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().
