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
The open() system call is used to convert a pathname into
a file descriptor (a small, non-negative integer for use
in subsequent I/O as with read, write, etc.). When the
call is successful, the file descriptor returned will be
the lowest file descriptor not currently open for the pro
cess. This call creates a new open file, not shared with
any other process. (But shared open files may arise via
the fork(2) system call.) The new file descriptor is set
to remain open across exec functions (see fcntl(2)). The
file offset is set to the beginning of the file.
The parameter flags is one of O_RDONLY, O_WRONLY or O_RDWR
which request opening the file read-only, write-only or
read/write, respectively, bitwise-or'd with zero or more
of the following:
O_CREAT
If the file does not exist it will be created. The
owner (user ID) of the file is set to the effective
user ID of the process. The group ownership (group
ID) is set either to the effective group ID of the
process or to the group ID of the parent directory
(depending on filesystem type and mount options,
and the mode of the parent directory, see, e.g.,
the mount options bsdgroups and sysvgroups of the
ext2 filesystem, as described in mount(8)).
O_EXCL When used with O_CREAT, if the file already exists
it is an error and the open will fail. In this con
text, a symbolic link exists, regardless of where
its points to. O_EXCL is broken on NFS file sys
tems, 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. If link() returns
0, the lock is successful. Otherwise, use stat(2)
on the unique file to check if its link count has
increased to 2, in which case the lock is also suc
cessful.
If pathname refers to a terminal device -- see
tty(4) -- it will not become the process's control
ling terminal even if the process does not have
one.
O_TRUNC
If the file already exists and is a regular file
and the open mode allows writing (i.e., is O_RDWR
or O_WRONLY) it will be truncated to length 0. If
the file is a FIFO or terminal device file, the
O_TRUNC flag is ignored. Otherwise the effect of
O_TRUNC is unspecified. (On many Linux versions it
will be ignored; on other versions it will return
an error.)
O_APPEND
The file is opened in append mode. Before each
write, the file pointer is positioned at the end of
the file, as if with lseek. O_APPEND may lead to
corrupted files on NFS file systems if more than
one process appends data to a file at once. This
is because NFS does not support appending to a
file, so the client kernel has to simulate it,
which can't be done without a race condition.
O_NONBLOCK or O_NDELAY
When possible, the file is opened in non-blocking
mode. Neither the open nor any subsequent opera
tions on the file descriptor which is returned will
cause the calling process to wait. For the han
dling of FIFOs (named pipes), see also fifo(4).
This mode need not have any effect on files other
than FIFOs.
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 RESTRIC
TIONS below, though.
O_NOFOLLOW
If pathname is a symbolic link, then the open
fails. This is a FreeBSD extension, which was
added to Linux in version 2.1.126. Symbolic links
in earlier components of the pathname will still be
followed. The headers from glibc 2.0.100 and later
include a definition of this flag; kernels before
2.1.126 will ignore it if used.
O_DIRECTORY
If pathname is not a directory, cause the open to
fail. This flag is Linux-specific, and was added
in kernel version 2.1.126, to avoid denial-of-
or tape device, but should not be used outside of
the implementation of opendir.
O_LARGEFILE
On 32-bit systems that support the Large Files Sys
tem, allow files whose sizes cannot be represented
in 31 bits to be opened.
Some of these optional flags can be altered using fcntl
after the file has been opened.
The argument mode specifies the permissions to use in case
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). Note that this mode only
applies to future accesses of the newly created file; the
open call that creates a read-only file may well return a
read/write file descriptor.
The following symbolic constants are provided for mode:
S_IRWXU
00700 user (file owner) has read, write and execute
permission
S_IRUSR (S_IREAD)
00400 user has read permission
S_IWUSR (S_IWRITE)
00200 user has write permission
S_IXUSR (S_IEXEC)
00100 user has execute permission
S_IRWXG
00070 group has read, write and execute permission
S_IRGRP
00040 group has read permission
S_IWGRP
00020 group has write permission
S_IXGRP
00010 group has execute permission
S_IRWXO
00007 others have read, write and execute permis
sion
S_IROTH
00004 others have read permission
00002 others have write permisson
S_IXOTH
00001 others have execute permission
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 appropri
ately). Note that open can open device special files, but
creat cannot create them - use mknod(2) instead.
On NFS file systems with UID mapping enabled, open may
return a file descriptor but e.g. read(2) requests are
denied with EACCES. This is because the client performs
open by checking the permissions, but UID mapping is per
formed by the server upon read and write requests.
If the file is newly created, its atime, ctime, mtime
fields are set to the current time, and so are the ctime
and mtime fields of the parent directory. Otherwise, if
the file is modified because of the O_TRUNC flag, its
ctime and mtime fields are set to the current time.
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.
EACCES The requested access to the file is not allowed, or
one of the directories in pathname did not allow
search (execute) permission, or the file did not
exist yet and write access to the parent directory
is not allowed.
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, or O_DIRECTORY was specified
and pathname was not a directory.
FIFO and no process has the file open for reading.
Or, the file is a device special file and no corre
sponding device exists.
ENODEV pathname refers to a device special file and no
corresponding device exists. (This is a Linux ker
nel bug - in this situation ENXIO must be
returned.)
EROFS pathname refers to a file on a read-only filesystem
and write access was requested.
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.
ELOOP Too many symbolic links were encountered in resolv
ing pathname, or O_NOFOLLOW was specified but path
name was a symbolic link.
ENOSPC pathname was to be created but the device contain
ing pathname has no room for the new file.
ENOMEM Insufficient kernel memory was available.
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.
CONFORMING TO
SVr4, SVID, POSIX, X/OPEN, BSD 4.3 The O_NOFOLLOW and
O_DIRECTORY flags are Linux-specific. One may have to
define the _GNU_SOURCE macro to get their definitions.
RESTRICTIONS
There are many infelicities in the protocol underlying
NFS, affecting amongst others O_SYNC and O_NDELAY.
POSIX provides for three different variants of synchro
nised I/O, corresponding to the flags O_SYNC, O_DSYNC and
O_RSYNC. Currently (2.1.130) these are all synonymous
under Linux.
SEE ALSO
read(2), write(2), fcntl(2), close(2), link(2), mknod(2),
mount(2), stat(2), umask(2), unlink(2), socket(2),
fopen(3), fifo(4)
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