C Knowledge

Question 1

Standard ‘MIN’ Macro

Answer 1

define MIN(A,B) ((A) <= (B) ? (A) : (B))

Question 2

Conditional expression with ternary operator

Answer 2

expr1 ? expr2 : expr3
expr1 is evaluated first
If true (non-Zero), then expr2 is evaluated.
Otherwise, expr3 is evaluated.

Question 3

Code an infinite loop

Answer 3

while(1)
for(;;) //KnR preferred method, necessary for Lint
Loop:
goto Loop;

Question 4

What are the three uses of the keyword static?

Answer 4

a. A variable declared static within the body of a function maintains its value between function calls
b. A variable declared static within a module, but outside a function, is accessible by all functions within that module. (localized global)
c. Functions declared static w/in a module may only be called by other functions within that module. The scope of the function is localized to the module in which it was declared.

Question 5

What does the keyword const mean?

Answer 5

essentially “ready-only”

Question 6

const int a;
or
int const a;

Answer 6

read-only integer

Question 7

const int *a;

Answer 7

pointer to a read-only integer. (integer isn’t modifiable, but pointer is)

Question 8

int * const a;

Answer 8

read-only pointer to a modifiable integer

Question 9

int const * a const;

Answer 9

constant pointer to a constant integer

Question 10

What does the keyword volatile mean?

Answer 10

( volatile == a qualifier) A volatile variable is one that can change unexpectedly. Tells optimizer that the variable must reload the variable every time it is used instead of holding a copy in a register.

Question 11

Give three examples of using the volatile keyword.

Answer 11

1. Hardware registers in peripherals (e.g., status registers)
2. Non-stack variables referenced within an interrupt service routine.
3. Variables shared by multiple tasks in a multi-threaded application

Question 12

Can a parameter be both const and volatile?

Answer 12

Yes. Example: read-only status register. Volatile because it can change unexpectedly, const because the program should not attempt to modify it.

Question 13

Can a pointer be volatile?

Answer 13

Yes. Example: when an interrupt service routine modifies a pointer to a buffer

Question 14

What is wrong with following function:

int square(volatile int *ptr)
{
return *ptr * *ptr;
}

Answer 14

Since *ptr is volatile and could change unexpectedly, it could produce a return value that is not a square. Function should set a local variable with the value of *ptr and then square the local variable.

Question 15

Efficient way to produce Bit manipulation?

Answer 15

define SET_BIT(x,y) (x |= (1«y))

Set Bit |=
Clr Bit &= ~

#define CLR_BIT(x,y) (x &= ~(1«y))

Question 16

Set integer value at absolute address 0x67a9 to the value of 0xaa55 on pure ANSI compiler

Answer 16

int *ptr;
ptr = (int *) 0x67a9;
*ptr = 0xaa55;

Question 17

Can an interrupt service routine return a value?

Answer 17

no

Question 18

Can ISRs be passed a value?

Answer 18

no

Question 19

What is the goal of the body of an ISR?

Answer 19

short and sweet (less than five lines of code?)

Question 20

Re-entrant?

Answer 20

One function is said to be a reentrant function if there is a provision to interrupt that function in the course of execution, then service the ISR (Interrupt Service Routine) and then resume the task

Question 21

Expressions involving signed and unsigned types __________

Answer 21

have all operands promoted to unsigned types

Question 22

tilde ~

Answer 22

creates complement of operand (0s become ones, ones become 0s)

Question 23

What is #error?

Answer 23

A preprocessor directive that causes compilation to terminate and prints the user defined error message to stderr.
#error <writer></writer>

Filename (line_number): Error!
Ennnn: <writer></writer>

Question 24

When would a programmer use #error?

Answer 24

if (INT_MAX != 32767)

Incomplete code: add to section of code that is incomplete to guide user after return to project

Compiler dependent code: use to check if compiler meets minimums for certain aspects
#if (LDBL_DIG < 12)
#error ** long doubles need 12 digit resolution **
#endif

#error This file only works with 16 bit int
#endif

Conditionally compiled code: