1. Tech

Your suggestion is on its way!

An email with a link to:


was emailed to:

Thanks for sharing About.com with others!

Linux / Unix Command: open
Command Library


open, creat - open and possibly create a file or device


#include <<A HREF="file:/usr/include/sys/types.h">sys/types.h>
#include <<A HREF="file:/usr/include/sys/stat.h">sys/stat.h>
#include <<A HREF="file:/usr/include/fcntl.h">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);


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.

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:

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)).
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 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.
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.
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.)
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.
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.
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.
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.
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.
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. Transfer sizes, and the alignment of user buffer and file offset must all be multiples of the logical block size of the file system.
This flag is supported on a number of Unix-like systems; support was added under Linux in kernel version 2.4.10.
A semantically similar interface for block devices is described in raw(8).
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.
On 32-bit systems that support the Large Files System, 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:

00700 user (file owner) has read, write and execute permission
00400 user has read permission
00200 user has write permission
00100 user has execute permission
00070 group has read, write and execute permission
00040 group has read permission
00020 group has write permission
00010 group has execute permission
00007 others have read, write and execute permission
00004 others have read permission
00002 others have write permisson
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.


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.


pathname already exists and O_CREAT and O_EXCL were used.
pathname refers to a directory and the access requested involved writing (that is, O_WRONLY or O_RDWR is set).
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.
pathname was too long.
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.
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.
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.
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.)
pathname refers to a file on a read-only filesystem and write access was requested.
pathname refers to an executable image which is currently being executed and write access was requested.
pathname points outside your accessible address space.
Too many symbolic links were encountered in resolving pathname, or O_NOFOLLOW was specified but pathname was a symbolic link.
pathname was to be created but the device containing pathname has no room for the new file.
Insufficient kernel memory was available.
The process already has the maximum number of files open.
The limit on the total number of files open on the system has been reached.


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.


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.


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)

Important: Use the man command (% man) to see how a command is used on your particular computer.

>> Linux/Unix Command Library

>> Shell Command Library

  1. About.com
  2. Tech
  3. Linux
  4. Linux Commands and Shell Commands
  5. open - Linux Command - Unix Command

©2015 About.com. All rights reserved.