No, C does not have a boolean variable type.
One can use ints, chars, #defines or enums to achieve the same in
C.
#define TRUE 1
#define FALSE 0
enum bool {false, true};
An enum may be good if the debugger shows the
names of enum constants when examining variables.
Q Where may
variables be defined in C?
Outside a function definition (global scope,
from the point of definition downward in the source code). Inside a block before
any statements other than variable declarations (local scope with respect to the
block).
Q What does the typedef
keyword do?
This keyword provides a short-hand way to write
variable declarations. It is not a true data typing mechanism, rather, it is
syntactic "sugar coating".
For example
typedef struct node
{
int
value;
struct node *next;
}mynode;
This can later be used to declare variables
like this
mynode *ptr1;
& not by the lengthy
expression
struct node *ptr1;
There are three main reasons for using
typedefs:
* It makes the writing of complicated declarations a lot
easier. This helps in eliminating a lot of clutter in the code.
*
It helps in achieving portability in programs. That is, if we use typedefs for
data types that are machine dependent, only the typedefs need to change when the
program is ported to a new platform.
* It helps in providing better
documentation for a program. For example, a node of a doubly linked list is
better understood as ptrToList than just a pointer to a complicated
structure.
Q What is the difference between constants defined
through #define & the constant keyword?
A constant is similar to a variable in the
sense that it represents a memory location (or simply, a value). It is different
from a normal variable, in that it cannot change it's value in the proram - it
must stay for ever stay constant. In general, constants are a useful because
they can prevent program bugs & logical errors(errors are explained later).
Unintended modifications are prevented from occurring. The compiler will catch
attempts to reassign new values to constants.
Constants may be defined
using the preprocessor directive #define. They may also be defined using the
const keyword.
So whats the difference between these
two?
#define ABC 5
&
const int
abc = 5;
There are two main advantages of the second one
over the first technique. First, the type of the constant is defined. "pi" is
float. This allows for some type checking by the compiler. Second, these
constants are variables with a definite scope. The scope of a variable relates
to parts of your program in which it is defined.
There is also one good
use of the important use of the const keyword. Suppose you want to make use of
some structure data in some function. You will pass a pointer to that structure
as argument to that function. But to make sure that your structure is readonly
inside the function you can declare the structure argument as const in function
prototype. This will prevent any accidental modification of the structure values
inside the function.
Q What are Trigraph
characters?
These are used when you keyboard does not
support some special characters
??= #
??( [
??) ]
??< {
??> }
??!
|
??/ \
??' ^
??-
~
Q How are floating point numbers stored? Whats the IEEE
format?
IEEE Standard 754 floating point is the most
common representation today for real numbers on computers, including Intel-based
PC's, Macintoshes, & most Unix platforms.
IEEE floating point
numbers have three basic components: the sign, the exponent, & the mantissa.
The mantissa is composed of the fraction & an implicit leading digit
(explained below). The exponent base(2) is implicit & need not be
stored.
The following figure shows the layout for single (32-bit) &
double (64-bit) precision floating-point values. The number of bits for each
field are shown (bit ranges are in square brackets):
Sign Exponent Fraction
Bias
--------------------------------------------------
Single Precision 1 [31] 8 [30-23] 23 [22-00] 127
Double Precision 1
[63] 11 [62-52] 52 [51-00] 1023
The sign bit is as simple as it gets. 0 denotes
a positive number; 1 denotes a negative number. Flipping the value of this bit
flips the sign of the number.
The exponent field needs to represent
both positive & negative exponents. To do this, a bias is added to the
actual exponent in order to get the stored exponent. For IEEE single-precision
floats, this value is 127. Thus, an exponent of zero means that 127 is stored in
the exponent field. A stored value of 200 indicates an exponent of (200-127), or
73. For reasons discussed later, exponents of -127 (all 0s) & +128 (all 1s)
are reserved for special numbers. For double precision, the exponent field is 11
bits, & has a bias of 1023.
