OPEN

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
CONFORMING TO
RESTRICTIONS
SEE ALSO

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, write, 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.

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. 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. 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_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. 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

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).

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 RESTRICTIONS 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-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_LARGEFILE

On 32-bit systems that support the Large Files System, allow files whose sizes cannot be represented in 31 bits to be opened. The Linux kernel does not yet have the support for this (as of 2.1.130), but the flag definition is there and the userspace LFS interfaces are present in the glibc 2.1 test releases.

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).

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 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). 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.

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.

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.

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.)

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 resolving pathname, or O_NOFOLLOW was specified but pathname was a symbolic link.

ENOSPC

pathname was to be created but the device containing 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

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.

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)