c - Program misbehaves 25% of the time -

inspired this topic, decided write simple program that.
logic isn't complicated , have working program 75% of time.. amount of asked numbers defined #define bufsize x, x can arbitrary int.
problem arises when ((bufsize+1) % sizeof(int)) == 0.

so example, if bufsize=10, program behaves correctly, when bufsize=11 odd behaviour.

here sourcecode:

#include <stdio.h> #include <stdlib.h> #define bufsize 7  int max(int *buf);  int main() {     int bufsize = bufsize, *buf = malloc(sizeof(int[bufsize]));      // read values     int *ptr = buf;     while(--bufsize + 1)     {         printf("input %d: ", bufsize - bufsize);         scanf("%d", ptr);         ++ptr;     }      // reset pointer , determine max     ptr = buf;     printf("\nmax: %d\n", max(ptr));     // cleanup     free(buf);     ptr = null;     buf = null;      exit(exit_success); }  int max(int *buf) {     int max = 0;     while(*buf)     {         printf("%d\n", *buf);         if(*buf > max) max = *buf;         ++buf;     }     return max; } 

and sample output bufsize=2 (correct) , bufsize=3 (incorrect).

suze:/home/born05/htdocs/experiments/c# gcc input.c && ./a.out input 1: 12 input 2: 23 12 23  max: 23  suze:/home/born05/htdocs/experiments/c# gcc input.c && ./a.out input 1: 12 input 2: 23 input 3: 34 12 23 34 135153  max: 135153 

i have feeling extremely logical can't put finger on exact cause of misbehaviour. point out (perhaps obvious) flaw me?

it's pure luck works values of bufsize. (in fact, me, breaks on bufsize=2). here's why -- this:

while(*buf) 

is not appropriate way check end of buffer. load value @ address pointed buf , see if contents zero. since you're never explicitly putting 0 @ end of buffer, that's never going true, , loop run potentially forever, reading memory past end of buf array , invoking undefined behavior.

you either need allocate element @ end of buf array , set 0 (but program won't work right if user enters 0 input), or explicitly pass size of buf max function , use determine when should stop looping.