C For Swimmers : Mastering C step-by-step

Unix Programming - Beginners Level

Compiling "C" And "C++" Programs On Unix Systems - gcc/g++

1. Preface
2. Compiling A Single-Source "C" Program
3. Running The Resulting Program
4. Creating Debug-Ready Code
5. Creating Optimized Code
6. Getting Extra Compiler Warnings
7. Compiling A Single-Source "C++" Program
8. Compiling A Multi-Source "C" Program
9. Getting a Deeper Understanding - Compilation Steps

This document tries to give the reader basic knowledge in compiling C and C++ programs on a Unix system. If you've no knowledge as to how to compile C programs under Unix (for instance, you did that until now on other operating systems), you'd better read this tutorial first, and then write a few programs before you try to get to gdb, makefiles or C libraries.

If you're already familiar with that, it's recommended to learn about makefiles, and then go and learn other C programming topics and practice the usage of makefiles, before going on to read about C libraries. This last issue is only relevant to larger projects, while makefiles make sense even for a small program composed of but a few source files.

As a policy, we'll stick with the basic features of programming tools mentioned here, so that the information will apply to more than a single tool version. This way, you might find the information here useful, even if the system you're using does not have the GNU tools installed.

In this lovely tutorial, we'll deal with compilation of a C program, using the compiler directly from the command line. It might be that you'll eventually use a more sophisticated interface (an IDE - Integrated Development Environment) of some sort, but the common denominator you'll always find is the plain command line interface. Further more, even if you use an IDE, it could help you understand how things work "behind the scenes". We'll see how to compile a program, how to combine several source files into a single program, how to add debug information and how to optimize code.

The easiest case of compilation is when you have all your source code set in a single file. This removes any unnecessary steps of synchronizing several files or thinking too much. Lets assume there is a file named 'single_main.c' that we want to compile. We will do so using a command line similar to this:

cc single_main.c

Note that we assume the compiler is called "cc". If you're using a GNU compiler, you'll write 'gcc' instead. If you're using a Solaris system, you might use 'acc', and so on. Every compiler might show its messages (errors, warnings, etc.) differently, but in all cases, you'll get a file 'a.out' as a result, if the compilation completed successfully. Note that some older systems (e.g. SunOs) come with a C compiler that does not understand ANSI-C, but rather the older 'K&R' C style. In such a case, you'll need to use gcc (hopefully it is installed), or learn the differences between ANSI-C and K&R C (not recommended if you don't really have to), or move to a different system.

You might complain that 'a.out' is a too generic name (where does it come from anyway? - well, that's a historical name, due to the usage of something called "a.out format" for programs compiled on older Unix systems). Suppose that you want the resulting program to be called "single_main". In that case, you could use the following line to compile it:

cc single_main.c -o single_main

Every compiler I've met so far (including the glorious gcc) recognized the '-o' flag as "name the resulting executable file 'single_main'".

Once we created the program, we wish to run it. This is usually done by simply typing its name, as in:

single_main

However, this requires that the current directory be in our PATH (which is a variable telling our Unix shell where to look for programs we're trying to run). In many cases, this directory is not placed in our PATH. Aha! - we say. Then lets show this computer who is smarter, and thus we try:

./single_main

This time we explicitly told our Unix shell that we want to run the program from the current directory. If we're lucky enough, this will suffice. However, yet one more obstacle could block our path - file permission flags.

When a file is created in the system, it is immediately given some access permission flags. These flags tell the system who should be given access to the file, and what kind of access will be given to them. Traditional Unix systems use 3 kinds of entities to which they grant (or deny) access: The user which owns the file, the group which owns the file, and everybody else. Each of these entities may be given access to read the file ('r'), write to the file ('w') and execute the file ('x').

Now, when the compiler created the program file for us, we became owners of the file. Normally, the compiler would make sure that we get all permissions to the file - read, write and execute. It might be, thought that something went wrong, and the permissions are set differently. In that case, we can set the permissions of the file properly (the owner of a file can normally change the permission flags of the file), using a command like this:

chmod u+rwx single_main

This means "the user ('u') should be given ('+') permissions read ('r'), write ('w') and execute ('x') to the file 'single_main'. Now we'll surely be able to run our program. Again, normally you'll have no problem running the file, but if you copy it to a different directory, or transfer it to a different computer over the network, it might loose its original permissions, and thus you'll need to set them properly, as shown above. Note too that you cannot just move the file to a different computer an expect it to run - it has to be a computer with a matching operating system (to understand the executable file format), and matching CPU architecture (to understand the machine-language code that the executable file contains).

Finally, the run-time environment has to match. For example, if we compiled the program on an operating system with one version of the standard C library, and we try to run it on a version with an incompatible standard C library, the program might crush, or complain that it cannot find the relevant C library. This is especially true for systems that evolve quickly (e.g. Linux with libc5 vs. Linux with libc6), so beware.

