trace a program

[braddy]

Hi ,
Please I need help to trace this program:

Show the output for the following program

#include <stdio.h>
#include <string.h>
char * show(char *s);
void main() {
  char signs[3][30] = {"Watch Your Step",
                       "Slippery When Wet",
                       "Danger - High Voltage"};
  char *tricky;
  tricky = show(signs[1]);
  printf("%.6s\n", tricky);
  tricky = show(tricky);
  printf("%.6s\n", tricky);
}
char * show(char *s) {
  char *keep = s + 4;
  printf("%c %.8s\n", *keep, s+2);
  strncpy(++keep, s, 10);           
  return keep-3;
}

Output (Exactly as it would appear)

Line 1 p _ i p p e r y _ W _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

Line 2 i p p S l i _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

Line 3 l _ p S l i p p S l _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

Line 4 p S l i p p _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

I get the first line( the print inside the function)
But I dont get the second line.
My problem is the command"strncpy(++keep, s, 10);"
Since I have problem with that , I cannot continue tracing the program. Can someone help me please to get Line 2?

thank you for your help
B.

[quzah]

Sorry, I stop at 'void main'.


Quzah.

[Salem]

> strncpy(++keep, s, 10);
Since keep and s point to the same array, this is undefined.

       The  strcpy()  function  copies  the  string pointed to by src (including the terminating ‘\0’
       character) to the array pointed to by dest.  The strings may not overlap, and the  destination
       string dest must be large enough to receive the copy.

       The  strncpy() function is similar, except that not more than n bytes of src are copied. Thus,
       if there is no null byte among the first n bytes of src, the result will  not  be  null-termi-
       nated.

Even if you succeed in "tracing" it, it won't do you any good because there is no guarantee that the same explanation will work for anyone else.

As Quzah pointed out, your program became undefined at void main.

[dwks]

If you want to copy overlapping strings, check out memmove().

And use int main(), not void.