code: 9ferno

ref: 44ce0097b612a1fefd754065bdf8d9d2e5ef60c8
dir: /man/2/sys-open/

.TH SYS-OPEN 2
.SH NAME
open, create \- open a file for reading or writing, create file
.SH SYNOPSIS
.EX
include "sys.m";
sys := load Sys Sys->PATH;

create: fn(file: string, omode, perm: int): ref FD;
open:   fn(file: string, omode: int):       ref FD;
.EE
.SH DESCRIPTION
.B Open
opens the
.I file
for I/O and returns an associated file descriptor.
.I Omode
is one of
.BR Sys->OREAD ,
.BR Sys->OWRITE ,
or
.BR Sys->ORDWR ,
asking for permission to read, write, or read and write, respectively.
There are two values that can be OR'd with those to form
.IR omode :
.B Sys->OTRUNC
says to truncate the file before opening it, which requires write permission
even if
.I omode
is
.BR Sys->OREAD ;
and
.B Sys->ORCLOSE
says to remove the file when it is closed (ie, when the last reference
to this file descriptor goes away).
.PP
.B Open
returns
.B nil
if the file does not exist, if the file name is unacceptable, if the user does not have
permission to open it as requested
(see
.IR sys-stat (2)
for a description of permissions),
or if any other error occurs.
.PP
.B Create
creates a new
.I file
or prepares to rewrite an existing
.IR file ,
opens it according to
.I omode
(as described for
.BR open ),
and returns an associated file descriptor.
.PP
If the file is new,
the owner is set to the
.I "user id
of the creating process group,
the group to that of the containing directory,
and the permissions to
.I perm
ANDed with the permissions of the containing directory.
The bits in 
.I perm
are the same as those
in the file mode returned by 
.IR sys-stat (2).
.PP
If the file already exists,
it is truncated to 0 length,
but the permissions, owner, and group remain unchanged.
.PP
The created file will be a directory if the
.B Sys->DMDIR
bit is set in
.IR perm ,
and
.I omode
is
.BR Sys->OREAD .
The file will be exclusive-use if the
.B Sys->DMEXCL
bit is set in
.I perm
and the underlying file server supports it;
see
.IR open (5)
for details.
It will be append-only if the
.B Sys->DMAPPEND
bit is set, and the underlying file server supports it.
.PP
.B Create
returns
.B nil
if the path up to the last element of
.I file
cannot be evaluated, if the file name is unacceptable,
if the user does not have write permission in the final directory,
if the file already exists and does not permit the access defined by
.IR omode,
or if any other error occurs.
.PP
If the file is new and the directory in which it is created is
a union directory (see
.IR sys-intro (2))
then the constituent directory where the file is created
depends on the structure of the union: see
.IR sys-bind (2).
.PP
Since create may succeed even if the file exists, a special mechanism
is necessary for applications that require an atomic create operation.
If the
.B Sys->OEXCL
bit is set in the mode for a create, the call succeeds only if
the file does not already exist; see
.IR open (5)
for details.
.PP
There is no explicit ``close'' routine:
when the last reference to the file descriptor is released,
the system closes the associated file.
For devices and network protocols where shutdown must be guaranteed,
write a
.B hangup
message to the associated control file and use the return value of the
.B write
to verify closure.
.SH SEE ALSO
.IR sys-intro (2),
.IR sys-bind (2),
.IR sys-stat (2)