Normally, when we write a program, we want to be able to debug it - that is, test it using a debugger that allows running it step by step, setting a break point before a given command is executed, looking at contents of variables during program execution, and so on. In order for the debugger to be able to relate between the executable program and the original source code, we need to tell the compiler to insert information to the resulting executable program that'll help the debugger. This information is called "debug information". In order to add that to our program, lets compile it differently:

cc -g single_main.c -o single_main

The '-g' flag tells the compiler to use debug info, and is recognized by mostly any compiler out there. You will note that the resulting file is much larger than that created without usage of the '-g' flag. The difference in size is due to the debug information. We may still remove this debug information using the strip command, like this:

strip single_main

You'll note that the size of the file now is even smaller than if we didn't use the '-g' flag in the first place. This is because even a program compiled without the '-g' flag contains some symbol information (function names, for instance), that the strip command removes. You may want to read strip's manual page (man strip) to understand more about what this command does.

After we created a program and debugged it properly, we normally want it to compile into an efficient code, and the resulting file to be as small as possible. The compiler can help us by optimizing the code, either for speed (to run faster), or for space (to occupy a smaller space), or some combination of the two. The basic way to create an optimized program would be like this:

cc -O single_main.c -o single_main

The '-O' flag tells the compiler to optimize the code. This also means the compilation will take longer, as the compiler tries to apply various optimization algorithms to the code. This optimization is supposed to be conservative, in that it ensures us the code will still perform the same functionality as it did when compiled without optimization (well, unless there are bugs in our compiler). Usually can define an optimization level by adding a number to the '-O' flag. The higher the number - the better optimized the resulting program will be, and the slower the compiler will complete the compilation. One should note that because optimization alters the code in various ways, as we increase the optimization level of the code, the chances are higher that an improper optimization will actually alter our code, as some of them tend to be non-conservative, or are simply rather complex, and contain bugs. For example, for a long time it was known that using a compilation level higher than 2 (or was it higher than 3?) with gcc results bugs in the executable program. After being warned, if we still want to use a different optimization level (lets say 4), we can do it this way:

cc -O4 single_compile.c -o single_compile

And we're done with it. If you'll read your compiler's manual page, you'll soon notice that it supports an almost infinite number of command line options dealing with optimization. Using them properly requires thorough understanding of compilation theory and source code optimization theory, or you might damage your resulting code. A good compilation theory course (preferably based on "the Dragon Book" by Aho, Sethi and Ulman) could do you good.

Normally the compiler only generates error messages about erroneous code that does not comply with the C standard, and warnings about things that usually tend to cause errors during runtime. However, we can usually instruct the compiler to give us even more warnings, which is useful to improve the quality of our source code, and to expose bugs that will really bug us later. With gcc, this is done using the '-W' flag. For example, to get the compiler to use all types of warnings it is familiar with, we'll use a command line like this:

cc -Wall single_source.c -o single_source

This will first annoy us - we'll get all sorts of warnings that might seem irrelevant. However, it is better to eliminate the warnings than to eliminate the usage of this flag. Usually, this option will save us more time than it will cause us to waste, and if used consistently, we will get used to coding proper code without thinking too much about it. One should also note that some code that works on some architecture with one compiler, might break if we use a different compiler, or a different system, to compile the code on. When developing on the first system, we'll never see these bugs, but when moving the code to a different platform, the bug will suddenly appear. Also, in many cases we eventually will want to move the code to a new system, even if we had no such intentions initially.

Note that sometimes '-Wall' will give you too many errors, and then you could try to use some less verbose warning level. Read the compiler's manual to learn about the various '-W' options, and use those that would give you the greatest benefit. Initially they might sound too strange to make any sense, but if you are (or when you will become) a more experienced programmer, you will learn which could be of good use to you.

Now that we saw how to compile C programs, the transition to C++ programs is rather simple. All we need to do is use a C++ compiler, in place of the C compiler we used so far. So, if our program source is in a file named 'single_main.cc' ('cc' to denote C++ code. Some programmers prefer a suffix of 'C' for C++ code), we will use a command such as the following:

g++ single_main.cc -o single_main

Or on some systems you'll use "CC" instead of "g++" (for example, with Sun's compiler for Solaris), or "aCC" (HP's compiler), and so on. You would note that with C++ compilers there is less uniformity regarding command line options, partially because until recently the language was evolving and had no agreed standard. But still, at least with g++, you will use "-g" for debug information in the code, and "-O" for optimization.

