Linux with Operating System Concepts

Directive
Meaning
Example
#define
Macro substitution definition to replace a 
given string with another string
#define MAX 1000
#define getmax(a,b) 
((a 
>
b)?(a):(b))
#include
Include header file into this file before 
compilation
#include 
<
stdio.h 
>
#include “myheader.h”
#ifdef… #endif
If parameter has been defined, perform 
the following task
#ifdef MAX
int a[MAX];
#endif
#ifndef…#endif
If parameter is not defined, perform the 
following task
#ifndef MAX
#define MAX 1000
#endif
#if…#else…#endif
Test given parameter, perform associated 
action (#elif clauses are also permitted)
#if MAX 
>
500
int a[500];
#else
int a[MAX];
#endif

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Software Installation and Maintenance
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sense? For instance, if z is an array, you cannot apply 
++
to it whereas if x is an int, you 
cannot add 1.5 to it.
As with the previous two phases, semantic errors can be caught during this phase. 
While lexical and syntactic errors are misuses of the programming language itself, seman-
tic errors are misapplications of the programming language.
If the semantic analysis completes without error, the code can be converted. Typically, 
as an intermediate stage, the code is first converted to assembly language; thus we have an 
assembly component. Next, the assembly code is converted by assembler to machine lan-
guage code (executable code). Even here, gcc is not finished. Optionally, you can ask that 
gcc optimize the code. This is particularly useful for today’s processors that offer a number 
of parallel processing hardware elements that can execute program code more effectively 
if that code is compiled specifically to match the architecture. Optimization itself may 
require several additional passes through the code. The result is an executable program (or 
multiple executable programs).
13.6.4 Linking
Earlier in this chapter, we examined the role of the library file and the need to track down 
dependencies. The C/C
++
programming languages use library files extensively. This allows 
you to utilize pieces of precompiled code without having to write your own versions. 
Libraries in both C and C
++
are extensive and include such necessary functions as input 
and output operations and string and character operations but also include random num-
ber generation, dynamic memory allocation functions, math functions, type conversion 
operations, and diagnostic and testing functions to name just a few.
If (x > y–z) {
x = x+15;
[If-op] [Condition]
[If-clause]
[Block]
x > y–z
x 
y
z
–
<
[Assign]
[Id] [Relational-op] [Expr]
[Id] [arith-op] [id]
[Id] [assign-op] [expr];
x = [Id] [arith-op] [literal] 
[Id] [incr-op];
z++
x
+ 1.5
[Assign] }
{
z++;
)
(
}
FIGURE 13.9 
Parse tree for C instruction.

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Linux with Operating System Concepts
In order to utilize a function written in an external file, you must include the function’s 
prototype. This is a definition of the function, similar to a function header. For instance, 
the function prototype for the printf output function is
int printf(const char *format, . . .);
meaning that this function will return an int value and expects to receive at least one 
parameter, a char pointer (string). The … indicates that optional parameters may also be 
passed to the function.
In order to simplify the use of library functions, header files collect together information 
that the compiler will need in order to use those functions. These will include at a minimum 
the function prototypes but may also include constants and data types defined for use in 
those functions as well as global variables. These are collectively placed into a text file known 
as a header file, labeled with a .h extension. You include header files by using 
#include 
<