OPEN(2) BSD System Calls Manual OPEN(2)
open, openat -- open or create a file for reading, writing or executing
Standard C Library (libc, -lc)
open(const char *path, int flags, ...);
openat(int fd, const char *path, int flags, ...);
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 specify-
ing 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. In this case 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().
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_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
O_SYNC synchronous writes
O_NOFOLLOW do not follow symlinks
O_NOCTTY don't assign controlling terminal
O_TTY_INIT restore default terminal attributes
O_DIRECTORY error if file is not a directory
O_CLOEXEC set FD_CLOEXEC upon open
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() sys-
tem call would result in the process being blocked for some reason (e.g., waiting for car-
rier on a dialup line), open() returns immediately. The descriptor remains in non-blocking
mode for subsequent operations.
If O_FSYNC is used in the mask, all writes will immediately be written to disk, the kernel
will not cache written data and all writes on the descriptor will not return until the data
to be written completes.
O_SYNC is a synonym for O_FSYNC required by POSIX.
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
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 dras-
tically 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 com-
patibility. 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.
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.
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.
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
[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 mod-
[EROFS] The named file resides on a read-only file system, and the file is to be
[EROFS] O_CREAT is specified and the named file would reside on a read-only file
[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
[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
[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
[EIO] An I/O error occurred while making the directory entry or allocating the
inode for O_CREAT.
[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, O_RDWR and O_EXEC.
[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.
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)
The open() function appeared in Version 6 AT&T UNIX. The openat() function was introduced
in FreeBSD 8.0.
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 direc-
tory currently permits searches. The present implementation of the openat checks the cur-
rent permissions of directory instead.
BSD February 7, 2013 BSD