OPEN(2) System Calls OPEN(2)

NAME

open, openat - open a file

SYNOPSIS

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

int
open(const char *path, int oflag [, mode_t mode]);

int
openat(int fildes, const char *path, int oflag [, mode_t mode]);

DESCRIPTION

The open() function establishes the connection between a file and a
file descriptor. It creates 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 openat() function is identical to the open() function except that
the path argument is interpreted relative to the starting point implied
by the fildes argument. If the fildes argument has the special value
AT_FDCWD, a relative path argument will be resolved relative to the
current working directory. If the path argument is absolute, the
fildes argument is ignored.

The open() function returns 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 does
not share it with any other process in the system.

The file offset used to mark the current position within the file is
set to the beginning of the file.

The file status flags and file access modes of the open file
description are set according to the value of oflag. The mode argument
is used only when O_CREAT is specified (see below).

Values for oflag are constructed by a bitwise-inclusive-OR of flags
from the following list, defined in fcntl.h(3HEAD). Applications must
specify exactly one of the first three values (file access modes) below
in the value of oflag:

O_RDONLY
Open for reading only.

O_WRONLY
Open for writing only.

O_RDWR Open for reading and writing. The result is undefined if this
flag is applied to a FIFO.

Any combination of the following may be used:

O_APPEND
If set, the file offset is set to the end of the file prior to
each write.

O_CREAT
Create the file if it does not exist. This flag requires that
the mode argument be specified.

If the file exists, this flag has no effect except as noted
under O_EXCL below. Otherwise, the file is created with the
user ID of the file set to the effective user ID of the
process. The group ID of the file is set to the effective
group IDs of the process, or if the S_ISGID bit is set in the
directory in which the file is being created, the file's group
ID is set to the group ID of its parent directory. If the
group ID of the new file does not match the effective group ID
or one of the supplementary groups IDs, the S_ISGID bit is
cleared.

The access permission bits (see stat.h(3HEAD)) of the file mode
are set to the value of mode, modified as follows (see
creat(2)): a bitwise-AND is performed on the file-mode bits and
the corresponding bits in the complement of the process's file
mode creation mask. Thus, all bits set in the process's file
mode creation mask (see umask(2)) are correspondingly cleared
in the file's permission mask. The "save text image after
execution bit" of the mode is cleared (see chmod(2)). When
bits other than the file permission bits are set, the effect is
unspecified. The mode argument does not affect whether the
file is open for reading, writing or for both.

O_DIRECT
Indicates that the file data is not going to be reused in the
near future. When possible, data is read or written directly
between the application's memory and the device when the data
is accessed with read(2) and write(2) operations. See
directio(3C) for more details.

O_DIRECTORY
Indicates that attempts to open path should fail unless path is
a directory. If both O_CREAT and O_DIRECTORY are specified
then the call will fail if it would result in a file being
created. If a directory already exists at path then it will
behave as if the O_DIRECTORY flag had not been present. If the
O_EXCL and O_CREAT flags are specified, then the call will
always fail as they imply a file should always be created.

O_DSYNC
Write I/O operations on the file descriptor complete as defined
by synchronized I/O data integrity completion.

O_EXCL If O_CREAT and O_EXCL are set, open() fails if the file exists.
The check for the existence of the file and the creation of the
file if it does not exist is 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() fails and sets
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_EXEC If set, indicates that the file should be opened for execute
permission. This option is only valid for regular files; an
error will be returned if the target is not a regular file.

O_LARGEFILE
If set, the offset maximum in the open file description is the
largest value that can be represented correctly in an object of
type off64_t.

O_NOCTTY
If set and path identifies a terminal device, open() does not
cause the terminal device to become the controlling terminal
for the process.

O_NOFOLLOW
If the path names a symbolic link, open() fails and sets errno
to ELOOP.

O_NOLINKS
If the link count of the named file is greater than 1, open()
fails and sets errno to EMLINK.

O_CLOEXEC
If set, the file descriptor returned will be closed prior to
any future exec(2) calls. This sets the FD_CLOEXEC flag on the
file descriptor. If not performed at open time, this can later
be set with the F_SETFD fcntl(2) command.

O_CLOFORK
If set, the file descriptor returned will be closed in any
child processses created with the fork(2) family of functions.
The file descriptor will remain open in the parent. This sets
the FD_CLOFORK flag on the file descriptor. If not performed
at open time, this can later be set with the F_SETFD fcntl(2)
command.

O_NONBLOCK O_NDELAY
These flags can affect subsequent reads and writes (see read(2)
and write(2)). If both O_NDELAY and O_NONBLOCK are set,
O_NONBLOCK takes precedence.

When opening a FIFO with O_RDONLY or O_WRONLY set:

+o If O_NONBLOCK or O_NDELAY is set, an open() for reading
only returns without delay. An open() for writing only
returns an error if no process currently has the file open
for reading.

