2:open
From Linux Man Pages
open, creat - open and possibly create a file or device
Contents |
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
Given a pathname for a file, open() returns a file descriptor, a small, non-negative integer for use in subse-
quent system calls (read(2), write(2), lseek(2), fcntl(2), etc.). The file descriptor returned by a successful
call will be the lowest-numbered file descriptor not currently open for the process.
The new file descriptor is set to remain open across an execve(2) (i.e., the FD_CLOEXEC file descriptor flag
described in fcntl(2) is initially disabled). The file offset is set to the beginning of the file (see
lseek(2)).
A call to open() creates a new open file description, an entry in the system-wide table of open files. This
entry records the file offset and the file status flags (modifiable via the fcntl() F_SETFL operation). A file
descriptor is a reference to one of these entries; this reference is unaffected if pathname is subsequently
removed or modified to refer to a different file. The new open file description is initially not shared with any
other process, but sharing may arise via fork(2).
The parameter flags must include one of the following access modes: O_RDONLY, O_WRONLY, or O_RDWR. These request
opening the file read-only, write-only, or read/write, respectively.
In addition, zero or more file creation flags and file status flags can be bitwise-or'd in flags. The file cre-
ation flags are O_CREAT, O_EXCL, O_NOCTTY, and O_TRUNC. The file status flags are all of the remaining flags
listed below. The distinction between these two groups of flags is that the file status flags can be retrieved
and (in some cases) modified using fcntl(2). The full list of file creation flags and file status flags is as
follows:
O_APPEND
The file is opened in append mode. Before each write(), the file offset 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 pro-
cess 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_ASYNC
Enable signal-driven I/O: generate a signal (SIGIO by default, but this can be changed via fcntl(2)) when
input or output becomes possible on this file descriptor. This feature is only available for terminals,
pseudo-terminals, sockets, and (since Linux 2.6) pipes and FIFOs. See fcntl(2) for further details.
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 pro-
cess 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 filesys-
tem, as described in mount(8)).
O_DIRECT
Try to minimize cache effects of the I/O to and from this file. In general this will degrade performance,
but it is useful in special situations, such as when applications do their own caching. File I/O is done
directly to/from user space buffers. The I/O is synchronous, i.e., at the completion of a read(2) or
write(2), data is guaranteed to have been transferred. Under Linux 2.4 transfer sizes, and the alignment
of user buffer and file offset must all be multiples of the logical block size of the file system. Under
Linux 2.6 alignment to 512-byte boundaries suffices.
A semantically similar (but deprecated) interface for block devices is described in raw(8).
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-service problems if opendir(3) is called on a FIFO or tape
device, but should not be used outside of the implementation of opendir.
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 it points to. 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 file system (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 successful.
O_LARGEFILE
(LFS) Allow files whose sizes cannot be represented in an off_t (but can be represented in an off64_t) to
be opened.
O_NOATIME
(Since Linux 2.6.8) Do not update the file last access time (st_atime in the inode) when the file is
read(2). This flag is intended for use by indexing or backup programs, where its use can significantly
reduce the amount of disk activity. This flag may not be effective on all filesystems. One example is
NFS, where the server maintains the access time.
O_NOCTTY
If pathname refers to a terminal device -- see tty(4) -- it will not become the process's controlling ter-
minal even if the process does not have one.
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.
O_NONBLOCK or O_NDELAY
When possible, 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. For the handling of
FIFOs (named pipes), see also fifo(7). For a discussion of the effect of O_NONBLOCK in conjunction with
mandatory file locks and with file leases, see fcntl(2).
O_SYNC The file is opened for synchronous I/O. Any write()s on the resulting file descriptor will block the call-
ing process until the data has been physically written to the underlying hardware. But see RESTRICTIONS
below.
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.
