.TH NEWCHAN 10
.SH NAME
newchan, chanfree, cclose, eqqid, eqchan, isdir, fdtochan, namec \- channel operations
.SH SYNOPSIS
.ta \w'\fLChan* 'u
.B
Chan*	newchan(void)
.PP
.B
void	chanfree(Chan *c)
.PP
.B
int	eqqid(Qid a, Qid b)
.PP
.B
int	eqchan(Chan *a, Chan *b, int pathonly)
.PP
.B
void	isdir(Chan *c)
.PP
.B
Chan*	fdtochan(Fgrp *f, int fd, int mode, int chkmnt, int iref)
.PP
.B
Chan*	namec(char *pathname, int amode, int omode, ulong perm)
.PP
.B
void	cclose(Chan *c)
.SH DESCRIPTION
A value of type
.B Chan
represents a kernel channel for I/O and name space operations.
It has the following public structure:
.IP
.EX
typedef struct Chan{
	ushort    type;       /* driver name */
	ulong     dev;        /* instance number */
	ushort    mode;       /* open mode */
	ushort    flag;       /* COPEN set once opened */
	ulong     offset;     /* current file offset */
	Qid       qid;        /* unique id (path, vers) */
	Cname*     name;	/* name by which it was accessed */
.EE
.PP
.I Newchan
returns a pointer to a newly allocated channel (sleeping if necessary until memory is available).
Device drivers do not normally call
.IR newchan
directly, but instead allocate channels using either
.IR devattach ,
when a process attaches to the device's root,
or
.IR devclone ,
when an existing channel is cloned;
see
.IR devattach (10).
.PP
.I Chanfree
frees the channel structure
.I c
for reuse.
.PP
.I Eqqid
returns 1 if
.B Qid
values
.I a
and
.I b
are equal
(ie,
both their
.B path
and
.B vers
members are equal);
it returns 0 otherwise.
.PP
.I Eqchan
returns 1 if
.I a
and
.I b
have the same
.BR qid ,
.BR type
and
.BR dev
members
(ie, they represent the same file);
it returns 0 otherwise.
If
.I pathonly
is non-zero, the comparison of the two
.B qid
members compares only their
.B path
values,
ignoring the version field
.BR vers .
.PP
.I Isdir
checks that a given channel
.I c
is a directory.
If so, it returns;
otherwise, it generates an
.IR error (10),
.BR Enotdir .
.PP
The
.B Fgrp
structure represents an array of open files, each
represented by a
.BR Chan ,
indexed by integer file descriptors.
A given
.B Fgrp
can be shared between processes.
.PP
.I Fdtochan
returns a pointer to the
.B Chan
corresponding to file descriptor
.I fd
in file descriptor group
.I f
(almost invariably
.BR up->fgrp ,
the file descriptor group for the current process).
If
.I mode
is a valid mode for
.IR sys-open (2),
typically
.BR OREAD ,
.B OWRITE
or
.BR ORDWR ,
it must correspond to the mode with which
.I fd
was originally opened; if
.I mode
is
.BR -1 ,
no check is made.
If
.I chkmnt
is non-zero,
.I c
must not be a channel in use by the mount driver
.IR mnt (3).
On successful return, if
.I iref
is non-zero, the channel's reference count has been incremented.
.I Fdtochan
calls
.IR error (10)
if it detects invalid uses, in particular an invalid file descriptor
.IR fd .
.PP
.I Namec
looks up a
.I pathname
in the current name space and returns a channel.
.I Amode
determines the mode of look up, and must be one of the constants below:
.TF Aaccess
.PD
.TP
.B Aaccess
Access file for information, as in the stat command or call.
.TP
.B Atodir
Access file as directory (the
.B CHDIR
bit of its
.B qid.path
must be set).
.TP
.B Aopen
Access for I/O.
.TP
.B Amount
Access directory to be mounted upon.
.TP
.B Acreate
File is to be created.
.PP
If
.I amode
is
.B Aopen
or
.BR Acreate ,
.I omode
should be a mode suitable for
.IR sys-open (2);
if
.BR Acreate ,
.I perm
should be valid file permissions.
In all other cases,
.I omode
and
.I perm
can be zero.
.PP
.I Cclose
decrements the reference count on
.IR c ;
if no further references remain, it
calls the corresponding device's
.B Dev.close
to close the channel, and frees
.IR c .
.SH DIAGNOSTICS
Most functions call
.IR error (10)
on any sort of error.
.SH SOURCE
.B /sys/src/9/port/chan.c
.SH SEE ALSO
.IR ref (10)