OPEN

Section: Linux Programmer's Manual (2)
Updated: 2004-12-08
Index Return to Main Contents

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

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(2), write(2), etc.). When the call is successful, the file descriptor returned will be the lowest file descriptor not currently open for the process. 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_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_ASYNC: 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, and sockets. 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 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_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 the read(2) or write(2) system call, 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 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 context, a symbolic link exists, regardless of where its 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 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 terminal 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. 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_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(4). This mode need not have any effect on files other than FIFOs.
O_SYNC: The file is opened for synchronous I/O. Any write()s on the resulting file descriptor will block the calling process until the data has been physically written to the underlying hardware. See RESTRICTIONS below, though.
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 (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 permission
S_IROTH: 00004 others have read permission
S_IWOTH: 00002 others have write permisson
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). 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 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

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.

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, SVID, POSIX, X/OPEN, BSD 4.3. 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 various 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.

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

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.

This document was created by man2html, using the manual pages.
Time: 00:11:36 GMT, May 13, 2005