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
00400 user has read permission
S_IWUSR
00200 user has write permission
S_IXUSR
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 permission
S_IROTH
00004 others have read permission
S_IWOTH
00002 others have write permission
S_IXOTH
00001 others have execute permission
mode must 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).
NOTES
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 performed by the server upon read and write requests. If the file is newly created, its st_atime, st_ctime, st_mtime fields (respectively, time of last access, time of last status change, and time of last modification; see stat(2)) are set to the current time, and so are the st_ctime and st_mtime fields of the parent directory. Otherwise, if the file is modified because of the O_TRUNC flag, its st_ctime and st_mtime fields are set to the current time.
ERRORS
EACCES The requested access to the file is not allowed, or search permission is denied for one of the directories
in the path prefix of pathname, or the file did not exist yet and write access to the parent directory is
not allowed. (See also path_resolution(2).)
EEXIST pathname already exists and O_CREAT and O_EXCL were used.
EFAULT pathname points outside your accessible address space.
EISDIR pathname refers to a directory and the access requested involved writing (that is, O_WRONLY or O_RDWR is
set).
ELOOP Too many symbolic links were encountered in resolving pathname, or O_NOFOLLOW was specified but pathname
was a symbolic link.
EMFILE The process already has the maximum number of files open.
ENAMETOOLONG
pathname was too long.
ENFILE The system limit on the total number of open files has been reached.
ENODEV pathname refers to a device special file and no corresponding device exists. (This is a Linux kernel bug;
in this situation ENXIO must be returned.)
ENOENT O_CREAT is not set and the named file does not exist. Or, a directory component in pathname does not
exist or is a dangling symbolic link.
ENOMEM Insufficient kernel memory was available.
ENOSPC pathname was to be created but the device containing pathname has no room for the new file.
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.
ENXIO O_NONBLOCK | O_WRONLY is set, the named file is a FIFO and no process has the file open for reading. Or,
the file is a device special file and no corresponding device exists.
EOVERFLOW
pathname refers to a regular file, too large to be opened; see O_LARGEFILE above.
EPERM The O_NOATIME flag was specified, but the effective user ID of the caller did not match the owner of the
file and the caller was not privileged (CAP_FOWNER).
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.
EWOULDBLOCK
The O_NONBLOCK flag was specified, and an incompatible lease was held on the file (see fcntl(2)).
NOTE
Under Linux, the O_NONBLOCK flag indicates that one wants to open but does not necessarily have the intention to read or write. This is typically used to open devices in order to get a file descriptor for use with ioctl(2).
CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001. The O_NOATIME, O_NOFOLLOW, and O_DIRECTORY flags are Linux-specific. One may have to define the _GNU_SOURCE macro to get their definitions.
The (undefined) effect of O_RDONLY | O_TRUNC varies among implementations. On many systems the file is actually truncated.
The O_DIRECT flag was introduced in SGI IRIX, where it has alignment restrictions similar to those of Linux 2.4. IRIX has also a fcntl(2) call to query appropriate alignments, and sizes. FreeBSD 4.x introduced a flag of same name, but without alignment restrictions. Support was added under Linux in kernel version 2.4.10. Older Linux kernels simply ignore this flag. One may have to define the _GNU_SOURCE macro to get its definition.
BUGS
"The thing that has always disturbed me about O_DIRECT is that the whole interface is just stupid, and was probably designed by a deranged monkey on some serious mind-controlling substances." -- Linus
Currently, it is not possible to enable signal-driven I/O by specifying O_ASYNC when calling open(); use fcntl(2) to enable this flag.
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 synchronised I/O, corresponding to the flags O_SYNC, O_DSYNC and O_RSYNC. Currently (2.1.130) these are all synonymous under Linux.
RELATED
close(2), dup(2), fcntl(2), link(2), lseek(2), mknod(2), mount(2), mmap(2), openat(2), path_resolution(2), read(2), socket(2), stat(2), umask(2), unlink(2), write(2), fopen(3), fifo(7), feature_test_macros(7)