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 FD_CLOEXEC file descriptor
flag associated with the new file descriptor is cleared.

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.

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, 2022 illumos