Thread: array and pointer problem

  1. #1
    Registered User
    Join Date
    Oct 2016
    Posts
    5

    array and pointer problem

    //c program

    Code:
    #include <stdio.h>
    
    
    int main(void) {
        static int arr[] = {97, 98, 99, 100, 101, 102, 103, 104 };
                           
         int * ptr = arr + 1;
                                                     
          print(++ptr, ptr--, ptr, ptr++, ++ptr);
                                                                                      
          return 0;
    }
    
    
    int print(int *a, int *b, int *c, int *d, int *e)
    {
       printf("%d %d %d %d %d \n", *a, *b, *c, *d, *e);
                     
    }
    Expected Output of program by me:
    99 99 98 98 100

    Actual Output of program:
    100 100 100 99 100


    Could you please explain the Actual output?

  2. #2
    Registered User
    Join Date
    Jun 2015
    Posts
    1,643
    Two problems:
    1. In C there's no guarantee about the order in which a function's arguments are evaluated.
    2. You're not allowed to modify the same variable more than once per "sequence point". Question 3.8

    Compiling your program with gcc and full warnings gives multiple "sequence point" warnings. (You also forgot to prototype your print function and are not returning the promised int.)

    Although an explanation for the exact values you received could be garnered from the assembly code, it would be essentially meaningless since this is undefined behaviour.

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