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OPEN(3P) POSIX Programmer's Manual OPEN(3P)
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
open, openat — open file relative to directory file descriptor
#include <sys/stat.h>
#include <fcntl.h>
int open(const char *path, int oflag, ...);
int openat(int fd, const char *path, int oflag, ...);
The open() function shall establish the connection between a file and
a file descriptor. It shall create an open file description that
refers to a file and a file descriptor that refers to that open file
description. The file descriptor is used by other I/O functions to
refer to that file. The path argument points to a pathname naming the
file.
The open() function shall return a file descriptor for the named file
that is the lowest file descriptor not currently open for that
process. The open file description is new, and therefore the file
descriptor shall not share it with any other process in the system.
The FD_CLOEXEC file descriptor flag associated with the new file
descriptor shall be cleared unless the O_CLOEXEC flag is set in
oflag.
The file offset used to mark the current position within the file
shall be set to the beginning of the file.
The file status flags and file access modes of the open file
description shall be set according to the value of oflag.
Values for oflag are constructed by a bitwise-inclusive OR of flags
from the following list, defined in <fcntl.h>. Applications shall
specify exactly one of the first five values (file access modes)
below in the value of oflag:
O_EXEC Open for execute only (non-directory files). The result
is unspecified if this flag is applied to a directory.
O_RDONLY Open for reading only.
O_RDWR Open for reading and writing. The result is undefined
if this flag is applied to a FIFO.
O_SEARCH Open directory for search only. The result is
unspecified if this flag is applied to a non-directory
file.
O_WRONLY Open for writing only.
Any combination of the following may be used:
O_APPEND If set, the file offset shall be set to the end of the
file prior to each write.
O_CLOEXEC If set, the FD_CLOEXEC flag for the new file descriptor
shall be set.
O_CREAT If the file exists, this flag has no effect except as
noted under O_EXCL below. Otherwise, the file shall be
created; the user ID of the file shall be set to the
effective user ID of the process; the group ID of the
file shall be set to the group ID of the file's parent
directory or to the effective group ID of the process;
and the access permission bits (see <sys/stat.h>) of
the file mode shall be set to the value of the argument
following the oflag argument taken as type mode_t
modified as follows: a bitwise AND is performed on the
file-mode bits and the corresponding bits in the
complement of the process' file mode creation mask.
Thus, all bits in the file mode whose corresponding bit
in the file mode creation mask is set are cleared. When
bits other than the file permission bits are set, the
effect is unspecified. The argument following the oflag
argument does not affect whether the file is open for
reading, writing, or for both. Implementations shall
provide a way to initialize the file's group ID to the
group ID of the parent directory. Implementations may,
but need not, provide an implementation-defined way to
initialize the file's group ID to the effective group
ID of the calling process.
O_DIRECTORY If path resolves to a non-directory file, fail and set
errno to [ENOTDIR].
O_DSYNC Write I/O operations on the file descriptor shall
complete as defined by synchronized I/O data integrity
completion.
O_EXCL If O_CREAT and O_EXCL are set, open() shall fail if the
file exists. The check for the existence of the file
and the creation of the file if it does not exist shall
be atomic with respect to other threads executing
open() naming the same filename in the same directory
with O_EXCL and O_CREAT set. If O_EXCL and O_CREAT are
set, and path names a symbolic link, open() shall fail
and set errno to [EEXIST], regardless of the contents
of the symbolic link. If O_EXCL is set and O_CREAT is
not set, the result is undefined.
O_NOCTTY If set and path identifies a terminal device, open()
shall not cause the terminal device to become the
controlling terminal for the process. If path does not
identify a terminal device, O_NOCTTY shall be ignored.
O_NOFOLLOW If path names a symbolic link, fail and set errno to
[ELOOP].
O_NONBLOCK When opening a FIFO with O_RDONLY or O_WRONLY set:
* If O_NONBLOCK is set, an open() for reading-only
shall return without delay. An open() for writing-
only shall return an error if no process currently
has the file open for reading.
* If O_NONBLOCK is clear, an open() for reading-only
shall block the calling thread until a thread opens
the file for writing. An open() for writing-only
shall block the calling thread until a thread opens
the file for reading.
