急急急急！！C的题目

这几道题只能用C不能用C＋＋
(1) List some ways for a C function to pass a string back to its caller. Say what advantages or disadvantages
each way might have.   

(2)The following function counts the number of bits that are set in an integer (i.e. the number of ’1′ bits). For
example if the input is 9 (in decimal) then the return value will be 2 (as 9 in binary is 1001 which has two 1
bits).  Explain how the function works. Do not just show that the function works for one example, but try to
prove that the function will work for all possible inputs.
 
   int count_bits (unsigned int n)
   {
     int count = 0;
     while (n) {
       n = (n-1) & n;
    count++;
     }
     return count;
   }

(3)Write a C function that multiplies two integers together and returns the result, i.e.
 
   int multiply (int a, int b)
   {
 
     …
   }
 
 You can not use the multiplication (*) operator in your implementation, since we will assume that this
function needs to run on an embedded CPU that has no multiply instruction. Thus function must also be as
efficient as possible.

(4)Write an implementation of the function my_strncat:
 
   void my_strncat (char *DST, const char *SRC, int MAX_LENGTH);
 
 ’my_strncat’ appends the null-terminated string in SRC to the end of the null-terminated string in DST. On
exit the length of the string DST, including its terminating null character, must not be more than
MAX_LENGTH.  In other words the string DST sits in a buffer of size MAX_LENGTH characters and this
buffer must not be overflowed. On exit the string DST must always be null-terminated.
 
 You should not use any C standard library functions.

(5) Write an implementation of the following two functions that are used to set up and check for timer events:
 
   void alarm_set (void (*callback)(void), int alarm_tick_count)
   alarm_check (int system_tick_count)
 
 The alarm_set() function is used to set an alarm. It requests that the callback function be called when the
system tick count reaches alarm_tick_count.
 
 The alarm_check() function is called by the operating system each time the system tick count is advanced (for
example, this could be every 1/100th of a second).
 
 The system must be able to track more than one alarm at a time – assume that we wish to accomodate
many alarms being set, though there is _no_ need for the alarm SETTING efficiency to be better than
O(n).  Also assume that the alarm check algorithm is time sensitive
 
 Please define any data structures that are required. Assume that the environment has limited memory, and that
both constrained memory usage and high throughput are important.