So you learned how to compile a single-source program properly (hopefully by now you played a little with the compiler and tried out a few examples of your own). Yet, sooner or later you'll see that having all the source in a single file is rather limiting, for several reasons:

• As the file grows, compilation time tends to grow, and for each little change, the whole program has to be re-compiled.
• It is very hard, if not impossible, that several people will work on the same project together in this manner.
• Managing your code becomes harder. Backing out erroneous changes becomes nearly impossible.

There are two possible ways to compile a multi-source C program. The first is to use a single command line to compile all the files. Suppose that we have a program whose source is found in files "main.c", "a.c" and "b.c" (found in directory "multi-source" of this tutorial). We could compile it this way:

cc main.c a.c b.c -o hello_world

This will cause the compiler to compile each of the given files separately, and then link them all together to one executable file named "hello_world". Two comments about this program:

1. If we define a function (or a variable) in one file, and try to access them from a second file, we need to declare them as external symbols in that second file. This is done using the C "extern" keyword.
2. The order of presenting the source files on the command line may be altered. The compiler (actually, the linker) will know how to take the relevant code from each file into the final program, even if the first source file tries to use a function defined in the second or third source file.

In order to overcome this limitation, we could divide the compilation process into two phases - compiling, and linking. Lets first see how this is done, and then explain:

cc -c main.cc
cc -c a.c
cc -c b.c
cc main.o a.o b.o -o hello_world

The first 3 commands have each taken one source file, and compiled it into something called "object file", with the same names, but with a ".o" suffix. It is the "-c" flag that tells the compiler only to create an object file, and not to generate a final executable file just yet. The object file contains the code for the source file in machine language, but with some unresolved symbols. For example, the "main.o" file refers to a symbol named "func_a", which is a function defined in file "a.c". Surely we cannot run the code like that. Thus, after creating the 3 object files, we use the 4th command to link the 3 object files into one program. The linker (which is invoked by the compiler now) takes all the symbols from the 3 object files, and links them together - it makes sure that when "func_a" is invoked from the code in object file "main.o", the function code in object file "a.o" gets executed. Further more, the linker also links the standard C library into the program, in this case, to resolve the "printf" symbol properly.

To see why this complexity actually helps us, we should note that normally the link phase is much faster than the compilation phase. This is especially true when doing optimizations, since that step is done before linking. Now, lets assume we change the source file "a.c", and we want to re-compile the program. We'll only need now two commands:

cc -c a.c
cc main.o a.o b.o -o hello_world

In our small example, it's hard to notice the speed-up, but in a case of having few tens of files each containing a few hundred lines of source-code, the time saving is significant; not to mention even larger projects.

Now that we've learned that compilation is not just a simple process, lets try to see what is the complete list of steps taken by the compiler in order to compile a C program.

1. Driver - what we invoked as "cc". This is actually the "engine", that drives the whole set of tools the compiler is made of. We invoke it, and it begins to invoke the other tools one by one, passing the output of each tool as an input to the next tool.
2. C Pre-Processor - normally called "cpp". It takes a C source file, and handles all the pre-processor definitions (#include files, #define macros, conditional source code inclusion with #ifdef, etc.) You can invoke it separately on your program, usually with a command like:
   
   cc -E single_source.c
   
   Try this and see what the resulting code looks like.
3. The C Compiler - normally called "cc1". This is the actual compiler, that translates the input file into assembly language. As you saw, we used the "-c" flag to invoke it, along with the C Pre-Processor, (and possibly the optimizer too, read on), and the assembler.
4. Optimizer - sometimes comes as a separate module and sometimes as the found inside the compiler module. This one handles the optimization on a representation of the code that is language-neutral. This way, you can use the same optimizer for compilers of different programming languages.
5. Assembler - sometimes called "as". This takes the assembly code generated by the compiler, and translates it into machine language code kept in object files. With gcc, you could tell the driver to generated only the assembly code, by a command like:
   
   cc -S single_source.c
   
6. Linker-Loader - This is the tool that takes all the object files (and C libraries), and links them together, to form one executable file, in a format the operating system supports. A Common format these days is known as "ELF". On SunOs systems, and other older systems, a format named "a.out" was used. This format defines the internal structure of the executable file - location of data segment, location of source code segment, location of debug information and so on.

As you see, the compilation is split in to many different phases. Not all compiler employs exactly the same phases, and sometimes (e.g. for C++ compilers) the situation is even more complex. But the basic idea is quite similar - split the compiler into many different parts, to give the programmer more flexibility, and to allow the compiler developers to re-use as many modules as possible in different compilers for different languages (by replacing the preprocessor and compiler modules), or for different architectures (by replacing the assembly and linker-loader parts).

A copy of the original document can be found at http://users.actcom.co.il/~choo/lupg/tutorials/index.html

This document is copyright (c) 1998-2002 by guy keren.

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