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Man Pages
OPEN(2) FreeBSD System Calls Manual OPEN(2)

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

Standard C Library (libc, -lc)

#include <fcntl.h>

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

int
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 specifying the O_CREAT flag). In this case () 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 () 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 () is called with an absolute path, it ignores the fd argument.

In capsicum(4) capability mode, () 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 'ing the following values:

open for reading only
open for writing only
open for reading and writing
open for execute only
open for search only (an alias for O_EXEC typically used with O_DIRECTORY)
do not block on open
set file pointer to the end of the file before each write
create file if it does not exist
truncate size to 0
fail if O_CREAT is set and the file exists
atomically obtain a shared lock
atomically obtain an exclusive lock
read and write directly from the backing store
synchronous data and metadata writes (historical synonym for O_SYNC)
synchronous data and metadata writes
synchronous data writes
do not follow symlinks
ignored
ignored
error if file is not a directory
automatically close file on execve(2)
verify the contents of the file with mac_veriexec(4)
(openat(2) only) path resolution must not cross the fd directory
record only the target path in the opened descriptor
(openat(2) only) open file referenced by fd if path is empty

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 () 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, () will fail even if the symbolic link points to a non-existent name.

If O_NONBLOCK is specified and the () 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 () 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 () 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 () 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 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 () 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 a directory file descriptor for () 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 following descriptor-level operations:

fcntl(2)
but advisory locking is not allowed
dup(2)
 
close(2)
 
fstat(2)
 
fexecve(2)
 
can be passed over a unix(4) socket using a SCM_RIGHTS message
kqueue(2)
only with EVFILT_VNODE
readlinkat(2)
 
__acl_get_fd(2)
 
__acl_aclcheck_fd(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 () 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.

If successful, () 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(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 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.

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:

[]
A component of the path prefix is not a directory.
[]
A component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 characters.
[]
is not set and the named file does not exist.
[]
A component of the path name that must exist does not exist.
[]
Search permission is denied for a component of the path prefix.
[]
The required permissions (for reading and/or writing) are denied for the given flags.
[]
is specified and write permission is denied.
[]
is specified, the file does not exist, and the directory in which it is to be created does not permit writing.
[]
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.
[]
The named file has its immutable flag set and the file is to be modified.
[]
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.
[]
Too many symbolic links were encountered in translating the pathname.
[]
The named file is a directory, and the arguments specify it is to be modified.
[]
The named file is a directory, and the flags specified O_CREAT without O_DIRECTORY.
[]
The named file resides on a read-only file system, and the file is to be modified.
[]
is specified and the named file would reside on a read-only file system.
[]
The process has already reached its limit for open file descriptors.
[]
The system file table is full.
was specified and the target is a symbolic link. POSIX specifies a different error for this case; see the note in STANDARDS below.
[]
The named file is a character special or block special file, and the device associated with this special file does not exist.
[]
is set, the named file is a fifo, O_WRONLY is set, and no process has the file open for reading.
[]
The open() operation was interrupted by a signal.
[]
or O_EXLOCK is specified but the underlying file system does not support locking.
[]
The named file is a special file mounted through a file system that does not support access to it (for example, NFS).
[]
and one of O_SHLOCK or O_EXLOCK is specified and the file is locked.
[]
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.
[]
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.
[]
An I/O error occurred while making the directory entry or allocating the inode for O_CREAT.
[]
Corrupted data was detected while reading from the file system.
[]
The file is a pure procedure (shared text) file that is being executed and the open() system call requests write access.
[]
The path argument points outside the process's allocated address space.
[]
and O_EXCL were specified and the file exists.
[]
An attempt was made to open a socket (not currently implemented).
[]
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.
[]
is specified, and the last component of the path argument is invalid on the file system on which the file is being created.
[]
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.
[]
is specified and the file is not a directory.
[]
is specified and the process is in capability mode.
[]
open() was called and the process is in capability mode.
[]
path is an absolute path and the process is in capability mode.
[]
path is an absolute path and O_RESOLVE_BENEATH is specified.
[]
path contains a ".." component leading to a directory outside of the directory hierarchy specified by fd and the process is in capability mode.
[]
path contains a ".." component leading to a directory outside of the directory hierarchy specified by fd and O_RESOLVE_BENEATH is specified.
[]
path contains a ".." component, the vfs.lookup_cap_dotdot sysctl(3) is set, and the process is in capability mode.

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)

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.

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.

The mode argument is variadic and may result in different calling conventions than might otherwise be expected.

February 28, 2025 FreeBSD 14.3-RELEASE
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