TH OPEN 2
SH NAME
open, create, close \- open a file for reading or writing, create file
SH SYNOPSIS
B #include <u.h>
br
B #include <libc.h>
PP
B
int open(char *file, int omode)
PP
B
int create(char *file, int omode, ulong perm)
PP
B
int close(int fd)
SH DESCRIPTION
I Open
opens the
I file
for I/O and returns an associated file descriptor.
I Omode
is one of
BR OREAD ,
BR OWRITE ,
BR ORDWR ,
or
BR 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
IR omode :
B OTRUNC
says to truncate the file
to zero length before opening it;
B OCEXEC
says to close the file when an
IR exec (2)
or
I execl
system call is made;
and
B ORCLOSE
says to remove the file when it is closed (by everyone who has a copy of the file descriptor).
I Open
fails if the file does not exist or the user does not have
permission to open it for the requested purpose
(see
IR stat (2)
for a description of permissions).
The user must have write permission on the
I file
if the
B OTRUNC
bit is set.
For the
I open
system call
(unlike the implicit
I open
in
IR exec (2)),
B OEXEC
is actually identical to
BR OREAD .
PP
I Create
creates a new
I file
or prepares to rewrite an existing
IR file ,
opens it according to
I omode
(as described for
IR 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
I 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
B DMDIR
bit is set in
IR perm ,
an exclusive-use file if the
B DMEXCL
bit is set, and an append-only file if the
B 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
IR open (5).
PP
I Create
fails if the path up to the last element of
I 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
IR 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
IR intro (2))
then the constituent directory where the file is created
depends on the structure of the union: see
IR bind (2).
PP
Since
I create
may succeed even if the file exists, a special mechanism is necessary
for those applications that require an atomic create operation.
If the
B OEXCL
RB ( 0x1000 )
bit is set in the
I mode
for a
IR create,
the call succeeds only if the file does not already exist;
see
IR open (5)
for details.
PP
I Close
closes the file associated with a file descriptor.
Provided the file descriptor is a valid open descriptor,
I close
is guaranteed to close it; there will be no error.
Files are closed automatically upon termination of a process;
I close
allows the file descriptor to be reused.
SH SOURCE
B /sys/src/libc/9syscall
SH SEE ALSO
IR intro (2),
IR bind (2),
IR stat (2)
SH DIAGNOSTICS
These functions set
IR errstr .
