C program using inotify help in understanding code

Question

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.

Answer 1

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).