OPEN(2)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 descrip‐
tion 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>. Applications must spec‐
ify 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
<sys/stat.h>) of the file mode are set to the value of mode, modi‐
fied 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 correspond‐
ingly 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_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_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() calls.
O_NONBLOCK or 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, 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 sup‐
ports 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() func‐
tion 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_SYNC
Write I/O operations on the file descriptor complete as defined by
synchronized I/O file integrity completion (see fcntl.h(3HEAD) def‐
inition of O_SYNC).
O_TRUNC
If the file exists and is a regular file, and the file is success‐
fully 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 pres‐
ence 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 success‐
ful 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_NON‐
BLOCK 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 opera‐
tion 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 module
(see connld(7M)) has been pushed on the pipe, open() blocks until the
server process has issued an I_RECVFD ioctl() (see streamio(7I)) to
receive the file descriptor.
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. Portable applications must call unlockpt(3C) before opening
the slave 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 automat‐
ically 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
Upon successful completion, the open() function opens the file and
return a non-negative integer representing the lowest numbered unused
file descriptor. Otherwise, −1 is returned, errno is set to indicate
the error, and no files are created or modified.
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(5) for special considerations when open‐
ing files owned by UID 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.
EBADF
The file descriptor provided to openat() is invalid.
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
exists.
EILSEQ
The path argument includes non-UTF8 characters and the
file system accepts only file names where all charac‐
ters 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
The system does not support synchronized I/O for this
file, or the O_XATTR flag was supplied and the underly‐
ing 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 res‐
olution 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.
ENOLINK
The path argument points to a remote machine, and the
link to that machine is no longer active.
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.
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.
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 spe‐
cial 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
a 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 repre‐
sented correctly in an object of type off_t or O_LARGE‐
FILE is set and the size of the file cannot be repre‐
sented 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 slave side of a pseudo-ter‐
minal 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.
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 speci‐
fies 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>
#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);
}
...
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>
#define LOCKFILE "/etc/ptmp"
...
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)) == -1)
{
perror("Cannot open output file\n"); exit(1);
}
...
USAGE
The open() function has a transitional interface for 64-bit file off‐
sets. See lf64(5). Note that using open64() is equivalent to using
open() with O_LARGEFILE set in oflag.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌────────────────────┬───────────────────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├────────────────────┼───────────────────────────────┤
│Interface Stability │ Committed │
├────────────────────┼───────────────────────────────┤
│MT-Level │ Async-Signal-Safe │
├────────────────────┼───────────────────────────────┤
│Standard │ For open(), see standards(5). │
└────────────────────┴───────────────────────────────┘
SEE ALSOIntro(2), chmod(2), close(2), creat(2), dup(2), exec(2), fcntl(2),
getmsg(2), getrlimit(2), lseek(2), putmsg(2), read(2), stat(2),
umask(2), write(2), attropen(3C), fcntl.h(3HEAD), stat.h(3HEAD),
unlockpt(3C), attributes(5), lf64(5), privileges(5), standards(5),
connld(7M), streamio(7I)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.
Nov 16, 2013 OPEN(2)
