II.18: What is the benefit of using const for declaring constants?

Answer:
The benefit of using the const keyword is that the compiler might be able to make optimizations based on the knowledge that the value of the variable will not change. In addition, the compiler will try to ensure that the values won’t be changed inadvertently.
Of course, the same benefits apply to #defined constants. The reason to use const rather than #define to define a constant is that a const variable can be of any type (such as a struct, which can’t be represented by a #defined constant). Also, because a const variable is a real variable, it has an address that can be used, if needed, and it resides in only one place in memory (some compilers make a new copy of a #defined character string each time it is used—see IX.9).

Reference:
II.7: Can a variable be both const and volatile?
II.8: When should the const modifier be used?
II.14: When should a type cast not be used?
IX.9: What is the difference between a string and an array?

II.17: Can static variables be declared in a header file?

Answer:
You can’t declare a static variable without defining it as well (this is because the storage class modifiers static and extern are mutually exclusive). A static variable can be defined in a header file, but this would cause each source file that included the header file to have its own private copy of the variable, which is probably not what was intended.

Reference:
II.16: What is the difference between declaring a variable and defining a variable?

II.16: What is the difference between declaring a variable and

Answer:
Declaring a variable means describing its type to the compiler but not allocating any space for it. Defining a variable means declaring it and also allocating space to hold the variable. You can also initialize a variable at the time it is defined. Here is a declaration of a variable and a structure, and two variable definitions, one with initialization:

extern int decl1; /* this is a declaration */
struct decl2 {
int member;
}; /* this just declares the type--no variable mentioned */
int def1 = 8; /* this is a definition */
int def2; /* this is a definition */

To put it another way, a declaration says to the compiler, “Somewhere in my program will be a variable with this name, and this is what type it is.” A definition says, “Right here is this variable with this name and this type.”

NOTE
One way to remember what each term means is to remember that the Declaration of Independence didn’t actually make the United States independent (the Revolutionary War did that); it just stated that it was independent.
A variable can be declared many times, but it must be defined exactly once. For this reason, definitions do not belong in header files, where they might get #included into more than one place in your program.

Reference:
II.17: Can static variables be declared in a header file?

II.15: Is it acceptable to declare/define a variable in a C header?

Answer:
A global variable that must be accessed from more than one file can and should be declared in a header file.
In addition, such a variable must be defined in one source file. Variables should not be defined in header files, because the header file can be included in multiple source files, which would cause multiple definitions of the variable. The ANSI C standard will allow multiple external definitions, provided that there is only one initialization. But because there’s really no advantage to using this feature, it’s probably best to avoid it and maintain a higher level of portability.

NOTE
Don’t confuse declaring and defining variables. II.16 states the differences between these
two actions.
“Global” variables that do not have to be accessed from more than one file should be declared static and should not appear in a header file.

Reference:
II.16: What is the difference between declaring a variable and defining a variable?
II.17: Can static variables be declared in a header file?

II.14: When should a type cast not be used?

Answer:
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.
Reference:
II.6: When should the volatile modifier be used?
II.8: When should the const modifier be used?

II.13: When should a type cast be used?

Answer:
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. If you have read II.12, the following listing should look familiar. The variable f1 is set to the result of dividing the integer i by the integer j. The result is 0, because integer division is used. The variable f2 is set to the result of dividing i by j as well.
However, the (float) type cast causes i to be converted to a float. That in turn causes floating-point division to be used (see II.11) and gives the result 0.75.

#include
main()
{
int i = 3;
int j = 4;
float f1 = i / j;
float f2 = (float) i / j;
printf(“3 / 4 == %g or %g depending on the type used.\n”,
f1, f2);
}

The second case is to cast pointer types to and 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));

Reference:
II.6: When should the volatile modifier be used?
II.8: When should the const modifier be used?
II.11: Are there any problems with performing mathematical operations on different variable types?
II.12: What is operator promotion?
II.14: When should a type cast not be used?
VII.5: What is a void pointer?
VII.6: When is a void pointer used?
VII.21: What is the heap?
VII.27: Can math operations be performed on a void pointer?

