flags for st_mode of stat system call

Question

I'm trying to understand the flags for the st_mode field of the stat structure of that stat command, but I'm having such a hard time! I found this example here, but I really don't understand this code fragment:

if ( mode & S_IRUSR ) str[1] = 'r';    /* 3 bits for user  */
if ( mode & S_IWUSR ) str[2] = 'w';
if ( mode & S_IXUSR ) str[3] = 'x';

if ( mode & S_IRGRP ) str[4] = 'r';    /* 3 bits for group */
if ( mode & S_IWGRP ) str[5] = 'w';
if ( mode & S_IXGRP ) str[6] = 'x';

if ( mode & S_IROTH ) str[7] = 'r';    /* 3 bits for other */
if ( mode & S_IWOTH ) str[8] = 'w';
if ( mode & S_IXOTH ) str[9] = 'x';

I know that "&" is the bitwise AND operator, but nothing else. I don't even know what to ask.

PD: Sorry about the previous questions I asked. I don't know how to mark a question answered or anything like that :(

Answer 1

mode is a bitfield which is a common way to pack data. Think of each bit in the field as being a toggle switch which can be set to off or on. To check if the toggle is on, you check to see if the appropriate bit has been set using the & operator. You can set bits using | and clear them using ~ bitwise operations.

Answer 2

Well, the POSIX spec for <sys/stat.h> enumerates everything you can learn from the st_mode field of struct stat.

Is your question "What can this field tell me", or is it "How do I extract the information", or both?