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Characters from the executable character set are output to the active position on the screen or in a file. The active position is defined by the ANSI C standard as the spot where the next output character will appear. After a character is output, the active position advances to the next position on the current line (to the left or right).
The Compaq C compiler moves the active position from left to right across an output line.
The C language was initially designed as a small, portable programming language used to implement an operating system. In its history, C has evolved into a powerful tool for writing all types of programs, and includes mechanisms to achieve most programming goals. C offers:
To help you take full advantage of C's features, the following sections provide a guide to the basic concepts of the language:
These sections represent an expanded glossary of selected C terms and
basic concepts. Understanding these concepts will provide a good
foundation for a working knowledge of C, and will help show the
relationship of these concepts to more complex ones in the language.
2.1 Blocks
A block in C is a section of code surrounded by braces { }. Understanding the definition of a block is very important to understanding many other C concepts, such as scope, visibility, and external or internal declarations.
The following example shows two blocks, one defined inside the other:
main ()
{ /* This brace marks the beginning of the outer block */
int x;
if (x!=0)
{ /* This brace marks the beginning of the inner block */
x = x++;
return x;
}; /* This brace marks the end of the inner block */
} /* This brace marks the end of the outer block */
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A block is also a form of a compound statement; a set of
related C statements enclosed in braces. Declarations of objects used
in the program can appear anywhere within a block and affect the
object's scope and visibility. Section 2.3 discusses scope;
Section 2.4 discusses visibility.
2.2 Compilation Units
A compilation unit is C source code that is compiled and treated as one logical unit. The compilation unit is usually one or more entire files, but can also be a selected portion of a file if, for example, the #ifdef preprocessor directive is used to select specific code sections. Declarations and definitions within a compilation unit determine the scope of functions and data objects.
Files included by using the #include preprocessor directive become part of the compilation unit. Source lines skipped because of the conditional inclusion preprocessor directives are not included in the compilation unit.
Compilation units are important in determining the scope of identifiers, and in determining the linkage of identifiers to other internal and external identifiers. Section 2.3 discusses scope. Section 2.8 discusses linkage.
A compilation unit can refer to data or functions in other compilation units in the following ways:
Programs composed of more than one compilation unit can be separately
compiled, and later linked to produce the executable program. A legal C
compilation unit consists of at least one external declaration, as
defined in Section 4.3.
A translation unit with no declarations is accepted with a compiler
warning in all modes except for the strict ANSI standard mode.
2.3 Scope
The scope of an identifier is the range of the program in which the declared identifier has meaning. An identifier has meaning if it is recognized by the compiler. Scope is determined by the location of the identifier's declaration. Trying to access an identifier outside of its scope results in an error. Every declaration has one of four kinds of scope:
An enumeration constant's scope begins at the defining enumerator in an
enumerator list. The scope of a statement label includes the entire
function body. The scope of any other type of identifier begins at the
identifier itself in the identifier's declaration. See the following
sections for information on when an identifier's scope ends.
2.3.1 File Scope
An identifier whose declaration is located outside any block or function parameter list has file scope. An identifier with file scope is visible from the declaration of the identifier to the end of the compilation unit, unless hidden by an inner block declaration. In the following example, the identifier off has file scope:
int off = 5; /* Declares (and defines) the integer
identifier off. */
main ()
{
int on; /* Declares the integer identifier on. */
on = off + 1; /* Uses off, declared outside the function
block of main. This point of the
program is still within the
active scope of off. */
if (on<=100)
{
int off = 0;/* This declaration of off creates a new object
that hides the former object of the same name.
The scope of the new off lasts through the
end of the if block. */
off = off + on;
return off;
}
}
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An identifier appearing within a block or in a parameter list of a function definition has block scope and is visible within the block, unless hidden by an inner block declaration.
Block scope begins at the identifier declaration and ends at the closing brace (}) completing the block. In the following example, the identifier red has block scope and blue has file scope:
int blue = 5; /* blue: file scope */
main ()
{
int x = 0 , y = 0; /* x and y: block scope */
int red = 10; /* red: block scope */
x = red + blue;
}
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Only statement labels have function scope (see Chapter 7). An identifier with function scope is unique throughout the function in which it is declared. Labeled statements are used as targets for goto statements and are implicitly declared by their syntax, which is the label followed by a colon (:) and a statement. For example:
int func1(int x, int y, int z)
{
label: x += (y + z); /* label has function scope */
if (x > 1) goto label;
}
int func2(int a, int b, int c)
{
if (a > 1) goto label; /* illegal jump to undefined label */
}
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See Section 7.1 for more information on statement labels.
2.3.4 Function Prototype Scope
An identifier that appears within a function prototype's list of parameter declarations has function prototype scope. The scope of such an identifier begins at the identifier's declaration and terminates at the end of the function prototype declaration list. For example:
int students ( int david, int susan, int mary, int john ); |
In this example, the identifiers (
david, susan, mary
, and
john
) have scope beginning at their declarations and ending at the closing
parenthesis. The type of the function
students
is "function returning
int
with four
int
parameters." In effect, these identifiers are merely placeholders
for the actual parameter names to be used after the function is defined.
