what does this ":" in the structure ?

[David d'Angers]

hi group

i read this in the linux version "tree" program

struct _info {
char *name;
char *lnk;
u_char isdir : 1;
u_char issok : 1;
u_char isexe : 1;
u_char isfifo : 1;
u_char orphan : 1;
u_short mode, lnkmode, uid, gid;
off_t size;
time_t atime, ctime, mtime;
dev_t dev;
ino_t inode;
#ifdef __EMX__
long attr;
#endif
};

can anyone tells me why is there a ":" in the structure and its use ?

thx

[Richard Heathfield]

David d'Angers said:

hi group

i read this in the linux version "tree" program

struct _info {
char *name;
char *lnk;
u_char isdir : 1;

Look up "bitfield" or "bit-field" in your C reference book. Bitfields are
sometimes used when a direct mapping to hardware bits is desired, but this
usage is inherently non-portable (not least because which way round the
bits go depends on the implementation). They are also sometimes used,
rather more portably, in an attempt to pack more than one numeric value
into a byte or group of bytes.

There is much more to be said about bitfields (mostly bad!, in my opinion),
but I think this short answer does at least address your question
sufficiently to enable you to discover more about them for yourself.

--
Richard Heathfield <http://www.cpax.org.uk>
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[David d'Angers]

thx, i've found the corresponding section in the classical "the c
programming language"

[Malcolm McLean]

"Richard Heathfield" <rj*@see.sig.invalidwrote in message

They are also sometimes used,
rather more portably, in an attempt to pack more than one numeric value
into a byte or group of bytes.

That's the proper use of a bitfield. It's as portable as you can expect it
to be. The problem is that the compiler doesn't know whether you want
efficiency of packing or efficiency of access, and there is often a tension
between the two.

--
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