Page 115 - C-Language

executed. Binary executables have no special suffix on Unix operating systems, although
they generally end in .exe on Windows.
e.g., foo foo.exe

5. Libraries: A library is a compiled binary but is not in itself an an executable (i.e., there is no
main() function in a library). A library contains functions that may be used by more than one
program. A library should ship with header files which contain prototypes for all functions in
the library; these header files should be referenced (e.g; #include <library.h>) in any source
file that uses the library. The linker then needs to be referred to the library so the program
can successfully compiled. There are two types of libraries: static and dynamic.

• Static library: A static library (.a files for POSIX systems and .lib files for Windows —
not to be confused with DLL import library files, which also use the .lib extension) is
statically built into the program . Static libraries have the advantage that the program
knows exactly which version of a library is used. On the other hand, the sizes of
executables are bigger as all used library functions are included.

e.g., libfoo.a foo.lib
• Dynamic library: A dynamic library (.so files for most POSIX systems, .dylib for OSX
and .dll files for Windows) is dynamically linked at runtime by the program. These are
also sometimes referred to as shared libraries because one library image can be
shared by many programs. Dynamic libraries have the advantage of taking up less disk
space if more than one application is using the library. Also, they allow library updates
(bug fixes) without having to rebuild executables.

e.g., foo.so foo.dylib foo.dll

The Preprocessor

Before the C compiler starts compiling a source code file, the file is processed in a preprocessing
phase. This phase can be done by a separate program or be completely integrated in one
executable. In any case, it is invoked automatically by the compiler before compilation proper
begins. The preprocessing phase converts your source code into another source code or
translation unit by applying textual replacements. You can think of it as a "modified" or "expanded"
source code. That expanded source may exist as a real file in the file system, or it may only be
stored in memory for a short time before being processed further.

Preprocessor commands start with the pound sign ("#"). There are several preprocessor
commands; two of the most important are:

1. Defines:

#define is mainly used to define constants. For instance,

#define BIGNUM 1000000
int a = BIGNUM;

becomes

int a = 1000000;

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