C语言高级测试（二）---答案

  C语言高级测试（二）---答案

Answer With Detailed Explanation _____________________________________________________________ Answer 1. The answer is (b) volatile variable isn't affected by the optimization. Its value after the longjump is the last value variable assumed. b last value is 5 hence 5 is printed. setjmp : Sets up for nonlocal goto /* setjmp.h*/ Stores context information such as register values so that the lomgjmp function can return control to the statement following the one calling setjmp.Returns 0 when it is initially called. Lonjjmp: longjmp Performs nonlocal goto /* setjmp.h*/ Transfers control to the statement where the call to setjmp (which initialized buf) was made. Execution continues at this point as if longjmp cannot return the value 0.A nonvolatile automatic variable might be changed by a call to longjmp.When you use setjmp and longjmp, the only automatic variables guaranteed to remain valid are those declared volatile. Note: Test program without volatile qualifier (result may very) Answer 2. The answer is (a) The members of structures have address in increasing order of their declaration. If a pointer to a structure is cast to the type of a pointer to its first member, the result refers to the first member. Answer 3. The answer is (a) Non recursive version of the program int  what ( int x , int  n) {   int val;   int product;   product =1;   val =x;     while(n>0)   {     if (n%2 == 1)        product = product*val;     n = n/2;     val = val* val;   } } /* Code raise a number (x) to a large power (n) using binary doubling strategy */ Algorithm description (while n>0)  {   if  next most significant binary digit of  n( power)  is one   then multiply accumulated product by current val  ,   reduce n(power)  sequence by a factor of two using integer division .   get next val by multiply current value of itself                    } Answer 4. The answer is (c) type of a is array of int type of &a is pointer to array of int Taking a pointer to the element one beyond the end of an array is sure to work. Answer 5. The answer is (b) Answer 6. The answer is (c) The comma separates the elements of a function argument list. The comma is also used as an operator in comma expressions. Mixing the two uses of comma is legal, but you must use parentheses to distinguish them. the left operand E1 is evaluated as a void expression, then E2 is evaluated to give the result and type of the comma expression. By recursion, the expression E1, E2, ..., En results in the left-to-right evaluation of each Ei, with the value and type of En giving the result of the whole expression. c=a,b;  / *yields c=a* / d=(a,b); /* d =b  */ Answer 7. The answer is (a) /* ptr is pointer to array of 3 int */ Answer 8. The answer is (c) f1 and f2 return address of local variable ,when function exit local variable disappeared Answer 9. The answer is (c) sizeof operator gives the number of bytes required to store an object of the type of its operand . The operands is either an expression, which is not evaluated ( (++i + ++ i ) is not evaluated so i remain 3 and j is sizeof int that is 4) or a parenthesized type name. Answer 10. The answer is (a) void(*p[2]) ( int *, int); define array of pointer to function accept two argument that is pointer to int and return int. p[0] = f1; p[1] = f2 contain address of function .function name without parenthesis represent address of function Value and address of variable is passed to function only argument that is effected is a (address is passed). Because of call by value f1, f2 can not effect b Answer 11. The answer is (a) Answer 12. The answer is (c) C provide a facility called typedef for creating new data type names, for example declaration typedef char string Makes the name string a synonym for int .The type string can be used in declaration, cast, etc, exactly the same way that the type int can be. Notice that the type being declared in a typedef appears in the position of a variable name not after the word typedef. Answer 13. The answer is (c) If the type of an expression is "array of T" for some type T, then the value of the expression is a pointer to the first object in the array, and the type of the expression is altered to "pointer to T" So (buf+1)[5] is equvalent to *(buf +6) or buf[6] Answer 14. The answer is (d) p+=sizeof(int) point to argv[4] (p+=sizeof(int))[-1] points to argv[3] Answer 15. The answer is (c) When we call ripple value of the first argument passed to ripple is collected in the n that is 3. va_start initialize p to point to first unnamed argument that is 5 (first argument).Each call of va_arg return an argument and step p to the next argument. va_arg uses a type name to determine what type to return and how big a step to take Consider inner loop (; i; i&=i-1) k++ /* count number of  1 bit in i * in five number of 1 bits is (101) 2 i n seven number of 1 bits is (111) 3 hence k return 5 example let  i= 9 = 1001      i-1  = 1000            (i-1) +1 = i                1000                  +1               1 001 The right most 1 bit of i has corresponding 0 bit in i-1 this way i & i-1, in a two complement number system will delete the right most 1 bit I(repeat until I become 0 gives number of 1 bits) Answer 16. The answer is (b) Static variable count remain in existence rather than coming and going each time function is called so first call counter(0) count =0 second call counter(1) count = 0+1; third call counter(2) count = 1+2; /* count = count +i */ fourth call counter(3) count = 3+3; fifth call counter(4) count = 6+4; sixth call counter(5) count = 10+5;

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