine the only way to get the target system to recognise it would be to update ld-linux and the kernel. That's almost certainly not going to happen.
Is there an option I haven't found to disable these symbol types? Or perhaps is there some neat way around this, or something that I'm missing? I am beginning to suspect it will just have to be compiled on G++ 4.1.x, which will mean an old Linux installation or building that from source.
I was trying to deal with the same problem (which led me to finding this question) and after a bunch of research came to the definitive conclusion that no, you are not missing anything, there is no way around this besides compiling your own g++. See this recent question on the gcc-help mailing list:
http://gcc.gnu.org/ml/gcc-help/2013-01/msg00008.html
I compared gcc sources and found that you can go as high as stock 4.4, as unique symbols were added in 4.5. However on RHEL/CentOS 6 they default to 4.4 but patched unique symbol support into it, so as usual one must beware of distribution-specific gcc versions. For me this is a huge bummer as it means that things compiled on RHEL 6 can't be run on RHEL 5, even with a copy of libstdc++ made just for gcc 4.4 + RHEL 5.
Here's the message where unique symbol support was first proposed, by the way:
https://gcc.gnu.org/ml/gcc-patches/2009-07/msg01240.html
If you search around you'll find that people have complained about it on other lists for various reasons, but I guess it's here to stay.
I was reading an article about cross-compiling for OSX on linux, but it was quite hard to understand.
What tools do I need? And what configurations are necessary?
Are there any tools for creating packages too?
First you need odcctools, which contains assembler and linker and such (like binutils but capable of handling the Mach-O object format). Then you need the system libraries from the official SDK. You can download it from Apple, but must agree to some stuff and become a member to do so. And finally good old gcc. Quite easy in theory, but in reallity a horrible mess. The easiest way to go (that I know of) is to use I'm Cross!.
Update: I found a newer and better updated method called xchain. It requires more manual work than I'm Cross! thou.
Recently, I switched my development environment from Windows to Linux. So far, I have only used Visual Studio for C++ development, so many concepts, like make and Autotools, are new to me. I have read the GNU makefile documentation and got almost an idea about it. But I am kind of confused about Autotools.
As far as I know, makefiles are used to make the build process easier.
Why do we need tools like Autotools just for creating the makefiles? Since all knows how to create a makefile, I am not getting the real use of Autotools.
What is the standard? Do we need to use tools like this or would just handwritten makefiles do?
You are talking about two separate but intertwined things here:
Autotools
GNU coding standards
Within Autotools, you have several projects:
Autoconf
Automake
Libtool
Let's look at each one individually.
Autoconf
Autoconf easily scans an existing tree to find its dependencies and create a configure script that will run under almost any kind of shell. The configure script allows the user to control the build behavior (i.e. --with-foo, --without-foo, --prefix, --sysconfdir, etc..) as well as doing checks to ensure that the system can compile the program.
Configure generates a config.h file (from a template) which programs can include to work around portability issues. For example, if HAVE_LIBPTHREAD is not defined, use forks instead.
I personally use Autoconf on many projects. It usually takes people some time to get used to m4. However, it does save time.
You can have makefiles inherit some of the values that configure finds without using automake.
Automake
By providing a short template that describes what programs will be built and what objects need to be linked to build them, Makefiles that adhere to GNU coding standards can automatically be created. This includes dependency handling and all of the required GNU targets.
Some people find this easier. I prefer to write my own makefiles.
Libtool
Libtool is a very cool tool for simplifying the building and installation of shared libraries on any Unix-like system. Sometimes I use it; other times (especially when just building static link objects) I do it by hand.
There are other options too, see StackOverflow question Alternatives to Autoconf and Autotools?.
Build automation & GNU coding standards
In short, you really should use some kind of portable build configuration system if you release your code to the masses. What you use is up to you. GNU software is known to build and run on almost anything. However, you might not need to adhere to such (and sometimes extremely pedantic) standards.
If anything, I'd recommend giving Autoconf a try if you're writing software for POSIX systems. Just because Autotools produce part of a build environment that's compatible with GNU standards doesn't mean you have to follow those standards (many don't!) :) There are plenty of other options, too.
Edit
Don't fear m4 :) There is always the Autoconf macro archive. Plenty of examples, or drop in checks. Write your own or use what's tested. Autoconf is far too often confused with Automake. They are two separate things.
First of all, the Autotools are not an opaque build system but a loosely coupled tool-chain, as tinkertim already pointed out. Let me just add some thoughts on Autoconf and Automake:
Autoconf is the configuration system that creates the configure script based on feature checks that are supposed to work on all kinds of platforms. A lot of system knowledge has gone into its m4 macro database during the 15 years of its existence. On the one hand, I think the latter is the main reason Autotools have not been replaced by something else yet. On the other hand, Autoconf used to be far more important when the target platforms were more heterogeneous and Linux, AIX, HP-UX, SunOS, ..., and a large variety of different processor architecture had to be supported. I don't really see its point if you only want to support recent Linux distributions and Intel-compatible processors.
Automake is an abstraction layer for GNU Make and acts as a Makefile generator from simpler templates. A number of projects eventually got rid of the Automake abstraction and reverted to writing Makefiles manually because you lose control over your Makefiles and you might not need all the canned build targets that obfuscate your Makefile.
Now to the alternatives (and I strongly suggest an alternative to Autotools based on your requirements):
CMake's most notable achievement is replacing AutoTools in KDE. It's probably the closest you can get if you want to have Autoconf-like functionality without m4 idiosyncrasies. It brings Windows support to the table and has proven to be applicable in large projects. My beef with CMake is that it is still a Makefile-generator (at least on Linux) with all its immanent problems (e.g. Makefile debugging, timestamp signatures, implicit dependency order).
SCons is a Make replacement written in Python. It uses Python scripts as build control files allowing very sophisticated techniques. Unfortunately, its configuration system is not on par with Autoconf. SCons is often used for in-house development when adaptation to specific requirements is more important than following conventions.
If you really want to stick with Autotools, I strongly suggest to read Recursive Make Considered Harmful (archived) and write your own GNU Makefile configured through Autoconf.
The answers already provided here are good, but I'd strongly recommend not taking the advice to write your own makefile if you have anything resembling a standard C/C++ project. We need the autotools instead of handwritten makefiles because a standard-compliant makefile generated by automake offers a lot of useful targets under well-known names, and providing all these targets by hand is tedious and error-prone.
Firstly, writing a Makefile by hand seems a great idea at first, but most people will not bother to write more than the rules for all, install and maybe clean. automake generates dist, distcheck, clean, distclean, uninstall and all these little helpers. These additional targets are a great boon to the sysadmin that will eventually install your software.
Secondly, providing all these targets in a portable and flexible way is quite error-prone. I've done a lot of cross-compilation to Windows targets recently, and the autotools performed just great. In contrast to most hand-written files, which were mostly a pain in the ass to compile. Mind you, it is possible to create a good Makefile by hand. But don't overestimate yourself, it takes a lot of experience and knowledge about a bunch of different systems, and automake creates great Makefiles for you right out of the box.
Edit: And don't be tempted to use the "alternatives". CMake and friends are a horror to the deployer because they aren't interface-compatible to configure and friends. Every half-way competent sysadmin or developer can do great things like cross-compilation or simple things like setting a prefix out of his head or with a simple --help with a configure script. But you are damned to spend an hour or three when you have to do such things with BJam. Don't get me wrong, BJam is probably a great system under the hood, but it's a pain in the ass to use because there are almost no projects using it and very little and incomplete documentation. autoconf and automake have a huge lead here in terms of established knowledge.
So, even though I'm a bit late with this advice for this question: Do yourself a favor and use the autotools and automake. The syntax might be a bit strange, but they do a way better job than 99% of the developers do on their own.
For small projects or even for large projects that only run on one platform, handwritten makefiles are the way to go.
Where autotools really shine is when you are compiling for different platforms that require different options. Autotools is frequently the brains behind the typical
./configure
make
make install
compilation and install steps for Linux libraries and applications.