The mantissa, also known as the
significand, represents the precision bits of the number. It is composed of an
implicit leading bit & the fraction bits. To find out the value of the
implicit leading bit, consider that any number can be expressed in scientific
notation in many different ways. For example, the number five can be represented
as any of these:
5.00 × 100
0.05 × 10 ^
2
5000 × 10 ^ -3
In order to maximize the quantity of
representable numbers, floating-point numbers are typically stored in normalized
form. This basically puts the radix point after the first non-zero digit. In
normalized form, five is represented as 5.0 × 100. A nice little optimization is
available to us in base two, since the only possible non-zero digit is 1. Thus,
we can just assume a leading digit of 1, & don't need to represent it
explicitly. As a result, the mantissa has effectively 24 bits of resolution, by
way of 23 fraction bits.
So, to sum up:
1. The sign bit is 0 for positive, 1 for
negative.
2. The exponent's base is two.
3. The exponent
field contains 127 plus the true exponent for single-precision, or 1023 plus
the true exponent for double precision.
4. The first bit of the
mantissa is typically assumed to be 1.f, where f is the field of fraction
bits.
Q. When should the register modifier be
used?
The register modifier hints to the compiler
that the variable will be heavily used & should be kept in the CPU?s
registers, if possible, so that it can be accessed faster. There are several
restrictions on the use of the register modifier.
First, the variable
must be of a type that can be held in the CPU?s register. This usually means a
single value of a size less than or equal to the size of an integer. Some
machines have registers that can hold floating-point numbers as well. Second,
because the variable might not be stored in memory, its address cannot be taken
with the unary & operator. An attempt to do so is flagged as an error by the
compiler. Some additional rules affect how useful the register modifier is.
Because the number of registers is limited, & because some registers can
hold only certain types of data (such as pointers or floating-point numbers),
the number & types of register modifiers that will actually have any effect
are dependent on what machine the program will run on. Any additional register
modifiers are silently ignored by the compiler. Also, in some cases, it might
actually be slower to keep a variable in a register because that register then
becomes unavailable for other purposes or because the variable isn?t used enough
to justify the overhead of loading & storing it. So when should the register
modifier be used? The answer is never, with most modern compilers. Early C
compilers did not keep any variables in registers unless directed to do so,
& the register modifier was a valuable addition to the language. C compiler
design has advanced to the point, however, where the compiler will usually make
better decisions than the programmer about which variables should be stored in
registers. In fact, many compilers actually ignore the register modifier, which
is perfectly legal, because it is only a hint & not a directive.
Q. When should a type cast be
used?
There are two situations in which to use a type
cast.
The first use is to change the type of an operand to an
arithmetic operation so that the operation will be performed properly.
The second case is to cast pointer types to & from void * in order to
interface with functions that expect or return void pointers. For example, the
following line type casts the return value of the call to malloc() to be a
pointer to a foo structure.
struct foo *p = (struct foo *)
malloc(sizeof(struct foo));
A type cast should not be used to override a
const or volatile declaration. Overriding these type modifiers can cause the
program to fail to run correctly. A type cast should not be used to turn a
pointer to one type of structure or data type into another. In the
rare
events in which this action is beneficial, using a union to hold the values
makes the programmer?s intentions clearer.
Q. Can structures be assigned to
variables & passed to & from functions?
Yes, they can!
But note that when
structures are passed, returned or assigned, the copying is done only at one
level (The data pointed to by any pointer fields is not copied!.
Q What is the difference between the
declaration & the definition of a variable?.
The definition is the one that actually
allocates space, & provides an initialization value, if any.
There
can be many declarations, but there must be exactly one definition. A definition
tells the compiler to set aside storage for the variable. A declaration makes
the variable known to parts of the program that may wish to use it. A variable
might be defined & declared in the same statement.
Q Do Global variables start out as
zero?
Un initialized variables declared with the
"static" keyword are initialized to zero. Such variables are implicitly
initialized to the null pointer if they are pointers, & to 0.0F if they are
floating point numbers.
Local variables start out containing garbage,
unless they are explicitly initialized.
Memory obtained with malloc()
& realloc() is likely to contain junk, & must be initialized. Memory
obtained with calloc() is all-bits-0, but this is not necessarily useful for
pointer or floating-point values (This is in contrast to Global pointers &
Global floating point numbers, which start as zeroes of the right
type).
Q To what does the term storage class refer?
What are auto, static, extern, volatile, const classes?
