• Choose a category
• All
• Classic-Fiction

papaya$ gcc -I../include -L../lib -o wibble wibble.c -lstuff -lm

You might wonder what the linker does if a shared library libstuff.so and a static library libstuff.a are available. In this case, the linker always picks the shared library. To make it use the static one, you will have to name it explicitly on the command line:

papaya$ gcc -I../include -L../lib -o wibble wibble.c libstuff.a -lm

Another very useful tool for working with shared libraries is ldd. It tells you which shared libraries an executable program uses. Here's an example:

papaya$ ldd wibble

linux-gate.so.1 => (0xffffe000)

libstuff.so => libstuff.so (0x400af000)

libm.so.5 => /lib/libm.so.5 (0x400ba000)

libc.so.5 => /lib/libc.so.5 (0x400c3000)

The three fields in each line are the name of the library, the full path to the instance of the library that is used, and where in the virtual address space the library is mapped to. The first line is something arcane, part of the Linux loader implementation that you can happily ignore.

If ldd outputs not found for a certain library, you are in trouble and won't be able to run the program in question. You will have to search for a copy of that library. Perhaps it is a library shipped with your distribution that you opted not to install, or it is already on your hard disk but the loader (the part of the system that loads every executable program) cannot find it.

In the latter situation, try locating the libraries yourself and find out whether they're in a nonstandard directory. By default, the loader looks only in /lib and /usr/lib. If you have libraries in another directory, create an environment variable LD_LIBRARY_PATH and add the directories separated by colons. If you believe that everything is set up correctly, and the library in question still cannot be found, run the command ldconfig as root, which refreshes the linker system cache.

Using C++

If you prefer object-oriented programming, gcc provides complete support for C++ as well as Objective-C. There are only a few considerations you need to be aware of when doing C++ programming with gcc.

First, C++ source filenames should end in the extension .cpp (most often used), .C, or .cc. This distinguishes them from regular C source filenames, which end in .c. It is actually possible to tell gcc to compile even files ending in .c as C++ files, by using the command-line parameter -x c++, but that is not recommended, as it is likely to confuse you.

Second, you should use the g++ shell script in lieu of gcc when compiling C++ code. g++ is simply a shell script that invokes gcc with a number of additional arguments, specifying a link against the C++ standard libraries, for example. g++ takes the same arguments and options as gcc.

If you do not use g++, you'll need to be sure to link against the C++ libraries in order to use any of the basic C++ classes, such as the cout and cin I/O objects. Also be sure you have actually installed the C++ libraries and include files. Some distributions contain only the standard C libraries. gcc will be able to compile your C++ programs fine, but without the C++ libraries, you'll end up with linker errors whenever you attempt to use standard objects.

* * *

[*] On a variety of Unix systems, the authors have repeatedly found available documentation to be insufficient. With Linux, you can explore the very source code for the kernel, libraries, and system utilities. Having access to source code is more important than most programmers think.

[*] The name is a very clever pun, if you think about the tool for a while.

[*] It should be noted that some very knowledgeable programmers consider shared libraries harmful, for reasons too involved to be explained here. They say that we shouldn't need to bother in a time when most computers ship with 80-GB hard disks and at least 256 MB of memory preinstalled.

[*] In the ancient days of Linux, creating a shared library was a daunting task of which