C program using inotify help in understanding code

I was referring the man page for inotify and I came across this piece of code

               if (event->mask & IN_ISDIR)
                   printf(" [directory]\n");
               else
                   printf(" [file]\n");

where event is a pointer to struct inotify_event.

I couldn't understand this particular line if (event->mask & IN_ISDIR) and why a bitwise AND is used here.
How was it determined that the bitwise AND is supposed to be used here and nothing else? It was not mentioned in the man page for inotify.

Solution

This bitwise AND is masking out a specific bit (IN_ISDIR). It is testing whether or not this one bit is set or not. If this bit is set in event->mask, it evaluates to true.

For example,

#include <stdio.h>

#define FIRST_BIT  1
#define SECOND_BIT 2
#define THIRD_BIT  4

int main() {
  int x = 3;  /* 3 in binary is 011 */

  if ( x & FIRST_BIT )
    printf("First bit is set\n");
  if ( x & SECOND_BIT )
    printf("Second bit is set\n");
  if ( x & THIRD_BIT )
    printf("Third bit is set\n");
}

will give the output

    First bit is set
    Second bit is set

From inotify.h:

#define IN_ISDIR          0x40000000    /* event occurred against dir */

This value has only one bit set. (In binary, this is 01000000000000000000000000000000.) A bitwise AND with this (0x40000000) and some variable will evaluate to either 0 (if the variable has a 0 here), or 0x40000000 if the variable has a 1 in the same place. Any non-zero value is considered "true".

Logically, it is testing if the event was from a directory (instead of a file).