OPEN(2)OPEN(2)
NAME
open, create, close – open a file for reading or writing, create file
SYNOPSIS
#include <u.h>
#include <libc.h>
int open(char *file, int omode)
int create(char *file, int omode, ulong perm)
int close(int fd)
DESCRIPTION
Open
opens the
file
for I/O and returns an associated file descriptor.
Omode
is one of
OREAD,
OWRITE,
ORDWR,
or
OEXEC,
asking for permission to read, write, read and write, or execute, respectively.
In addition, there are three values that can be ORed with the
omode:
OTRUNC
says to truncate the file
to zero length before opening it;
OCEXEC
says to close the file when an
exec(2)
or
execl
system call is made;
and
ORCLOSE
says to remove the file when it is closed (by everyone who has a copy of the file descriptor).
Open
fails if the file does not exist or the user does not have
permission to open it for the requested purpose
(see
stat(2)
for a description of permissions).
The user must have write permission on the
file
if the
OTRUNC
bit is set.
For the
open
system call
(unlike the implicit
open
in
exec(2)),
OEXEC
is actually identical to
OREAD.
Create
creates a new
file
or prepares to rewrite an existing
file,
opens it according to
omode
(as described for
open),
and returns an associated file descriptor.
If the file is new,
the owner is set to the userid of the creating process group;
the group to that of the containing directory;
the permissions to
perm
ANDed with the permissions of the containing directory.
If the file already exists,
it is truncated to 0 length,
and the permissions, owner, and group remain unchanged.
The created file is a directory if the
DMDIR
bit is set in
perm,
an exclusive-use file if the
DMEXCL
bit is set, and an append-only file if the
DMAPPEND
bit is set.
Exclusive-use files may be open for I/O by only one client at a time,
but the file descriptor may become invalid if no I/O is done
for an extended period; see
open(5).
Create
fails if the path up to the last element of
file
cannot be evaluated, if the user doesn’t have write permission
in the final directory, if the file already exists and
does not permit the access defined by
omode,
or if there are no free file descriptors.
In the last case, the file may be created even when
an error is returned.
If the file is new and the directory in which it is created is
a union directory (see
intro(2))
then the constituent directory where the file is created
depends on the structure of the union: see
bind(2).
Since
create
may succeed even if the file exists, a special mechanism is necessary
for those applications that require an atomic create operation.
If the
OEXCL
(0x1000)
bit is set in the
mode
for a
create,
the call succeeds only if the file does not already exist;
see
open(5)
for details.
Close
closes the file associated with a file descriptor.
Provided the file descriptor is a valid open descriptor,
close
is guaranteed to close it; there will be no error.
Files are closed automatically upon termination of a process;
close
allows the file descriptor to be reused.
SOURCE
/sys/src/libc/9syscall
SEE
intro(2),
bind(2),
stat(2)
DIAGNOSTICS
These functions set
errstr.