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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 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().
In capsicum(4) capability mode, open() is not permitted. The path argument to openat() must be strictly relative to a file descriptor fd, as defined in sys/kern/vfs_lookup.c. path must not be an absolute path and must not contain ".." components. Additionally, no symbolic link in path may contain ".." components either. fd must not be AT_FDCWD.
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	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
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_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 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.
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:
[]
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 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.
[]
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 (e.g. 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.
[]
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, O_RDWR and O_EXEC.
[]
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 or contained a ".." component leading to a directory outside of the directory hierarchy specified by fd.

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() function appeared in Version 1 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 directory currently permits searches. The present implementation of the openat checks the current permissions of directory instead.
December 1, 2017 FreeBSD 12.0-RELEASE

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