+o If O_NONBLOCK and O_NDELAY are clear, an open() for reading
only blocks until a thread opens the file for writing. An
open() for writing only blocks the calling thread until a
thread opens the file for reading.

After both ends of a FIFO have been opened once, there is no
guarantee that further calls to open() O_RDONLY (O_WRONLY) will
synchronize with later calls to open() O_WRONLY (O_RDONLY)
until both ends of the FIFO have been closed by all readers and
writers. Any data written into a FIFO will be lost if both
ends of the FIFO are closed before the data is read.

When opening a block special or character special file that
supports non-blocking opens:

+o If O_NONBLOCK or O_NDELAY is set, the open() function
returns without blocking for the device to be ready or
available. Subsequent behavior of the device is device-
specific.

+o If O_NONBLOCK and O_NDELAY are clear, the open() function
blocks the calling thread until the device is ready or
available before returning.

Otherwise, the behavior of O_NONBLOCK and O_NDELAY is
unspecified.

O_RSYNC
Read I/O operations on the file descriptor 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 complete as defined by
synchronized I/O data integrity completion. If both O_SYNC and
O_RSYNC are set in oflag, all I/O operations on the file
descriptor complete as defined by synchronized I/O file
integrity completion.

O_SEARCH
If set, indicates that the directory should be opened for
searching. This option is only valid for a directory; an error
will be returned if the target is not a directory.

O_SYNC Write I/O operations on the file descriptor complete as defined
by synchronized I/O file integrity completion (see
fcntl.h(3HEAD)) definition of O_SYNC.

O_TRUNC
If the file exists and is a regular file, and the file is
successfully opened O_RDWR or O_WRONLY, its length is truncated
to 0 and the mode and owner are unchanged. It has no effect on
FIFO special files or terminal device files. Its effect on
other file types is implementation-dependent. The result of
using O_TRUNC with O_RDONLY is undefined.

O_XATTR
If set in openat(), a relative path argument is interpreted as
a reference to an extended attribute of the file associated
with the supplied file descriptor. This flag therefore
requires the presence of a legal fildes argument. If set in
open(), the implied file descriptor is that for the current
working directory. Extended attributes must be referenced with
a relative path; providing an absolute path results in a normal
file reference.

If O_CREAT is set and the file did not previously exist, upon
successful completion, open() marks for update the st_atime, st_ctime,
and st_mtime fields of the file and the st_ctime and st_mtime fields of
the parent directory.

If O_TRUNC is set and the file did previously exist, upon successful
completion, open() marks for update the st_ctime and st_mtime fields 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 O_NODELAY OR-ed with either O_RDONLY, O_WRONLY, or
O_RDWR. Other flag values are not applicable to STREAMS devices and
have no effect on them. The values O_NONBLOCK and O_NODELAY affect the
operation of STREAMS drivers and certain functions (see read(2),
getmsg(2), putmsg(2), and write(2)) applied to file descriptors
associated with STREAMS files. For STREAMS drivers, the implementation
of O_NONBLOCK and O_NODELAY is device-specific.

When open() is invoked to open a named stream, and the connld(4M)
module has been pushed on the pipe, open() blocks until the server
process has issued an I_RECVFD ioctl(2) (see streamio(4I)) to receive
the file descriptor.

If path names the manager side of a pseudo-terminal device, then it is
unspecified whether open() locks the subsidiary side so that it cannot
be opened. Portable applications must call unlockpt(3C) before opening
the subsidiary side.

If the file is a regular file and the local file system is mounted with
the nbmand mount option, then a mandatory share reservation is
automatically obtained on the file. The share reservation is obtained
as if fcntl(2) were called with cmd F_SHARE_NBMAND and the fshare_t
values set as follows:

f_access
Set to the type of read/write access for which the file
is opened.

f_deny F_NODNY

f_id The file descriptor value returned from open().

If path is a symbolic link and O_CREAT and O_EXCL are set, the link is
not followed.

Certain flag values can be set following open() as described in
fcntl(2).

The largest value that can be represented correctly in an object of
type off_t is established as the offset maximum in the open file
description.

RETURN VALUES

The open() and openat() functions open the file and, if successful,
return a non-negative integer representing the lowest numbered unused
file descriptor; otherwise the value -1 is returned and the global
variable errno is set to indicate the error and no files are created or
modified.

EXAMPLES

Example 1 Open 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. 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 *filename = "/tmp/file";
...
fd = open(filename, O_WRONLY | O_CREAT | O_TRUNC, mode);
...

Example 2 Open 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 application assumes that someone else is updating the
password file and exits.

#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <err.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)) < 0) {
err(1, "Cannot open %s. Try again later.", LOCKFILE);
}
...

Example 3 Open 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>
#include <err.h>
...
int pfd;
char filename[PATH_MAX+1];
...
if ((pfd = open(filename, O_WRONLY | O_CREAT | O_TRUNC,
S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)) < 0) {
err(1, "Cannot open output file");
}
...

ERRORS

The open() and openat() functions will fail if:

EACCES Search permission is denied on a component of the
path prefix.

