Updated: 2022/Sep/29
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OPEN(2) System Calls Manual OPEN(2)
NAME
open, openat - open or create a file for reading, writing or executing
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <fcntl.h>
int
open(const char *path, int flags, ...);
int
openat(int fd, const char *path, int flags, ...);
DESCRIPTION
The file name specified by path is opened for either execution or reading
and/or writing as specified by the argument flags and the file descriptor
returned to the calling process. The flags argument may indicate the
file is to be created if it does not exist (by specifying the O_CREAT
flag). In this case open() and openat() require an additional argument
mode_t mode, and the file is created with mode mode as described in
chmod(2) and modified by the process' umask value (see umask(2)).
The openat() function is equivalent to the open() function except in the
case where the path is relative. In that case, it is looked up from a
directory whose file descriptor was passed as fd. Search permission is
required on this directory. fd can be set to AT_FDCWD in order to
specify the current directory.
The flags are specified by or'ing the values listed below. Applications
must specify exactly one of these four values (file access methods):
O_RDONLY Open for reading only.
O_WRONLY Open for writing only.
O_EXEC Open for execute only.
O_RDWR Open for reading and writing.
Any combination of the following may be used as well:
O_NONBLOCK Do not block on open or for data to become available.
O_APPEND Append to the file on each write.
O_CREAT Create the file if it does not exist. The third
argument of type mode_t is used to compute the mode
bits of the file as described in chmod(2) and modified
by the process' umask value (see umask(2)).
O_TRUNC Truncate size to 0.
O_EXCL Error if O_CREAT and the file already exists.
O_SHLOCK Atomically obtain a shared lock.
O_EXLOCK Atomically obtain an exclusive lock.
O_NOFOLLOW If last path element is a symlink, don't follow it.
This option is provided for compatibility with other
operating systems, but its security value is
questionable.
O_CLOEXEC Set the close(2) on exec(3) flag.
O_NOSIGPIPE Return EPIPE instead of raising SIGPIPE.
O_DSYNC If set, write operations will be performed according
to synchronized I/O data integrity completion: each
write will wait for the file data to be committed to
stable storage.
O_SYNC If set, write operations will be performed according
to synchronized I/O file integrity completion: each
write will wait for both the file data and file status
to be committed to stable storage.
O_RSYNC If set, read operations will complete at the same
level of integrity which is in effect for write
operations: if specified together with O_SYNC, each
read will wait for the file status to be committed to
stable storage.
Combining O_RSYNC with O_DSYNC only, or specifying it
without any other synchronized I/O integrity
completion flag set, has no further effect.
O_ALT_IO Alternate I/O semantics will be used for read and
write operations on the file descriptor. Alternate
semantics are defined by the underlying layers and
will not have any alternate effect in most cases.
O_NOCTTY If the file is a terminal device, the opened device is
not made the controlling terminal for the session.
This flag has no effect on NetBSD, since the system
defaults to the abovementioned behaviour. The flag is
present only for standards conformance.
O_DIRECT If set on a regular file, data I/O operations will not
buffer the data being transferred in the kernel's
cache, but rather transfer the data directly between
user memory and the underlying device driver if
possible. This flag is advisory; the request may be
performed in the normal buffered fashion if certain
conditions are not met, e.g. if the request is not
sufficiently aligned or if the file is mapped.
To meet the alignment requirements for direct I/O, the
file offset, the length of the I/O and the address of
the buffer in memory must all be multiples of
DEV_BSIZE (512 bytes). If the I/O request is made
using an interface that supports scatter/gather via
struct iovec, each element of the request must meet
the above alignment constraints.
O_DIRECTORY Fail if the file is not a directory.
O_REGULAR Fail if the path does not refer to a regular file.
O_ASYNC Enable the SIGIO signal to be sent to the process
group when I/O is possible, e.g., upon availability of
data to be read.
Note: This is broken in open(); it must be set
explicitly with the F_SETFL command to fcntl(2).
Opening a file with O_APPEND set causes each write on the file to be
appended to the end. If O_TRUNC is specified and the file exists, the
file is truncated to zero length.
If O_EXCL is set with O_CREAT and the file already exists, open() returns
an error. This may be used to implement a simple exclusive access
locking mechanism. If O_EXCL is set and the last component of the
pathname is a symbolic link, open() will fail even if the symbolic link
points to a non-existent name.
If the O_NONBLOCK flag is specified, do not wait for the device or file
to be ready or available. If the open() call would result in the process
being blocked for some reason (e.g., waiting for carrier on a dialup
line), open() returns immediately. This flag also has the effect of
making all subsequent I/O on the open file non-blocking.
When opening a file, a lock with flock(2) semantics can be obtained by
setting O_SHLOCK for a shared lock, or O_EXLOCK for an exclusive lock.
