NEWCHAN(10.2)

NAME

newchan, chanfree, cclose, eqqid, eqchan, isdir, fdtochan, namec − channel operations

SYNOPSIS

Chan∗newchan(void);

voidchanfree(Chan ∗c)

inteqqid(Qid a, Qid b);

inteqchan(Chan ∗a, Chan ∗b, int pathonly);

voidisdir(Chan ∗c);

Chan∗fdtochan(Fgrp ∗f, int fd, int mode, int chkmnt, int iref);

Chan∗namec(char ∗pathname, int amode, int omode, ulong perm);

voidcclose(Chan ∗c);

DESCRIPTION

A value of type Chan represents a kernel channel for I/O and name space operations. It has the following public structure:

typedef struct Chan{
ulong     offset;     /∗ current file offset ∗/
ushort    type;       /∗ driver name ∗/
ulong     dev;        /∗ instance number ∗/
ushort    mode;       /∗ open mode ∗/
ushort    flag;       /∗ COPEN set once opened ∗/
Qid       qid;        /∗ unique id (path, vers) ∗/
Path∗     path;       /∗ see ptenter(10.2) ∗/
} Chan;

Newchan returns a pointer to a newly allocated channel (sleeping if necessary until memory is available). Device drivers do not normally call newchan directly, but instead allocate channels using either devattach, when a process attaches to the device’s root, or devclone, when an existing channel is cloned; see devattach(10.2).

Chanfree frees the channel structure c for reuse.

Eqqid returns 1 if Qid values a and b are equal (ie, both their path and vers members are equal); it returns 0 otherwise.

Eqchan returns 1 if a and b have the same qid, type and dev members (ie, they represent the same file); it returns 0 otherwise. If pathonly is non-zero, the comparison of the two qid members compares only their path values, ignoring the version field vers.

Isdir checks that a given channel c is a directory. If so, it returns; otherwise, it generates an error(10.2), Enotdir.

The Fgrp structure represents an array of open files, each represented by a Chan, indexed by integer file descriptors. A given Fgrp can be shared between processes.

Fdtochan returns a pointer to the Chan corresponding to file descriptor fd in file descriptor group f (almost invariably up->env->fgrp, the file descriptor group for the current process). If mode is a valid mode for sys-open(2), typically OREAD, OWRITE or ORDWR, it must correspond to the mode with which fd was originally opened; if mode is -1, no check is made. If chkmnt is non-zero, c must not be a channel in use by the mount driver mnt(3). On successful return, if iref is non-zero, the channel’s reference count has been incremented. Fdtochan calls error(10.2) if it detects invalid uses, in particular an invalid file descriptor fd.

Namec looks up a pathname in the current name space and returns a channel. Amode determines the mode of look up, and must be one of the constants below:

Aaccess Access file for information, as in the stat command or call.

Atodir Access file as directory (the CHDIR bit of its qid.path must be set).

Aopen Access for I/O.

Amount Access directory to be mounted upon.

Acreate File is to be created.

If amode is Aopen or Acreate, omode should be a mode suitable for sys-open(2); if Acreate, perm should be valid file permissions. In all other cases, omode and perm can be zero.

Cclose decrements the reference count on c; if no further references remain, it calls the corresponding device’s Dev.close to close the channel, and frees c.

DIAGNOSTICS

Most functions call error(10.2) on any sort of error.

SOURCE

/os/port/chan.c
/emu/chan.c

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