OPEN(2) FreeBSD 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 specifies a relative path or the O_EMPTY_PATH flag is
specified. For openat() and relative path, when fd references a
directory, and the O_EMPTY_PATH flag is not specified, the file to be
opened is determined relative to the directory associated with the file
descriptor fd instead of the current working directory. The flag
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 is used and the
behavior is identical to a call to open().
When openat() is called with an absolute path, it ignores the fd
argument.
When openat() is called with the fd argument that does not reference a
directory, the call fails unless O_EMPTY_PATH flag is specified, see
below.
In capsicum(4) capability mode, open() is not permitted. The path
argument to openat() must be strictly relative to a file descriptor fd;
that is, path must not be an absolute path and must not contain ".."
components which cause the path resolution to escape the directory
hierarchy starting at fd. Additionally, no symbolic link in path may
target absolute path or contain escaping ".." components. fd must not be
AT_FDCWD.
If the vfs.lookup_cap_dotdot sysctl(3) MIB is set to zero, ".."
components in the paths, used in capability mode, are completely
disabled. If the vfs.lookup_cap_dotdot_nonlocal MIB is set to zero, ".."
is not allowed if found on non-local filesystem.
The flags are formed by or'ing the following values:
O_RDONLY open for reading only
O_WRONLY open for writing only
O_RDWR open for reading and writing
O_EXEC open for execute only
O_SEARCH open for search only (an alias for O_EXEC typically
used with O_DIRECTORY)
O_NONBLOCK do not block on open
O_APPEND set file pointer to the end of the file before each
write
O_CREAT create file if it does not exist
O_TRUNC truncate size to 0
O_EXCL fail if O_CREAT is set and the file exists
O_SHLOCK atomically obtain a shared lock
O_EXLOCK atomically obtain an exclusive lock
O_DIRECT read and write directly from the backing store
O_FSYNC synchronous data and metadata writes (historical
synonym for O_SYNC)
O_SYNC synchronous data and metadata writes
O_DSYNC synchronous data writes
O_NOFOLLOW do not follow symlinks
O_NOCTTY ignored
O_TTY_INIT ignored
O_DIRECTORY error if file is not a directory
O_CLOEXEC automatically close file on execve(2)
O_CLOFORK automatically close file on any child process created
with fork(2)
O_VERIFY verify the contents of the file with mac_veriexec(4)
O_RESOLVE_BENEATH (openat(2) only) path resolution must not cross the fd
directory
O_PATH record only the target path in the opened descriptor
O_EMPTY_PATH (openat(2) only) open file referenced by fd if path is
empty
O_NAMEDATTR open a named attribute or named attribute directory
Exactly one of the flags O_RDONLY, O_WRONLY, O_RDWR, or O_EXEC must be
provided.
Opening a file with O_APPEND set causes each write on the resulting file
descriptor to be appended to the end of the file.
If O_TRUNC is specified and the file exists, the file is truncated to
zero length.
If O_CREAT is set, but file already exists, this flag has no effect
except when O_EXCL is set too, in this case open() fails with EEXIST.
This may be used to implement a simple exclusive access locking
mechanism. In all other cases, the file is created and the access
permission bits (see chmod(2)) of the file mode are set to the value of
the third argument taken as mode_t mode and passed through the umask(2).
This argument does not affect whether the file is opened for reading,
writing, or for both. The open' request for a lock on the file, created
with O_CREAT, will never fail provided that the underlying file system
supports locking; see also O_SHLOCK and O_EXLOCK below.
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 O_NONBLOCK is specified and the open() system call would block for
some reason (for example, waiting for carrier on a dialup line), open()
returns immediately. The descriptor remains in non-blocking mode for
subsequent operations.
If O_SYNC is used in the mask, all writes will immediately and
synchronously be written to disk. O_FSYNC is an historical synonym for
O_SYNC.
If O_DSYNC is used in the mask, all data and metadata required to read
the data will be synchronously written to disk, but changes to metadata
such as file access and modification timestamps may be written later.
If O_NOFOLLOW is used in the mask and the target file passed to open() is
a symbolic link then the open() will fail.
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.
O_DIRECT may be used to minimize or eliminate the cache effects of
reading and writing. The system will attempt to avoid caching the data
you read or write. If it cannot avoid caching the data, it will minimize
the impact the data has on the cache. Use of this flag can drastically
reduce performance if not used with care. The semantics of this flag are
filesystem dependent, and some filesystems may ignore it entirely.
O_NOCTTY may be used to ensure the OS does not assign this file as the
controlling terminal when it opens a tty device. This is the default on
FreeBSD, but is present for POSIX compatibility. The open() system call
will not assign controlling terminals on FreeBSD.
O_TTY_INIT may be used to ensure the OS restores the terminal attributes
when initially opening a TTY. This is the default on FreeBSD, but is
present for POSIX compatibility. The initial call to open() on a TTY
will always restore default terminal attributes on FreeBSD.
O_DIRECTORY may be used to ensure the resulting file descriptor refers to
a directory. This flag can be used to prevent applications with elevated
privileges from opening files which are even unsafe to open with
O_RDONLY, such as device nodes.