If creating a file with O_CREAT, the request for the lock will never fail
(provided that the underlying file system supports locking).
If open() is successful, the file pointer used to mark the current
position within the file is set to the beginning of the file.
When a new file is created it is given the group of the directory which
contains it.
The new descriptor is set to remain open across execve(2) system calls;
see close(2) and fcntl(2).
The system imposes a limit on the number of file descriptors open
simultaneously by one process. Calling getdtablesize(3) returns the
current system limit.
RETURN VALUES
If successful, open() and openat() returns a non-negative integer, termed
a file descriptor. Otherwise, a value of -1 is returned and errno is set
to indicate the error.
ERRORS
The named file is opened unless:
[EACCES] Search permission is denied for a component of the
path prefix, the required permissions (for reading
and/or writing) are denied for the given flags, or
O_CREAT is specified, the file does not exist, and the
directory in which it is to be created does not permit
writing.
[EDQUOT] O_CREAT is specified, the file does not exist, and the
directory in which the entry for the new file is being
placed cannot be extended because the user's quota of
disk blocks on the file system containing the
directory has been exhausted; or O_CREAT is specified,
the file does not exist, and the user's quota of
inodes on the file system on which the file is being
created has been exhausted.
[EEXIST] O_CREAT and O_EXCL were specified and the file exists.
[EFAULT] path points outside the process's allocated address
space.
[EFTYPE] O_NOFOLLOW was specified, but the last path component
is a symlink. Note: IEEE Std 1003.1-2008 ("POSIX.1")
specifies returning [ELOOP] for this case.
[EFTYPE] O_REGULAR is specified and the last path component is
not a regular file.
[EINTR] The open() operation was interrupted by a signal.
[EIO] An I/O error occurred while making the directory entry
or allocating the inode for O_CREAT.
[EISDIR] The named file is a directory, and the arguments
specify it is to be opened for writing.
[ELOOP] Too many symbolic links were encountered in
translating the pathname.
[EMFILE] The process has already reached its limit for open
file descriptors.
[ENAMETOOLONG] A component of a pathname exceeded {NAME_MAX}
characters, or an entire path name exceeded {PATH_MAX}
characters.
[ENFILE] The system file table is full.
[ENOENT] O_CREAT is not set and the named file does not exist,
or a component of the path name that must exist does
not exist.
[ENOSPC] O_CREAT is specified, the file does not exist, and the
directory in which the entry for the new file is being
placed cannot be extended because there is no space
left on the file system containing the directory; or
O_CREAT is specified, the file does not exist, and
there are no free inodes on the file system on which
the file is being created.
[ENOTDIR] A component of the path prefix is not a directory; or
O_DIRECTORY is specified and the last path component
is not a directory.
[ENXIO] The named file is a character special or block special
file, and the device associated with this special file
does not exist, or the named file is a FIFO,
O_NONBLOCK and O_WRONLY is set and no process has the
file open for reading.
[EOPNOTSUPP] O_SHLOCK or O_EXLOCK is specified but the underlying
file system does not support locking; or an attempt
was made to open a socket (not currently implemented).
[EPERM] The file's flags (see chflags(2)) don't allow the file
to be opened.
[EROFS] The named file resides on a read-only file system, and
the file is to be modified.
[ETXTBSY] The file is a pure procedure (shared text) file that
is being executed and the open() call requests write
access.
In addition, openat() will fail if:
[EBADF] path does not specify an absolute path and fd is
neither AT_FDCWD nor a valid file descriptor open for
reading or searching.
[EINVAL] An attempt was made to open a descriptor with an
illegal combination of O_RDONLY, O_WRONLY, O_RDWR, and
O_EXEC.
[ENOTDIR] path is not an absolute path and fd is a file
descriptor associated with a non-directory file.
SEE ALSO
chmod(2), close(2), dup(2), faccessat(2), fchmodat(2), fchownat(2),
fstatat(2), linkat(2), lseek(2), mkdirat(2), mkfifoat(2), mknodat(2),
read(2), readlinkat(2), symlinkat(2), umask(2), unlinkat(2),
utimensat(2), write(2), getdtablesize(3)
STANDARDS
The open() function conforms to IEEE Std 1003.1-1990 ("POSIX.1").
openat() conforms to IEEE Std 1003.1-2008 ("POSIX.1").
The flags values O_DSYNC, O_SYNC and O_RSYNC are extensions defined in
IEEE Std 1003.1b-1993 ("POSIX.1b").
The O_SHLOCK and O_EXLOCK flags are non-standard extensions and should
not be used if portability is of concern.
HISTORY
An open() function call appeared in Version 1 AT&T UNIX.
BUGS
O_ASYNC doesn't actually work as advertised with open; you must set
O_ASYNC explicitly with the F_SETFL command to fcntl(2).
NetBSD 10.99 March 5, 2023 NetBSD 10.99