Provided by: manpages-posix-dev_2017a-2_all 
PROLOG
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.
NAME
open, openat — open file
SYNOPSIS
#include <sys/stat.h>
#include <fcntl.h>
int open(const char *path, int oflag, ...);
int openat(int fd, const char *path, int oflag, ...);
DESCRIPTION
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, allocated as
described in Section 2.14, File Descriptor Allocation. 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, if O_DIRECTORY is not set the file shall be created as a
regular file; 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.1‐2017, 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 and O_DIRECTORY are set and the requested access mode is neither O_WRONLY nor
O_RDWR, the result is unspecified.
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 access mode of the open file description associated with the file
descriptor is not O_SEARCH, the function shall check whether directory searches are
permitted using the current permissions of the directory underlying the file descriptor.
If the access mode is 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().
RETURN VALUE
Upon successful completion, these functions shall open the file and return a non-negative
integer representing the 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.
ERRORS
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, or includes
O_CREAT without O_DIRECTORY.
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 without the trailing
<slash> characters would name 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 The access mode of the open file description associated with fd is not 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.
EOPNOTSUPP
The path argument names a socket.
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.
EXAMPLES
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);
}
...
APPLICATION USAGE
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.1‐2017, 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).
RATIONALE
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() 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.1‐2017 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.1‐2017, 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.
FUTURE DIRECTIONS
None.
SEE ALSO
chmod(), close(), creat(), dirfd(), dup(), exec, fcntl(), fdopendir(), link(), lseek(),
mkdtemp(), mknod(), read(), symlink(), umask(), unlockpt(), write()
The Base Definitions volume of POSIX.1‐2017, Chapter 11, General Terminal Interface,
<fcntl.h>, <sys_stat.h>, <sys_types.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1-2017, Standard for Information Technology -- Portable Operating System Interface
(POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by
the Institute of Electrical and Electronics Engineers, Inc and The Open Group. 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.opengroup.org/unix/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 .