That said, I find autotools to be a pain and I've been looking for a better system. Lately I've been using bjam, but that also has its drawbacks. Good luck finding what works for you.
Autotools are needed because Makefiles are not guaranteed to work the same across different platforms. If you handwrite a Makefile, and it works on your machine, there is a good chance that it won't on mine.
Do you know what unix your users will be using? Or even which distribution of Linux? Do you know where they want software installed? Do you know what tools they have, what architecture they want to compile on, how many CPUs they have, how much RAM and disk might be available to them?
The *nix world is a cross-platform landscape, and your build and install tools need to deal with that.
Mind you, the auto* tools date from an earlier epoch, and there are many valid complaints about them, but the several projects to replace them with more modern alternatives are having trouble developing a lot of momentum.
Lots of things are like that in the *nix world.
Autotools is a disaster.
The generated ./configure script checks for features that have not been present on any Unix system for last 20 years or so. To do this, it spends a huge amount of time.
Running ./configure takes for ages. Although modern server CPUs can have even dozens of cores, and there may be several such CPUs per server, the ./configure is single-threaded. We still have enough years of Moore's law left that the number of CPU cores will go way up as a function of time. So, the time ./configure takes will stay approximately constant whereas parallel build times reduce by a factor of 2 every 2 years due to Moore's law. Or actually, I would say the time ./configure takes might even increase due to increasing software complexity taking advantage of improved hardware.
The mere act of adding just one file to your project requires you to run automake, autoconf and ./configure which will take ages, and then you'll probably find that since some important files have changed, everything will be recompiled. So add just one file, and make -j${CPUCOUNT} recompiles everything.
And about make -j${CPUCOUNT}. The generated build system is a recursive one. Recursive make has for a long amount of time been considered harmful.
Then when you install the software that has been compiled, you'll find that it doesn't work. (Want proof? Clone protobuf repository from Github, check out commit 9f80df026933901883da1d556b38292e14836612, install it to a Debian or Ubuntu system, and hey presto: protoc: error while loading shared libraries: libprotoc.so.15: cannot open shared object file: No such file or directory -- since it's in /usr/local/lib and not /usr/lib; workaround is to do export LD_RUN_PATH=/usr/local/lib before typing make).
The theory is that by using autotools, you could create a software package that can be compiled on Linux, FreeBSD, NetBSD, OpenBSD, DragonflyBSD and other operating systems. The fact? Every non-Linux system to build packages from source has numerous patch files in their repository to work around autotools bugs. Just take a look at e.g. FreeBSD /usr/ports: it's full of patches. So, it would have been as easy to create a small patch for a non-autotools build system on a per project basis than to create a small patch for an autotools build system on a per project basis. Or perhaps even easier, as standard make is much easier to use than autotools.
The fact is, if you create your own build system based on standard make (and make it inclusive and not recursive, following the recommendations of the "Recursive make considered harmful" paper), things work in a much better manner. Also, your build time goes down by an order of magnitude, perhaps even two orders of magnitude if your project is very small project of 10-100 C language files and you have dozens of cores per CPU and multiple CPUs. It's also much easier to interface custom automatic code generation tools with a custom build system based on standard make instead of dealing with the m4 mess of autotools. With standard make, you can at least type a shell command into the Makefile.
So, to answer your question: why use autotools? Answer: there is no reason to do so. Autotools has been obsolete since when commercial Unix has become obsolete. And the advent of multi-core CPUs has made autotools even more obsolete. Why programmers haven't realized that yet, is a mystery. I'll happily use standard make on my build systems, thank you. Yes, it takes some amount of work to generate the dependency files for C language header inclusion, but the amount of work is saved by not having to fight with autotools.
I dont feel I am an expert to answer this but still give you a bit analogy with my experience.
Because upto some extent it is similar to why we should write Embedded Codes in C language(High Level language) rather then writing in Assembly Language.
Both solves the same purpose but latter is more lenghty, tedious ,time consuming and more error prone(unless you know ISA of the processor very well) .
Same is the case with Automake tool and writing your own makefile.
Writing Makefile.am and configure.ac is pretty simple than writing individual project Makefile.