This is a part of a variable declaration that
tells the compiler how to interpret the variable's symbol. It does not in itself
allocate storage, but it usually tells the compiler how the variable should be
stored. Storage class specifiers help you to specify the type of storage used
for data objects. Only one storage class specifier is permitted in a declaration
this makes sense, as there is only one way of storing things & if you omit
the storage class specifier in a declaration, a default is chosen. The default
depends on whether the declaration is made outside a function (external
declarations) or inside a function (internal declarations). For external
declarations the default storage class specifier will be extern & for
internal declarations it will be auto. The only exception to this rule is the
declaration of functions, whose default storage class specifier is always
extern.
Here are C's storage classes & what they
signify:
* auto - local variables.
* static -
variables are defined in a nonvolatile region of memory such that they retain
their contents though out the program's execution.
* register -
asks the compiler to devote a processor register to this variable in order to
speed the program's execution. The compiler may not comply & the variable
looses it contents & identity when the function it which it is defined
terminates.
* extern - tells the compiler that the variable is
defined in another module.
In C, const & volatile are type qualifiers.
The const & volatile type qualifiers are completely independent. A common
misconception is to imagine that somehow const is the opposite of volatile &
vice versa. This is wrong. The keywords const & volatile can be applied to
any declaration, including those of structures, unions, enumerated types or
typedef names. Applying them to a declaration is called qualifying the
declaration?that's why const & volatile are called type qualifiers, rather
than type specifiers.
* const means that something is not
modifiable, so a data object that is declared with const as a part of its type
specification must not be assigned to in any way during the run of a program.
The main intention of introducing const objects was to allow them to be put into
read-only store, & to permit compilers to do extra consistency checking in a
program. Unless you defeat the intent by doing naughty things with pointers, a
compiler is able to check that const objects are not modified explicitly by the
user. It is very likely that the definition of the object will contain an
initializer (otherwise, since you can't assign to it, how would it ever get a
value?), but this is not always the case. For example, if you were accessing a
hardware port at a fixed memory address & promised only to read from it,
then it would be declared to be const but not initialized.
*
volatile tells the compiler that other programs will be modifying this variable
in addition to the program being compiled. For example, an I/O device might need
write directly into a program or data space. Meanwhile, the program itself may
never directly access the memory area in question. In such a case, we would not
want the compiler to optimize-out this data area that never seems to be used by
the program, yet must exist for the program to function correctly in a larger
context. It tells the compiler that the object is subject to sudden change for
reasons which cannot be predicted from a study of the program itself, &
forces every reference to such an object to be a genuine reference.
* const volatile - Both constant & volatile.
The "volatile" modifier
The volatile
modifier is a directive to the compiler?s optimizer that operations involving
this variable should not be optimized in certain ways. There are two special
cases in which use of the volatile modifier is desirable. The first case
involves memory-mapped hardware (a device such as a graphics adaptor that
appears to the computer?s hardware as if it were part of the computer?s memory),
& the second involves shared memory (memory used by two or more programs
running simultaneously). Most computers have a set of registers that can be
accessed faster than the computer?s main memory. A good compiler will perform a
kind of optimization called ?redundant load & store removal.? The compiler
looks for places in the code where it can either remove an instruction to load
data from memory because the value is already in a register, or remove an
instruction to store data to memory because the value can stay in a register
until it is changed again anyway.
If a variable is a pointer to
something other than normal memory, such as memory-mapped ports on a
peripheral, redundant load & store optimizations might be detrimental. For
instance, here?s a piece of code that might be used to time some
operation:
time_t time_addition(volatile const struct
timer *t, int a)
{
int n;
int x;
time_t then;
x = 0;
then = t->value;
for
(n = 0; n < 1000; n++)
{
x = x + a;
}
return t->value - then;
}
In this code, the variable t->value is
actually a hardware counter that is being incremented as time passes. The
function adds the value of a to x 1000 times, & it returns the amount the
timer was incremented by while the 1000 additions were being performed. Without
the volatile modifier, a clever optimizer might assume that the value of t does
not change during the execution of the function, because there is no statement
that explicitly changes it. In that case, there?s no need to read it from memory
a second time & subtract it, because the answer will always be 0. The
compiler might therefore ?optimize? the function by making it always return 0.
If a variable points to data in shared memory, you also don?t want the compiler
to perform redundant load & store optimizations. Shared memory is normally
used to enable two programs to communicate with each other by having one program
store data in the shared portion of memory & the other program read the same
portion of memory. If the compiler optimizes away a load or store of shared
memory, communication between the two programs will be
affected.
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