II.12: What is operator promotion?

Answer:
If an operation is specified with operands of two different types, they are converted to the smallest type that can hold both values. The result has the same type as the two operands wind up having. To interpret the rules, read the following table from the top down, and stop at the first rule that applies.
If Either Operand Is And the Other Is Change Them To
long double any other type long double
double any smaller type double
float any smaller type float
unsigned long any integral type unsigned long
long unsigned > LONG_MAX unsigned long
long any smaller type long
unsigned any signed type unsigned
The following example code illustrates some cases of operator promotion. The variable f1 is set to 3 / 4.
Because both 3 and 4 are integers, integer division is performed, and the result is the integer 0. The variable f2 is set to 3 / 4.0. Because 4.0 is a float, the number 3 is converted to a float as well, and the result is the float 0.75.

#include
main()
{
float f1 = 3 / 4;
float f2 = 3 / 4.0;
printf(“3 / 4 == %g or %g depending on the type used.\n”,
f1, f2);
}

Reference:
II.11: Are there any problems with performing mathematical operations on different variable types?
II.13: When should a type cast be used?

II.11: Are there any problems with performing mathematical operations on different variable types?

Answer:
C has three categories of built-in data types: pointer types, integral types, and floating-point types.
Pointer types are the most restrictive in terms of the operations that can be performed on them. They are limited to - subtraction of two pointers, valid only when both pointers point to elements in the same array. The result is the same as subtracting the integer subscripts corresponding to the two pointers. + addition of a pointer and an integral type. The result is a pointer that points to the element which would be selected by that integer.
Floating-point types consist of the built-in types float, double, and long double. Integral types consist of char, unsigned char, short, unsigned short, int, unsigned int, long, and unsigned long. All of these types can have the following arithmetic operations performed on them:
+ Addition
- Subtraction
* Multiplication
/ Division
Integral types also can have those four operations performed on them, as well as the following operations:
% Modulo or remainder of division
<<>> Shift right
& Bitwise AND operation
Bitwise OR operation
^ Bitwise exclusive OR operation
! Logical negative operation
~ Bitwise “one’s complement” operation

Although C permits “mixed mode” expressions (an arithmetic expression involving different types), it actually converts the types to be the same type before performing the operations (except for the case of pointer arithmetic described previously). The process of automatic type conversion is called “operator promotion.” Operator promotion is explained in II.12.

Reference:II.12: What is operator promotion?

II.10: How can you determine the maximum value that a numeric variable can hold?

Answer:
The easiest way to find out how large or small a number that a particular type can hold is to use the values defined in the ANSI standard header file limits.h. This file contains many useful constants defining the values that can be held by various types, including these:
Value Description

CHAR_BIT Number of bits in a char
CHAR_MAX Maximum decimal integer value of a char
CHAR_MIN Minimum decimal integer value of a char
MB_LEN_MAX Maximum number of bytes in a multibyte character
INT_MAX Maximum decimal value of an int
INT_MIN Minimum decimal value of an int
LONG_MAX Maximum decimal value of a long
LONG_MIN Minimum decimal value of a long
SCHAR_MAX Maximum decimal integer value of a signed char
SCHAR_MIN Minimum decimal integer value of a signed char
SHRT_MAX Maximum decimal value of a short
SHRT_MIN Minimum decimal value of a short
UCHAR_MAX Maximum decimal integer value of unsigned char
UINT_MAX Maximum decimal value of an unsigned integer
ULONG_MAX Maximum decimal value of an unsigned long int
USHRT_MAX Maximum decimal value of an unsigned short int

For integral types, on a machine that uses two’s complement arithmetic (which is just about any machine you’re likely to use), a signed type can hold numbers from –2(number of bits – 1) to +2(number of bits – 1) – 1. An unsigned type can hold values from 0 to +2(number of bits) – 1. For instance, a 16-bit signed integer can hold numbers from
–215 (–32768) to +215 – 1 (32767).

