c中的运算符和优先级

Question

I came across the following program.

 #include<stdio.h> 
    int main() 
    { 
      char *s[] = { "knowledge","is","power"}; 
      char **p; 
      p = s; 
      printf("%s ", ++*p); 
      printf("%s ", *p++); 
      printf("%s ", ++*p); 

      return 0; 
    }

Output: nowledge nowledge s I'm sure
1.First printf prints nowledge
2.Second printf also prints nowledge because it contains a post increment so original value of first printf is retained.
Now at this stage I'm confused what exactly *p++ does. Now will p to point to is or owledge .

Answer 1

This output of this program is quite confusing because ++*p increments the value pointed to by p which makes different elements of the string array s to get incremented.

  printf("%s ", ++*p);

The value returned by *p gets incremented. p points to s[0] . Therefore will s[0] point to "nowledge" .

  printf("%s ", *p++); 

p still points to the first element of the char * array. Because we incremented the value of s[0] by one previously will s[0] still point to "nowledge" . p gets incremented after the dereference.

  printf("%s ", ++*p); 

The value returned by *p gets incremented. s[1] will get incremented and point to "s" .

Answer 2

The value at *p is of type *char (the value of a pointer being an address that is allocated the size of the type it is pointing to)

incrementing it would result in that value being added a sizeof(char)

It's like writing this :

  printf("%s ", ++(*p)); 

The value at p is of type **char

incrementing it would result in that value added a typeof(*char) .

the difference is that the value in the next address holding a *char is the address of the first char in of the next char array

I didn't look at the precedence sheet (which i'm sure you can find in simple google search) but from running the code you can understand that ++ precedes *

if you want to check it out run this :

 printf("%s ", (*p)++);

Answer 3

  char *s[] = { "knowledge","is","power"}; // s is an array of pointers
  char **p; // p is a pointer to a pointer to char
  p = s; 
  printf("%s ", ++*p); //(*p) pointing to first string of s, pre-increment so "nowledge" printed
  printf("%s ", *p++); // (*p) printed then post-increment  of p (not *p) hence jumped to next pointer in the array s
  printf("%s ", ++*p);