When opening a block special or character special file
that supports non-blocking opens:
* If O_NONBLOCK is set, the open() function shall
return without blocking for the device to be ready
or available. Subsequent behavior of the device is
device-specific.
* If O_NONBLOCK is clear, the open() function shall
block the calling thread until the device is ready
or available before returning.
Otherwise, the O_NONBLOCK flag shall not cause an
error, but it is unspecified whether the file status
flags will include the O_NONBLOCK flag.
O_RSYNC Read I/O operations on the file descriptor shall
complete at the same level of integrity as specified by
the O_DSYNC and O_SYNC flags. If both O_DSYNC and
O_RSYNC are set in oflag, all I/O operations on the
file descriptor shall complete as defined by
synchronized I/O data integrity completion. If both
O_SYNC and O_RSYNC are set in flags, all I/O operations
on the file descriptor shall complete as defined by
synchronized I/O file integrity completion.
O_SYNC Write I/O operations on the file descriptor shall
complete as defined by synchronized I/O file integrity
completion.
The O_SYNC flag shall be supported for regular files,
even if the Synchronized Input and Output option is not
supported.
O_TRUNC If the file exists and is a regular file, and the file
is successfully opened O_RDWR or O_WRONLY, its length
shall be truncated to 0, and the mode and owner shall
be unchanged. It shall have no effect on FIFO special
files or terminal device files. Its effect on other
file types is implementation-defined. The result of
using O_TRUNC without either O_RDWR or O_WRONLY is
undefined.
O_TTY_INIT If path identifies a terminal device other than a
pseudo-terminal, the device is not already open in any
process, and either O_TTY_INIT is set in oflag or
O_TTY_INIT has the value zero, open() shall set any
non-standard termios structure terminal parameters to a
state that provides conforming behavior; see the Base
Definitions volume of POSIX.1‐2008, Section 11.2,
Parameters that Can be Set. It is unspecified whether
O_TTY_INIT has any effect if the device is already open
in any process. If path identifies the slave side of a
pseudo-terminal that is not already open in any
process, open() shall set any non-standard termios
structure terminal parameters to a state that provides
conforming behavior, regardless of whether O_TTY_INIT
is set. If path does not identify a terminal device,
O_TTY_INIT shall be ignored.
If O_CREAT is set and the file did not previously exist, upon
successful completion, open() shall mark for update the last data
access, last data modification, and last file status change
timestamps of the file and the last data modification and last file
status change timestamps of the parent directory.
If O_TRUNC is set and the file did previously exist, upon successful
completion, open() shall mark for update the last data modification
and last file status change timestamps of the file.
If both the O_SYNC and O_DSYNC flags are set, the effect is as if
only the O_SYNC flag was set.
If path refers to a STREAMS file, oflag may be constructed from
O_NONBLOCK OR'ed with either O_RDONLY, O_WRONLY, or O_RDWR. Other
flag values are not applicable to STREAMS devices and shall have no
effect on them. The value O_NONBLOCK affects the operation of STREAMS
drivers and certain functions applied to file descriptors associated
with STREAMS files. For STREAMS drivers, the implementation of
O_NONBLOCK is device-specific.
The application shall ensure that it specifies the O_TTY_INIT flag on
the first open of a terminal device since system boot or since the
device was closed by the process that last had it open. The
application need not specify the O_TTY_INIT flag when opening pseudo-
terminals. If path names the master side of a pseudo-terminal
device, then it is unspecified whether open() locks the slave side so
that it cannot be opened. Conforming applications shall call
unlockpt() before opening the slave side.
The largest value that can be represented correctly in an object of
type off_t shall be established as the offset maximum in the open
file description.
The openat() function shall be equivalent to the open() function
except in the case where path specifies a relative path. In this case
the file to be opened is determined relative to the directory
associated with the file descriptor fd instead of the current working
directory. If the file descriptor was opened without O_SEARCH, the
function shall check whether directory searches are permitted using
the current permissions of the directory underlying the file
descriptor. If the file descriptor was opened with O_SEARCH, the
function shall not perform the check.