O_CLOEXEC may be used to set FD_CLOEXEC flag for the newly returned file
descriptor.
O_CLOFORK may be used to set FD_CLOFORK flag for the newly returned file
descriptor. The file will be closed on any child process created with
fork(2), vfork(2) or rfork(2) with the RFFDG flag, remaining open in the
parent. Both the O_CLOEXEC and O_CLOFORK flags can be modified with the
F_SETFD fcntl(2) command.
O_VERIFY may be used to indicate to the kernel that the contents of the
file should be verified before allowing the open to proceed. The details
of what "verified" means is implementation specific. The run-time linker
(rtld) uses this flag to ensure shared objects have been verified before
operating on them.
O_RESOLVE_BENEATH returns ENOTCAPABLE if any intermediate component of
the specified relative path does not reside in the directory hierarchy
beneath the starting directory. Absolute paths or even the temporal
escape from beneath of the starting directory is not allowed.
When a directory is opened with O_SEARCH, execute permissions are checked
at open time. The returned file descriptor may not be used for any read
operations like getdirentries(2). The primary use of this descriptor is
as the lookup descriptor for the *at() family of functions. If O_SEARCH
was not requested at open time, then the *at() functions use the current
directory permissions for the directory referenced by the descriptor at
the time of the *at() call.
O_PATH returns a file descriptor that can be used as the first argument
for openat() and other filesystem-related system calls collectively named
*at() taking a file descriptor argument, like fstatat(2) and others. The
other functionality of the returned file descriptor is limited to the
following descriptor-level operations:
connectat(2) for unix domain socket (see unix(4))
fcntl(2) but advisory locking is not allowed
dup(2)
close(2)
fstat(2)
fstatfs(2)
fchdir(2)
fchroot(2)
fexecve(2)
funlinkat(2) can be passed as the third argument
SCM_RIGHTS can be passed over a unix(4) socket using a
SCM_RIGHTS message
kqueue(2) only with EVFILT_VNODE
__acl_get_fd(2)
__acl_aclcheck_fd(2)
extattr(2)
capsicum(4) can be passed to cap_*_limit() and cap_*_get()
system calls (such as cap_rights_limit(2)).
Other operations like read(2), ftruncate(2), and any other that operate
on file and not on file descriptor (except fstat(2)), are not allowed.
A file descriptor created with the O_PATH flag can be opened as a normal
(operable) file descriptor by specifying it as the fd argument to
openat() with an empty path and the O_EMPTY_PATH flag. Such an open
behaves as if the current path of the file referenced by fd is passed,
except that path walk permissions are not checked. See also the
description of AT_EMPTY_PATH flag for fstatat(2) and related syscalls.
Conversely, a file descriptor fd referencing a filesystem file can be
converted to the O_PATH type of descriptor by using the following call
opath_fd = openat(fd, "", O_EMPTY_PATH | O_PATH);
If successful, open() returns a non-negative integer, termed a file
descriptor. It returns -1 on failure. The file descriptor value
returned is the lowest numbered descriptor currently not in use by the
process. The file pointer used to mark the current position within the
file is set to the beginning of the file.
If a sleeping open of a device node from devfs(4) is interrupted by a
signal, the call always fails with EINTR, even if the SA_RESTART flag is
set for the signal. A sleeping open of a fifo (see mkfifo(2)) is
restarted as normal.
When a new file is created, it is assigned the group of the directory
which contains it.
Unless O_CLOEXEC flag was specified, the new descriptor is set to remain
open across execve(2) system calls; see close(2), fcntl(2) and the
description of the O_CLOEXEC flag.
When the O_NAMEDATTR flag is specified for an openat() where the fd
argument is for a file object, a named attribute for the file object is
opened and not the file object itself. If the O_CREAT flag has been
specified as well, the named attribute will be created if it does not
exist. When the O_NAMEDATTR flag is specified for a open(), a named
attribute for the current working directory is opened and not the current
working directory. The path argument for this openat() or open() must be
a single component name with no embedded `/'. If the path argument is
`.' then the named attribute directory for the file object is opened.
(See named_attribute(7) for more information.)
The system imposes a limit on the number of file descriptors open
simultaneously by one process. The getdtablesize(2) system call returns
the current system limit.
RETURN VALUES
If successful, open() and openat() return a non-negative integer, termed
a file descriptor. They return -1 on failure, and set errno to indicate
the error.
ERRORS
The named file is opened unless:
[ENOTDIR] A component of the path prefix is not a directory.
[ENAMETOOLONG] A component of a pathname exceeded 255 characters, or
an entire path name exceeded 1023 characters.
[ENOENT] O_CREAT is not set and the named file does not exist.
[ENOENT] A component of the path name that must exist does not
exist.
[EACCES] Search permission is denied for a component of the
path prefix.
[EACCES] The required permissions (for reading and/or writing)
are denied for the given flags.
[EACCES] O_TRUNC is specified and write permission is denied.
[EACCES] O_CREAT is specified, the file does not exist, and the
directory in which it is to be created does not permit
writing.
[EPERM] O_CREAT is specified, the file does not exist, and the
directory in which it is to be created has its
immutable flag set, see the chflags(2) manual page for
more information.
