OPEN(2)

NAME
open, openat - open or create a file for reading or writing

LIBRARY
libc

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 reading and/or writing as specified by the argument flags and the lowest unused file descriptor in the process' file descriptor table is returned. 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 a third 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_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_NOFOLLOW	do not follow symlinks
    O_DIRECTORY	error if file is not a directory
    O_CLOEXEC	set FD_CLOEXEC upon open
    O_CLOFORK	set FD_CLOFORK 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() call would result in the process being blocked for some reason (e.g., waiting for carrier on a dialup line), open() returns immediately. The first time the process attempts to perform I/O on the open file it will block (not currently implemented).

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.

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 filesystem 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_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 atomically set the FD_CLOEXEC flag for the newly returned file descriptor.

O_CLOFORK may be used to atomically set the FD_CLOFORK flag for the newly returned file descriptor. The file will be closed on any child process created with fork(2) or vfork(2)

If successful, open() and openat() return 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.

Unless O_CLOEXEC 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. Getdtablesize(2) 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 or the path argument is not an absolute path and the fd argument is neither AT_FDCWD nor a file descriptor associated with a directory or O_DIRECTORY is specified and the file 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_CREAT is specified, the file does not exist, and the directory in which it is to be created does not permit writing.

[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 opened for writing.

[EROFS]
    The named file resides on a read-only file system, and the file is to be modified.

[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]
    The named file is a fifo, no process has it open for reading, and the arguments specify it is to be opened for writing.

[EINTR]
    The open() operation was interrupted by a signal.

[EOPNOTSUPP]
    O_SHLOCK or O_EXLOCK is specified but the underlying filesystem does not support locking.

[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.

[ETXTBSY]
    The file is a pure procedure (shared text) file that is being executed and the open() call requests write access.

[EFAULT]
    Path 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, and O_RDWR.

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

HISTORY
An open() function call appeared in AT&T UNIX. An openat() function call appeared first in Solaris and was ported to DragonFly 2.3. O_CLOFORK appeared in FreeBSD 15.0 and DragonFly 6.5.

BUGS
The Open Group Extended API Set 2 specification requires that the test for fd's searchability is based on whether it is open for searching, and not whether the underlying directory currently permits searches. The present implementation of openat() checks the current permissions of directory instead.