Reference:
X.1: What is the most efficient way to store flag values?
X.2: What is meant by “bit masking”?
X.6: How are 16- and 32-bit numbers stored?

II.9: How reliable are floating-point comparisons?

Answer:
Floating-point numbers are the “black art” of computer programming. One reason why this is so is that there is no optimal way to represent an arbitrary number. The Institute of Electrical and Electronic Engineers (IEEE) has developed a standard for the representation of floating-point numbers, but you cannot guarantee that every machine you use will conform to the standard.
Even if your machine does conform to the standard, there are deeper issues. It can be shown mathematically that there are an infinite number of “real” numbers between any two numbers. For the computer to distinguish between two numbers, the bits that represent them must differ. To represent an infinite number of different bit patterns would take an infinite number of bits. Because the computer must represent a large range of numbers in a small number of bits (usually 32 to 64 bits), it has to make approximate representations of most numbers.
Because floating-point numbers are so tricky to deal with, it’s generally bad practice to compare a floatingpoint number for equality with anything. Inequalities are much safer. If, for instance, you want to step through a range of numbers in small increments, you might write this:

#include
const float first = 0.0;
const float last = 70.0;
const float small = 0.007;
main()
{
float f;
for (f = first; f != last && f <>

However, rounding errors and small differences in the representation of the variable small might cause f to never be equal to last (it might go from being just under it to being just over it). Thus, the loop would go past the value last. The inequality f <>

float f;
for (f = first; f <>

You could even precompute the number of times the loop should be executed and use an integer to count iterations of the loop, as in this example:

float f;
int count = (last - first) / small;
for (f = first; count-- > 0; f += small)
;

Reference:
II.11: Are there any problems with performing mathematical operations on different variable
types?

II.8: When should the const modifier be used?

Answer:

There are several reasons to use const pointers. First, it allows the compiler to catch errors in which codeaccidentally changes the value of a variable, as inwhile

(*str = 0) /* programmer meant to write *str != 0 */

{/* some code here */str++;

}

in which the = sign is a typographical error. Without the const in the declaration of str, the program wouldcompile but not run properly.Another reason is efficiency. The compiler might be able to make certain optimizations to the code generatedif it knows that a variable will not be changed.Any function parameter which points to data that is not modified by the function or by any function it callsshould declare the pointer a pointer to const. Function parameters that are passed by value (rather thanthrough a pointer) can be declared const if neither the function nor any function it calls modifies the data.In practice, however, such parameters are usually declared const only if it might be more efficient for thecompiler to access the data through a pointer than by copying it.

Reference:

II.7: Can a variable be both const and volatile?

II.14: When should a type cast not be used?

II.18: What is the benefit of using const for declaring constants?

II.7: Can a variable be both const and volatile?

Answer:
Yes. The const modifier means that this code cannot change the value of the variable, but that does not mean that the value cannot be changed by means outside this code. For instance, in the example in II.6, the timer structure was accessed through a volatile const pointer. The function itself did not change the value of the timer, so it was declared const. However, the value was changed by hardware on the computer, so it was declared volatile. If a variable is both const and volatile, the two modifiers can appear in either order.
Reference:
II.6: When should the volatile modifier be used?
II.8: When should the const modifier be used?
II.14: When should a type cast not be used?

II.6: When should the volatile modifier be used?

Answer:
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), and 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 and 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 and 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 < x =" x">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, and 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 and 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 and 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 and 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.
Reference:
II.7: Can a variable be both const and volatile?
II.14: When should a type cast not be used?

II.5: When should the register modifier be used? Does it

Answer:
The register modifier hints to the compiler that the variable will be heavily used and 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, and because some registers can hold only certain types of data (such as pointers or floating-point numbers), the number and 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 and 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, and 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 and not a directive.
In the rare event that a program is too slow, and you know that the problem is due to a variable being stored in memory, you might try adding the register modifier as a last resort, but don’t be surprised if this action doesn’t change the speed of the program.
Reference:
II.6: When should the volatile modifier be used?