[EPERM] The named file has its immutable flag set and the file
is to be modified.
[EPERM] The named file has its append-only flag set, the file
is to be modified, and O_TRUNC is specified or
O_APPEND is not specified.
[ELOOP] Too many symbolic links were encountered in
translating the pathname.
[EISDIR] The named file is a directory, and the arguments
specify it is to be modified.
[EISDIR] The named file is a directory, and the flags specified
O_CREAT without O_DIRECTORY.
[EROFS] The named file resides on a read-only file system, and
the file is to be modified.
[EROFS] O_CREAT is specified and the named file would reside
on a read-only file system.
[EMFILE] The process has already reached its limit for open
file descriptors.
[ENFILE] The system file table is full.
[EMLINK] O_NOFOLLOW was specified and the target is a symbolic
link. POSIX specifies a different error for this
case; see the note in STANDARDS below.
[ENXIO] The named file is a character special or block special
file, and the device associated with this special file
does not exist.
[ENXIO] O_NONBLOCK is set, the named file is a fifo, O_WRONLY
is set, and no process has the file open for reading.
[EINTR] The open() operation was interrupted by a signal.
[EOPNOTSUPP] O_SHLOCK or O_EXLOCK is specified but the underlying
file system does not support locking.
[EOPNOTSUPP] The named file is a special file mounted through a
file system that does not support access to it (for
example, NFS).
[EWOULDBLOCK] O_NONBLOCK and one of O_SHLOCK or O_EXLOCK is
specified and the file is locked.
[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.
[ENOSPC] 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.
[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.
[EDQUOT] 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.
[EIO] An I/O error occurred while making the directory entry
or allocating the inode for O_CREAT.
[EINTEGRITY] Corrupted data was detected while reading from the
file system.
[ETXTBSY] The file is a pure procedure (shared text) file that
is being executed and the open() system call requests
write access.
[EFAULT] The path argument points outside the process's
allocated address space.
[EEXIST] O_CREAT and O_EXCL were specified and the file exists.
[EOPNOTSUPP] An attempt was made to open a socket (not currently
implemented).
[EINVAL] An attempt was made to open a descriptor with an
illegal combination of O_RDONLY, O_WRONLY, or O_RDWR,
and O_EXEC or O_SEARCH.
[EINVAL] O_CREAT is specified, and the last component of the
path argument is invalid on the file system on which
the file is being created.
[EBADF] The path argument does not specify an absolute path
and the fd argument is neither AT_FDCWD nor a valid
file descriptor open for searching.
[ENOTDIR] The path argument is not an absolute path and fd is
neither AT_FDCWD nor a file descriptor associated with
a directory.
[ENOTDIR] O_DIRECTORY is specified and the file is not a
directory.
[ECAPMODE] AT_FDCWD is specified and the process is in capability
mode.
[ECAPMODE] open() was called and the process is in capability
mode.
[ENOTCAPABLE] path is an absolute path and the process is in
capability mode.
[ENOTCAPABLE] path is an absolute path and O_RESOLVE_BENEATH is
specified.
[ENOTCAPABLE] path contains a ".." component leading to a directory
outside of the directory hierarchy specified by fd and
the process is in capability mode.
[ENOTCAPABLE] path contains a ".." component leading to a directory
outside of the directory hierarchy specified by fd and
O_RESOLVE_BENEATH is specified.
[ENOTCAPABLE] path contains a ".." component, the
vfs.lookup_cap_dotdot sysctl(3) is set, and the
process is in capability mode.
[ENOATTR] O_NAMEDATTR has been specified and the file object is
not a named attribute directory or named attribute.
SEE ALSO
chmod(2), close(2), dup(2), fexecve(2), fhopen(2), getdtablesize(2),
getfh(2), lgetfh(2), lseek(2), read(2), umask(2), write(2), fopen(3),
capsicum(4), unix(4), named_attribute(7)
STANDARDS
These functions are specified by IEEE Std 1003.1-2008 ("POSIX.1").
FreeBSD sets errno to EMLINK instead of ELOOP as specified by POSIX when
O_NOFOLLOW is set in flags and the final component of pathname is a
symbolic link to distinguish it from the case of too many symbolic link
traversals in one of its non-final components.
The Open Group Extended API Set 2 specification, that introduced the
*at() API, required that the test for whether fd is searchable is based
on whether fd is open for searching, not whether the underlying directory
currently permits searches. The present implementation of the openat
system call is believed to be compatible with IEEE Std 1003.1-2008, 2017
Edition ("POSIX.1"), which specifies that behavior for O_SEARCH, in the
absence of the flag the implementation checks the current permissions of
a directory.
HISTORY
The open() function appeared in Version 1 AT&T UNIX. The openat()
function was introduced in FreeBSD 8.0. O_DSYNC appeared in 13.0.
O_NAMEDATTR appeared in 15.0. O_CLOFORK appeared in FreeBSD 15.0.
BUGS
The mode argument is variadic and may result in different calling
conventions than might otherwise be expected.
FreeBSD 15.1-STABLE-HBSD May 17, 2025 OPEN(2)