The file exists and the permissions specified by
oflag are denied.

The file does not exist and write permission is
denied for the parent directory of the file to be
created.

O_TRUNC is specified and write permission is denied.

The {PRIV_FILE_DAC_SEARCH} privilege allows
processes to search directories regardless of
permission bits. The {PRIV_FILE_DAC_WRITE}
privilege allows processes to open files for writing
regardless of permission bits. See privileges(7)
for special considerations when opening files owned
by user ID 0 for writing. The {PRIV_FILE_DAC_READ}
privilege allows processes to open files for reading
regardless of permission bits.

EAGAIN A mandatory share reservation could not be obtained
because the desired access conflicts with an
existing f_deny share reservation (see fcntl(2)).

EDQUOT The file does not exist, O_CREAT is specified, and
either the directory where the new file entry is
being placed cannot be extended because the user's
quota of disk blocks on that file system has been
exhausted, or the user's quota of inodes on the file
system where the file is being created has been
exhausted.

EEXIST The O_CREAT and O_EXCL flags are set and the named
file already exists.

EILSEQ The path argument includes bytes that are not valid
UTF-8 characters, and the file system accepts only
file names where all characters are part of the
UTF-8 character codeset.

EINTR A signal was caught during open().

EFAULT The path argument points to an illegal address.

EINVAL Either the system does not support synchronized or
direct I/O for this file, or the O_XATTR flag was
supplied and the underlying file system does not
support extended file attributes.

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 Too many symbolic links were encountered in
resolving path.

A loop exists in symbolic links encountered during
resolution of the path argument.

The O_NOFOLLOW flag is set and the final component
of path is a symbolic link.

EMFILE There are currently {OPEN_MAX} file descriptors open
in the calling process.

EMLINK The O_NOLINKS flag is set and the named file has a
link count greater than 1.

EMULTIHOP Components of path require hopping to multiple
remote machines and the file system does not allow
it.

ENAMETOOLONG The length of the path argument exceeds {PATH_MAX}
or a pathname component is longer than {NAME_MAX}.

ENFILE The maximum allowable number of files is currently
open in the system.

ENOENT The O_CREAT flag is not set and the named file does
not exist; or the O_CREAT flag is set and either the
path prefix does not exist or the path argument
points to an empty string.

The O_CREAT and O_DIRECTORY flags were both set and
path did not point to a file.

ENOEXEC The O_EXEC flag is set and path does not point to a
regular file.

ENOLINK The path argument points to a remote machine, and
the link to that machine is no longer active.

ENOSR Th 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.

ENOSYS The device specified by path does not support the
open operation.

ENOTDIR A component of the path prefix is not a directory or
a relative path was supplied to openat(), the
O_XATTR flag was not supplied, and the file
descriptor does not refer to a directory. The
O_SEARCH flag was passed and path does not refer to
a directory.

The O_DIRECTORY flag was set and the file was not a
directory.

ENXIO The O_NONBLOCK flag is set, the named file is a
FIFO, the O_WRONLY flag is set, and no process has
the file open for reading; or the named file is a
character special or block special file and the
device associated with this special file does not
exist or has been retired by the fault management
framework.

EOPNOTSUPP An attempt was made to open a path that corresponds
to an AF_UNIX socket.

EOVERFLOW The named file is a regular file and either
O_LARGEFILE is not set and the size of the file
cannot be represented correctly in an object of type
off_t or O_LARGEFILE is set and the size of the file
cannot be represented correctly in an object of type
off64_t.

EROFS The named file resides on a read-only file system
and either O_WRONLY, O_RDWR, O_CREAT (if file does
not exist), or O_TRUNC is set in the oflag argument.

The openat() function will fail if:

EBADF The fildes argument is not a valid open file
descriptor or is not AT_FTCWD.

The open() function may fail if:

EAGAIN The path argument names the subsidiary side of a
pseudo-terminal device that is locked.

EINVAL The value of the oflag argument is not valid.

ENAMETOOLONG Pathname resolution of a symbolic link produced an
intermediate result whose length 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.

USAGE

The open() function has a transitional interface for 64-bit file
offsets. See lf64(7). Note that using open64() is equivalent to using
open(with) O_LARGEFILE set in oflag.

INTERFACE STABILITY

Committed

MT LEVEL

Async-Signal-Safe

SEE ALSO

chmod(2), close(2), creat(2), dup(2), exec(2), fcntl(2), getmsg(2),
getrlimit(2), Intro(2), lseek(2), putmsg(2), read(2), stat(2),
umask(2), write(2), attropen(3C), directio(3C), unlockpt(3C),
fcntl.h(3HEAD), stat.h(3HEAD), streamio(4I), connld(4M), attributes(7),
lf64(7), privileges(7), standards(7)

NOTES

Hierarchical Storage Management (HSM) file systems can sometimes cause
long delays when opening a file, since HSM files must be recalled from
secondary storage.

illumos February 5, 2024 illumos