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. For openat() and relative
path, 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.
In capsicum(4) capability mode, open() is not permitted. The path
argument to openat() must be strictly relative to a file descriptor fd.
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 specified 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
O_NONBLOCK do not block on open
O_APPEND append on each write
O_CREAT create file if it does not exist
O_TRUNC truncate size to 0
O_EXCL error if create and file exists
O_SHLOCK atomically obtain a shared lock
O_EXLOCK atomically obtain an exclusive lock
O_DIRECT eliminate or reduce cache effects
O_FSYNC synchronous writes (historical synonym for O_SYNC)
O_SYNC synchronous 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 set FD_CLOEXEC upon open
O_VERIFY verify the contents of the file
O_RESOLVE_BENEATH path resolution must not cross the fd directory
O_PATH record only the target path in the opened descriptor
O_EMPTY_PATH openat, open file referenced by fd if path is empty
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 and the open() system call would result in
the process being blocked for some reason (e.g., 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.
If creating a file with O_CREAT, the request for the lock will never fail
(provided that the underlying file system supports locking).
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.
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_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 fd is opened with O_SEARCH, execute permissions are checked at open
time. The fd may not be used for any read operations like
getdirentries(2). The primary use for this descriptor will be as the
lookup descriptor for the *at() family of functions.
O_PATH returns a file descriptor that can be used as a directory file
descriptor for openat(2) and other system calls taking a file descriptor
argument, like fstatat(2) and others. The other functionality of the
returned file descriptor is limited to the descriptor-level operations.
It can be used for
fcntl(2) but advisory locking is not allowed
dup(2)
close(2)
fstat(2)
fexecve(2)
SCM_RIGHTS can be passed over a unix(4) socket using a
SCM_RIGHTS message
kqueue(2) using for EVFILT_VNODE
readlinkat(2)
But 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 into normal
(operable) file descriptor by specifying it as the fd argument to
openat() with empty path and flag O_EMPTY_PATH. Such an open behaves as
if the current path of the file referenced by fd is passed, except that
the path walk permissions are not checked. See also the description of
AT_EMPTY_PATH flag for fstatat(2) and related syscalls.
If successful, open() returns a non-negative integer, termed a file
descriptor. It returns -1 on failure. 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(5) 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 given 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 O_CLOEXEC
description.
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.
[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 (e.g.
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] The O_RESOLVE_BENEATH flag is specified and path is
absolute.
[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, or contained a ".."
component leading to a directory outside of the
directory hierarchy specified by fd, and the process
is in capability mode.
[ENOTCAPABLE] The O_RESOLVE_BENEATH flag was provided, and the
relative path escapes the fd directory.
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)
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.
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.
BUGS
The Open Group Extended API Set 2 specification requires 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 checks the current
permissions of directory instead.
The mode argument is variadic and may result in different calling
conventions than might otherwise be expected.
FreeBSD 13.1-RELEASE-p6 April 22, 2022 FreeBSD 13.1-RELEASE-p6
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