The oflag parameter and the optional fourth parameter correspond
exactly to the parameters of open().
If openat() is passed the special value AT_FDCWD in the fd parameter,
the current working directory shall be used and the behavior shall be
identical to a call to open().
Upon successful completion, these functions shall open the file and
return a non-negative integer representing the lowest numbered unused
file descriptor. Otherwise, these functions shall return −1 and set
errno to indicate the error. If −1 is returned, no files shall be
created or modified.
These functions shall fail if:
EACCES Search permission is denied on a component of the path prefix,
or the file exists and the permissions specified by oflag are
denied, or the file does not exist and write permission is
denied for the parent directory of the file to be created, or
O_TRUNC is specified and write permission is denied.
EEXIST O_CREAT and O_EXCL are set, and the named file exists.
EINTR A signal was caught during open().
EINVAL The implementation does not support synchronized I/O for this
file.
EIO The path argument names a STREAMS file and a hangup or error
occurred during the open().
EISDIR The named file is a directory and oflag includes O_WRONLY or
O_RDWR.
ELOOP A loop exists in symbolic links encountered during resolution
of the path argument, or O_NOFOLLOW was specified and the path
argument names a symbolic link.
EMFILE All file descriptors available to the process are currently
open.
ENAMETOOLONG
The length of a component of a pathname is longer than
{NAME_MAX}.
ENFILE The maximum allowable number of files is currently open in the
system.
ENOENT O_CREAT is not set and a component of path does not name an
existing file, or O_CREAT is set and a component of the path
prefix of path does not name an existing file, or path points
to an empty string.
ENOENT or ENOTDIR
O_CREAT is set, and the path argument contains at least one
non-<slash> character and ends with one or more trailing
<slash> characters. If path names an existing file, an
[ENOENT] error shall not occur.
ENOSR The path argument names a STREAMS-based file and the system is
unable to allocate a STREAM.
ENOSPC The directory or file system that would contain the new file
cannot be expanded, the file does not exist, and O_CREAT is
specified.
ENOTDIR
A component of the path prefix names an existing file that is
neither a directory nor a symbolic link to a directory; or
O_CREAT and O_EXCL are not specified, the path argument
contains at least one non-<slash> character and ends with one
or more trailing <slash> characters, and the last pathname
component names an existing file that is neither a directory
nor a symbolic link to a directory; or O_DIRECTORY was
specified and the path argument resolves to a non-directory
file.
ENXIO O_NONBLOCK is set, the named file is a FIFO, O_WRONLY is set,
and no process has the file open for reading.
ENXIO The named file is a character special or block special file,
and the device associated with this special file does not
exist.
EOVERFLOW
The named file is a regular file and the size of the file
cannot be represented correctly in an object of type off_t.
EROFS The named file resides on a read-only file system and either
O_WRONLY, O_RDWR, O_CREAT (if the file does not exist), or
O_TRUNC is set in the oflag argument.
The openat() function shall fail if:
EACCES fd was not opened with O_SEARCH and the permissions of the
directory underlying fd do not permit directory searches.
EBADF The path argument does not specify an absolute path and the fd
argument is neither AT_FDCWD nor a valid file descriptor open
for reading or searching.
ENOTDIR
The path argument is not an absolute path and fd is a file
descriptor associated with a non-directory file.
These functions may fail if:
EAGAIN The path argument names the slave side of a pseudo-terminal
device that is locked.
EINVAL The value of the oflag argument is not valid.
ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during
resolution of the path argument.
ENAMETOOLONG
The length of a pathname exceeds {PATH_MAX}, or pathname
resolution of a symbolic link produced an intermediate result
with a length that exceeds {PATH_MAX}.
ENOMEM The path argument names a STREAMS file and the system is
unable to allocate resources.
ETXTBSY
The file is a pure procedure (shared text) file that is being
executed and oflag is O_WRONLY or O_RDWR.
The following sections are informative.
Opening a File for Writing by the Owner
The following example opens the file /tmp/file, either by creating it
(if it does not already exist), or by truncating its length to 0 (if
it does exist). In the former case, if the call creates a new file,
the access permission bits in the file mode of the file are set to
permit reading and writing by the owner, and to permit reading only
by group members and others.