2.4 Visibility
An identifier is visible only within a certain region of the program. An identifier has visibility over its entire scope, unless a subsequent declaration of the same identifier in an enclosed block overrides, or hides, the previous declaration. Visibility affects the ability to access a data object or other identifier, because an identifier can be used only where it is visible.
Once an identifier is used for a specific purpose, it cannot be used for another purpose within the same scope, unless the second use of the identifier is in a different name space. Section 2.15 describes the name space restrictions. For example, declarations of two different data objects using the same name as an identifier is illegal within the same scope.
When the scope of one of two identical identifiers is contained within the other (nested), the identifier with inner scope remains visible, while the identifier with wider scope becomes hidden for the duration of the inner identifier's scope.
In the following example, the identifier number is used twice: once as an integer variable and once as a floating-point variable. For the duration of the function main , the integer number is hidden by the floating-point number .
#include <math.h>
int number; /* number is declared as an integer variable */
main ()
{
float x;
float number; /* This declaration of number occurs in an inner
block, and "hides" the outer declaration.
The inner declaration creates a new object */
x = sqrt (number);/* x receives a floating-point value */
}
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The actual order in which expressions are evaluated is not specified for most of the operators in C. Because this sequence of evaluation is determined within the compiler depending on context, some unexpected results may occur when using certain operators. These unexpected results are caused by side effects.
Any operation that affects an operand's storage has a side effect. Side effects can be deliberately induced by the programmer to produce a desired result; in fact, the assignment operator depends on the side effect of altered storage to do its job. C guarantees that all side effects of a given expression will be completed by the next sequence point in the program. Sequence points are checkpoints in the program at which the compiler ensures that operations in an expression are concluded.
The most important sequence point is the semicolon marking the end of a statement. All expressions and their side effects are completely evaluated when the semicolon is reached. Other sequence points are as follows:
These operations do guarantee the order, or sequence, of evaluation (expr1), expr2, and expr3 are expressions). For each of these operators, the evaluation of expression expr1 is guaranteed to occur before the evaluation of expression expr2 (or expr3, in the case of the conditional expression).
Relying on the execution order of side effects, when none is guaranteed, is a risky practice because results are inconsistent and not portable. Undesirable side effects usually occur when the same data object is used in two or more places in the same expression, where at least one use produces a side effect. For example, the following code fragment produces inconsistent results because the order of evaluation of operands to the assignment operator is undefined.
int x[4] = { 0, 0, 0, 0 };
int i = 1;
x[i] = i++;
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If the increment of i occurs before the subscript is evaluated, the value of x[2] is 1. If the subscript is evaluated first, the value of x[1] is 1.
A function call also has side effects. In the following example, the order in which f1(y) and f2(z) are called is undefined:
int y = 0;
int z = 0;
int x = 0;
int f1(int s)
{
printf ("Now in f1\n");
y += 7; /* Storage of y affected */
return y;
}
int f2(int t)
{
printf ("Now in f2\n");
z += 3; /* Storage of z affected */
return z;
}
main ()
{
x = f1(y) + f2(z); /* Undefined calling order */
}
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The
printf
functions can be executed in any order even though the value of
x
will always be 10.
2.6 Incomplete Type
An identifier can be initially declared as having an incomplete type. An incomplete type declaration describes the object, but lacks the information needed to determine the object's size. For example, a declaration of an array of unknown size is an incomplete type declaration:
extern int x[]; |
The incomplete type may be completed in a subsequent declaration. Incomplete types are most commonly used when forward referencing arrays, structures, and unions. ( Section 2.12 discusses forward references.) An object of an aggregate type cannot contain a member of an incomplete type; therefore, an aggregate object (a structure or array member) cannot contain itself, because the aggregate type is not complete until the end of its declaration. The following example shows how an incomplete structure type is declared and later completed:
struct s
{ struct t *pt }; /* Incomplete structure declaration */
.
.
.
struct t
{ int a;
float *ps }; /* Completion of structure t */
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The
void
type is a special case of an incomplete type. It is an incomplete type
that cannot be completed, and is used to signify that a function
returns no value. Section 3.5 has more information on the
void
type.
2.7 Compatible and Composite Types
Compatibility between types refers to the similarity of two types to each other. Type compatibility is important during type conversions and operations. All valid declarations in the same scope that refer to the same object or function must have compatible types. Two types are compatible if they fit any of the following categories:
int tree (int);
int tree1 (char);
int tree2 (x)
char x; /* char promotes to int in old-style
function parameters, and so is
compatible with tree */
{
...
};
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The following types, which may appear to be compatible, are not:
A composite type is constructed from two compatible types and is compatible with both of the two types. Composite types satisfy the following conditions:
Consider the following file-scope declarations:
int f(int (*) (), double (*) [3]); int f(int (*) (char *), double (*)[]); |
They result in the following composite type for the function:
int f(int (*) (char *), double (*)[3]); |
The previous composite type rules apply recursively to types derived from composite types.
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