OPEN
Section: Linux Programmer's Manual (2)
Updated: 21 July 1993
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NAME
open, creat - open and possibly create a file or device
SYNOPSIS
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int open(const char *pathname, int flags);
int open(const char *pathname, int flags, mode_t mode);
int creat(const char *pathname, mode_t mode);
DESCRIPTION
open
attempts to open a file and return a file descriptor (a small,
non-negative integer for use in
read, write, etc.)
flags
is one of
O_RDONLY, O_WRONLY or O_RDWR
which request opening the file read-only, write-only or read/write,
respectively.
flags
may also be
bitwise-or'd
with one or more of the following:
- O_CREAT
-
If the file does not exist it will be created.
- O_EXCL
-
When used with
O_CREAT,
if the file already exists it is an error and the
open
will fail.
See BUGS below, though.
- O_NOCTTY
-
If
pathname
refers to a terminal device --- see
tty(4)
--- it will not become the process's controlling terminal even if the
process does not have one.
- O_TRUNC
-
If the file already exists it will be truncated.
- O_APPEND
-
The file is opened in append mode. Initially, and before each
write,
the file pointer is positioned at the end of the file, as if
with
lseek.
- O_NONBLOCK or O_NDELAY
-
The file is opened in non-blocking mode. Neither the
open
nor any subsequent operations on the file descriptor which is
returned will cause the calling process to wait.
- O_SYNC
-
The file is opened for synchronous I/O. Any
writes
on the resulting file descriptor will block the calling process until
the data has been physically written to the underlying hardware.
See BUGS below, though.
Some of these optional flags can be altered using
fcntl
after the file has been opened.
mode
specifies the permissions to use if a new file is created. It is
modified by the process's
umask
in the usual way: the permissions of the created file are
(mode & ~umask).
mode
should always be specified when
O_CREAT
is in the
flags,
and is ignored otherwise.
creat
is equivalent to
open
with
flags
equal to
O_CREAT|O_WRONLY|O_TRUNC.
RETURN VALUE
open and creat
return the new file descriptor, or -1 if an error occurred (in which case,
errno
is set appropriately).
ERRORS
- EEXIST
-
pathname
already exists and
O_CREAT and O_EXCL
were used.
- EISDIR
-
pathname
refers to a directory and the access requested involved writing.
- ETXTBSY
-
pathname
refers to an executable image which is currently being executed and
write access was requested..
- EFAULT
-
pathname points outside your accessible address space.
- EACCES
-
The requested access to the file is not allowed, or one of the
directories in
pathname
did not allow search (execute) permission.
- ENAMETOOLONG
-
pathname was too long.
- ENOENT
-
A directory component in
pathname
does not exist or is a dangling symbolic link.
- ENOTDIR
-
A component used as a directory in
pathname
is not, in fact, a directory.
- EMFILE
-
The process already has the maximum number of files open.
- ENFILE
-
The limit on the total number of files open on the system has been
reached.
- ENOMEM
-
Insufficient kernel memory was available.
- EROFS
-
pathname
refers to a file on a read-only filesystem and write access was
requested.
- ELOOP
-
pathname
contains a reference to a circular symbolic link, ie a symbolic link
whose expansion contains a reference to itself.
- ENOSPC
-
pathname
was to be created but the device containing
pathname
has no room for the new file.
CONFORMING TO
SVID, AT&T, POSIX, X/OPEN, BSD 4.3
BUGS
O_SYNC
is not currently implemented (as of Linux 0.99pl7).
There are many infelicities in the protocol underlying NFS, affecting
amongst others
O_SYNC, O_NDELAY, and O_APPEND.
O_EXCL
is broken on NFS file systems, programs which rely on it for performing
locking tasks will contain a race condition. The solution for performing
atomic file locking using a lockfile is to create a unique file on the same
fs (e.g., incorporating hostname and pid), use
link(2)
to make a link to the lockfile and use
stat(2)
on the unique file to check if its link count has increased to 2. Do not
use the return value of the link() call.
SEE ALSO
read(2), write(2), fcntl(2), close(2),
unlink(2), mknod(2), stat(2), umask(2),
mount(2), socket(2), socket(2), fopen(3),
link(2).
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- CONFORMING TO
-
- BUGS
-
- SEE ALSO
-
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