If the call to open() is successful, the file is opened for writing.
#include <fcntl.h>
...
int fd;
mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH;
char *pathname = "/tmp/file";
...
fd = open(pathname, O_WRONLY | O_CREAT | O_TRUNC, mode);
...
Opening a File Using an Existence Check
The following example uses the open() function to try to create the
LOCKFILE file and open it for writing. Since the open() function
specifies the O_EXCL flag, the call fails if the file already exists.
In that case, the program assumes that someone else is updating the
password file and exits.
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#define LOCKFILE "/etc/ptmp"
...
int pfd; /* Integer for file descriptor returned by open() call. */
...
if ((pfd = open(LOCKFILE, O_WRONLY | O_CREAT | O_EXCL,
S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)) == -1)
{
fprintf(stderr, "Cannot open /etc/ptmp. Try again later.\n");
exit(1);
}
...
Opening a File for Writing
The following example opens a file for writing, creating the file if
it does not already exist. If the file does exist, the system
truncates the file to zero bytes.
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#define LOCKFILE "/etc/ptmp"
...
int pfd;
char pathname[PATH_MAX+1];
...
if ((pfd = open(pathname, O_WRONLY | O_CREAT | O_TRUNC,
S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)) == -1)
{
perror("Cannot open output file\n"); exit(1);
}
...
POSIX.1‐2008 does not require that terminal parameters be
automatically set to any state on first open, nor that they be reset
after the last close. It is possible for a non-conforming application
to leave a terminal device in a state where the next process to use
that device finds it in a non-conforming state, but has no way of
determining this. To ensure that the device is set to a conforming
initial state, applications which perform a first open of a terminal
(other than a pseudo-terminal) should do so using the O_TTY_INIT flag
to set the parameters associated with the terminal to a conforming
state.
Except as specified in this volume of POSIX.1‐2008, the flags allowed
in oflag are not mutually-exclusive and any number of them may be
used simultaneously. Not all combinations of flags make sense. For
example, using O_SEARCH | O_CREAT will successfully open a pre-
existing directory for searching, but if there is no existing file by
that name, then it is unspecified whether a regular file will be
created. Likewise, if a non-directory file descriptor is successfully
returned, it is unspecified whether that descriptor will have execute
permissions as if by O_EXEC (note that it is unspecified whether
O_EXEC and O_SEARCH have the same value).
Some implementations permit opening FIFOs with O_RDWR. Since FIFOs
could be implemented in other ways, and since two file descriptors
can be used to the same effect, this possibility is left as
undefined.
See getgroups(3p) about the group of a newly created file.
The use of open() to create a regular file is preferable to the use
of creat(), because the latter is redundant and included only for
historical reasons.
The use of the O_TRUNC flag on FIFOs and directories (pipes cannot be
open()-ed) must be permissible without unexpected side-effects (for
example, creat() on a FIFO must not remove data). Since terminal
special files might have type-ahead data stored in the buffer,
O_TRUNC should not affect their content, particularly if a program
that normally opens a regular file should open the current
controlling terminal instead. Other file types, particularly
implementation-defined ones, are left implementation-defined.
POSIX.1‐2008 permits [EACCES] to be returned for conditions other
than those explicitly listed.
The O_NOCTTY flag was added to allow applications to avoid
unintentionally acquiring a controlling terminal as a side-effect of
opening a terminal file. This volume of POSIX.1‐2008 does not specify
how a controlling terminal is acquired, but it allows an
implementation to provide this on open() if the O_NOCTTY flag is not
set and other conditions specified in the Base Definitions volume of
POSIX.1‐2008, Chapter 11, General Terminal Interface are met.
In historical implementations the value of O_RDONLY is zero. Because
of that, it is not possible to detect the presence of O_RDONLY and
another option. Future implementations should encode O_RDONLY and
O_WRONLY as bit flags so that:
O_RDONLY | O_WRONLY == O_RDWR
O_EXEC and O_SEARCH are specified as two of the five file access
modes. Since O_EXEC does not apply to directories, and O_SEARCH only
applies to directories, their values need not be distinct. Since
O_RDONLY has historically had the value zero, implementations are not
able to distinguish between O_SEARCH and O_SEARCH | O_RDONLY, and
similarly for O_EXEC.
