OPEN(2) System Calls Manual OPEN(2)

open, openat — open or create a file for reading or writing

#include <fcntl.h>

int
open(const char *path, int oflag, ...);

int
openat(int fd, const char *path, int oflag, ...);

The file name specified by path is opened for reading and/or writing, as specified by the argument oflag; the file descriptor is returned to the calling process.

The oflag argument may indicate that the file is to be created if it does not exist (by specifying the O_CREAT flag). In this case, () and openat() require an additional argument mode_t mode; the file is created with mode mode as described in chmod(2) and modified by the process' umask value (see umask(2)).

The () 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 oflag argument and the optional fourth argument correspond exactly to the arguments for open(). If openat() is passed the special value AT_FDCWD in the fd argument, the current working directory is used and the behavior is identical to a call to open().

The flags specified for the oflag argument must include exactly one of the following file access modes:

O_RDONLY	open for reading only
O_WRONLY	open for writing only
O_RDWR		open for reading and writing
O_SEARCH	open directory for searching
O_EXEC		open for execute only

In addition any combination of the following values can be 'ed in oflag:

O_NONBLOCK	do not block on open or for data to become available
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 O_CREAT and the file exists
O_SHLOCK	atomically obtain a shared lock
O_EXLOCK	atomically obtain an exclusive lock
O_DIRECTORY	restrict open to a directory
O_NOFOLLOW	do not follow symlinks
O_SYMLINK	allow open of symlinks
O_EVTONLY	descriptor requested for event notifications only
O_CLOEXEC	mark as close-on-exec
O_NOFOLLOW_ANY	do not follow symlinks in the entire path.

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, () returns an error. This may be used to implement a simple exclusive-access locking mechanism. If O_EXCL is set with O_CREAT 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 () 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 filesystem supports locking).

If O_DIRECTORY is used in the mask and the target file passed to () is not a directory then the open() will fail.

If O_NOFOLLOW is used in the mask and the target file passed to () is a symbolic link then the open() will fail.

If O_SYMLINK is used in the mask and the target file passed to () is a symbolic link then the open() will be for the symbolic link itself, not what it links to.

The O_EVTONLY flag is only intended for monitoring a file for changes (e.g. kqueue). Note: when this flag is used, the opened file will not prevent an unmount of the volume that contains the file.

The O_CLOEXEC flag causes the file descriptor to be marked as close-on-exec, setting the FD_CLOEXEC flag. The state of the file descriptor flags can be inspected using the F_GETFD fcntl. See fcntl(2).

If O_NOFOLLOW_ANY is used in the mask and any component of the path passed to () is a symbolic link then the open() will fail.

If successful, () 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.

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 system calls; see close(2) and fcntl(2).

The system imposes a limit on the number of file descriptors that can be held open simultaneously by one process.

A file's metadata can be updated even if the file was opened in read-only mode. Getdtablesize(2) returns the current system limit.

If successful, open() returns a non-negative integer, termed a file descriptor. It returns -1 on failure, and sets errno to indicate the error.

The named file is opened unless:

[]
Search permission is denied for a component of the path prefix.
[]
The required permissions (for reading and/or writing or search or executing) are denied for the given flags.
[]
is specified, the file does not exist, and the directory in which it is to be created does not permit writing.
[]
is specified and write permission is denied.
[]
path specifies the slave side of a locked pseudo-terminal device.
[]
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.
[]
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.
[]
and O_EXCL are specified and the file exists.
[]
Path points outside the process's allocated address space.
[]
The open() operation is interrupted by a signal.
[]
The value of oflag is not valid.
[]
An I/O error occurs while making the directory entry or allocating the inode for O_CREAT.
[]
The named file is a directory, and the arguments specify that it is to be opened for writing.
[]
The named file is a directory, and the arguments specify that it is to be opened for executing.
[]
Too many symbolic links are encountered in translating the pathname. This is taken to be indicative of a looping symbolic link.
[]
The process has already reached its limit for open file descriptors.
[]
A component of a pathname exceeds {NAME_MAX} characters, or an entire path name exceeded {PATH_MAX} characters.
[]
The system file table is full.
[]
was specified and the target is not a directory.
[]
was specified and the target is not a directory.
[]
was specified and the target is a symbolic link.
[]
was specified and and a component of the path is a symbolic link.
[]
is not set and the named file does not exist.
[]
A component of the path name that must exist does not exist.
[]
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.
[]
is specified, the file does not exist, and there are no free inodes on the file system on which the file is being created.
[]
A component of the path prefix is not a directory.
[]
A component of the pathname refers to a “dataless” directory that requires materialization and the I/O policy of the current thread or process disallows dataless directory materialization (see getiopolicy_np(3)).
[]
The named file is a character-special or block-special file and the device associated with this special file does not exist.
[]
O_NONBLOCK and O_WRONLY are set, the file is a FIFO, and no process has it open for reading.
[]
or O_EXLOCK is specified, but the underlying filesystem does not support locking.
[]
An attempt is made to open a socket (not currently implemented).
[]
The named file is a regular file and its size does not fit in an object of type off_t.
[]
The named file resides on a read-only file system, and the file is to be modified.
[]
The file is a pure procedure (shared text) file that is being executed and the open() call requests write access.
[]
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.
[]
The path argument is not an absolute path and fd is neither AT_FDCWD nor a file descriptor associated with a directory.
[]
The filename does not match the encoding rules.
[]
O_SHLOCK or O_EXLOCK is specified, but the file is locked and the O_NONBLOCK option was specified.

open() on a terminal device (i.e., /dev/console) will now make that device a controlling terminal for the process. Use the O_NOCTTY flag to open a terminal device without changing your controlling terminal.

chmod(2), close(2), dup(2), getdtablesize(2), lseek(2), read(2), umask(2), write(2)

An open() function call appeared in Version 6 AT&T UNIX. The openat() function was introduced in OS X 10.10

June 3, 2021 BSD 4