In general, the open() function follows the symbolic link if path
names a symbolic link. However, the open() function, when called with
O_CREAT and O_EXCL, is required to fail with [EEXIST] if path names
an existing symbolic link, even if the symbolic link refers to a
nonexistent file. This behavior is required so that privileged
applications can create a new file in a known location without the
possibility that a symbolic link might cause the file to be created
in a different location.
For example, a privileged application that must create a file with a
predictable name in a user-writable directory, such as the user's
home directory, could be compromised if the user creates a symbolic
link with that name that refers to a nonexistent file in a system
directory. If the user can influence the contents of a file, the user
could compromise the system by creating a new system configuration or
spool file that would then be interpreted by the system. The test for
a symbolic link which refers to a nonexisting file must be atomic
with the creation of a new file.
In addition, the open() function refuses to open non-directories if
the O_DIRECTORY flag is set. This avoids race conditions whereby a
user might compromise the system by substituting a hard link to a
sensitive file (e.g., a device or a FIFO) while a privileged
application is running, where opening a file even for read access
might have undesirable side-effects.
In addition, the open() function does not follow symbolic links if
the O_NOFOLLOW flag is set. This avoids race conditions whereby a
user might compromise the system by substituting a symbolic link to a
sensitive file (e.g., a device) while a privileged application is
running, where opening a file even for read access might have
undesirable side-effects.
The POSIX.1‐1990 standard required that the group ID of a newly
created file be set to the group ID of its parent directory or to the
effective group ID of the creating process. FIPS 151‐2 required that
implementations provide a way to have the group ID be set to the
group ID of the containing directory, but did not prohibit
implementations also supporting a way to set the group ID to the
effective group ID of the creating process. Conforming applications
should not assume which group ID will be used. If it matters, an
application can use chown() to set the group ID after the file is
created, or determine under what conditions the implementation will
set the desired group ID.
The purpose of the openat() function is to enable opening files in
directories other than the current working directory without exposure
to race conditions. Any part of the path of a file could be changed
in parallel to a call to open(), resulting in unspecified behavior.
By opening a file descriptor for the target directory and using the
openat() function it can be guaranteed that the opened file is
located relative to the desired directory. Some implementations use
the openat() function for other purposes as well. In some cases, if
the oflag parameter has the O_XATTR bit set, the returned file
descriptor provides access to extended attributes. This functionality
is not standardized here.
None.
chmod(3p), close(3p), creat(3p), dirfd(3p), dup(3p), exec(1p),
fcntl(3p), fdopendir(3p), link(3p), lseek(3p), mkdtemp(3p),
mknod(3p), read(3p), symlink(3p), umask(3p), unlockpt(3p), write(3p)
The Base Definitions volume of POSIX.1‐2008, Chapter 11, General
Terminal Interface, fcntl.h(0p), sys_stat.h(0p), sys_types.h(0p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) In the event of any discrepancy between this version and
the original IEEE and The Open Group Standard, the original IEEE and
The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 OPEN(3P)
Pages that refer to this page: fcntl.h(0p), stropts.h(0p), cp(1p), sh(1p), aio_fsync(3p), chmod(3p), close(3p), creat(3p), dbm_clearerr(3p), dirfd(3p), dup(3p), exec(3p), fchmod(3p), fcntl(3p), fdatasync(3p), fdopen(3p), fdopendir(3p), freopen(3p), fstatvfs(3p), ftruncate(3p), getrlimit(3p), grantpt(3p), ioctl(3p), lockf(3p), lseek(3p), mkdtemp(3p), mknod(3p), openat(3p), posix_fallocate(3p), posix_openpt(3p), posix_spawn(3p), posix_spawn_file_actions_addclose(3p), ptsname(3p), read(3p), symlink(3p), tempnam(3p), tmpnam(3p), truncate(3p), umask(3p), unlockpt(3p), write(3p)