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IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .Dd January 3, 2025 .Dt OPEN 2 .Os .Sh NAME .Nm open , openat .Nd open or create a file for reading, writing or executing .Sh LIBRARY .Lb libc .Sh SYNOPSIS .In fcntl.h .Ft int .Fn open "const char *path" "int flags" "..." .Ft int .Fn openat "int fd" "const char *path" "int flags" "..." .Sh DESCRIPTION The file name specified by .Fa path is opened for either execution or reading and/or writing as specified by the argument .Fa flags and the file descriptor returned to the calling process. The .Fa flags argument may indicate the file is to be created if it does not exist (by specifying the .Dv O_CREAT flag). In this case .Fn open and .Fn openat require an additional argument .Fa "mode_t mode" , and the file is created with mode .Fa mode as described in .Xr chmod 2 and modified by the process' umask value (see .Xr umask 2 ) . .Pp The .Fn openat function is equivalent to the .Fn open function except in the case where the .Fa path specifies a relative path. For .Fn openat and relative .Fa path , the file to be opened is determined relative to the directory associated with the file descriptor .Fa fd instead of the current working directory. The .Fa flag parameter and the optional fourth parameter correspond exactly to the parameters of .Fn open . If .Fn openat is passed the special value .Dv AT_FDCWD in the .Fa fd parameter, the current working directory is used and the behavior is identical to a call to .Fn open . .Pp When .Fn openat is called with an absolute .Fa path , it ignores the .Fa fd argument. .Pp In .Xr capsicum 4 capability mode, .Fn open is not permitted. The .Fa path argument to .Fn openat must be strictly relative to a file descriptor .Fa fd ; that is, .Fa path must not be an absolute path and must not contain ".." components which cause the path resolution to escape the directory hierarchy starting at .Fa fd . Additionally, no symbolic link in .Fa path may target absolute path or contain escaping ".." components. .Fa fd must not be .Dv AT_FDCWD . .Pp If the .Dv vfs.lookup_cap_dotdot .Xr sysctl 3 MIB is set to zero, ".." components in the paths, used in capability mode, are completely disabled. If the .Dv vfs.lookup_cap_dotdot_nonlocal MIB is set to zero, ".." is not allowed if found on non-local filesystem. .Pp The .Fa flags are formed by .Em or Ns 'ing the following values: .Pp .Bl -tag -width O_RESOLVE_BENEATH .It Dv O_RDONLY open for reading only .It Dv O_WRONLY open for writing only .It Dv O_RDWR open for reading and writing .It Dv O_EXEC open for execute only .It Dv O_SEARCH open for search only (an alias for .Dv O_EXEC typically used with .Dv O_DIRECTORY ) .It Dv O_NONBLOCK do not block on open .It Dv O_APPEND set file pointer to the end of the file before each write .It Dv O_CREAT create file if it does not exist .It Dv O_TRUNC truncate size to 0 .It Dv O_EXCL fail if .Dv O_CREAT is set and the file exists .It Dv O_SHLOCK atomically obtain a shared lock .It Dv O_EXLOCK atomically obtain an exclusive lock .It Dv O_DIRECT read and write directly from the backing store .It Dv O_FSYNC synchronous data and metadata writes .Pq historical synonym for Dv O_SYNC .It Dv O_SYNC synchronous data and metadata writes .It Dv O_DSYNC synchronous data writes .It Dv O_NOFOLLOW do not follow symlinks .It Dv O_NOCTTY ignored .It Dv O_TTY_INIT ignored .It Dv O_DIRECTORY error if file is not a directory .It Dv O_CLOEXEC automatically close file on .Xr execve 2 .It Dv O_VERIFY verify the contents of the file with .Xr mac_veriexec 4 .It Dv O_RESOLVE_BENEATH .Pq Xr openat 2 only path resolution must not cross the .Fa fd directory .It Dv O_PATH record only the target path in the opened descriptor .It Dv O_EMPTY_PATH .Pq Xr openat 2 only open file referenced by .Fa fd if path is empty .El .Pp Exactly one of the flags .Dv O_RDONLY , .Dv O_WRONLY , .Dv O_RDWR , or .Dv O_EXEC must be provided. .Pp Opening a file with .Dv O_APPEND set causes each write on the resulting file descriptor to be appended to the end of the file. .Pp If .Dv O_TRUNC is specified and the file exists, the file is truncated to zero length. .Pp If .Dv O_EXCL is set with .Dv O_CREAT and the file already exists, .Fn open fails with .Er EEXIST . This may be used to implement a simple exclusive access locking mechanism. If .Dv O_EXCL is set and the last component of the pathname is a symbolic link, .Fn open will fail even if the symbolic link points to a non-existent name. .Pp If .Dv O_NONBLOCK is specified and the .Fn open system call would block for some reason (for example, waiting for carrier on a dialup line), .Fn open returns immediately. The descriptor remains in non-blocking mode for subsequent operations. .Pp If .Dv O_SYNC is used in the mask, all writes will immediately and synchronously be written to disk. .Dv O_FSYNC is an historical synonym for .Dv O_SYNC . .Pp If .Dv 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. .Pp If .Dv O_NOFOLLOW is used in the mask and the target file passed to .Fn open is a symbolic link then the .Fn open will fail. .Pp When opening a file, a lock with .Xr flock 2 semantics can be obtained by setting .Dv O_SHLOCK for a shared lock, or .Dv O_EXLOCK for an exclusive lock. If creating a file with .Dv O_CREAT , the request for the lock will never fail (provided that the underlying file system supports locking). .Pp .Dv 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. .Pp .Dv 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 .Fx , but is present for POSIX compatibility. The .Fn open system call will not assign controlling terminals on .Fx . .Pp .Dv O_TTY_INIT may be used to ensure the OS restores the terminal attributes when initially opening a TTY. This is the default on .Fx , but is present for POSIX compatibility. The initial call to .Fn open on a TTY will always restore default terminal attributes on .Fx . .Pp .Dv 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 .Dv O_RDONLY , such as device nodes. .Pp .Dv O_CLOEXEC may be used to set .Dv FD_CLOEXEC flag for the newly returned file descriptor. .Pp .Dv 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 .Dq verified means is implementation specific. The run-time linker (rtld) uses this flag to ensure shared objects have been verified before operating on them. .Pp .Dv O_RESOLVE_BENEATH returns .Er 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. .Pp When a directory is opened with .Dv O_SEARCH , execute permissions are checked at open time. The returned file descriptor may not be used for any read operations like .Xr getdirentries 2 . The primary use of this descriptor is as the lookup descriptor for the .Fn *at family of functions. If .Dv O_SEARCH was not requested at open time, then the .Fn *at functions use the current directory permissions for the directory referenced by the descriptor at the time of the .Fn *at call. .Pp .Dv O_PATH returns a file descriptor that can be used as a directory file descriptor for .Fn openat and other system calls taking a file descriptor argument, like .Xr fstatat 2 and others. The other functionality of the returned file descriptor is limited to the following descriptor-level operations: .Pp .Bl -tag -width __acl_aclcheck_fd -offset indent -compact .It Xr fcntl 2 but advisory locking is not allowed .It Xr dup 2 .It Xr close 2 .It Xr fstat 2 .It Xr fstatfs 2 .It Xr fchdir 2 .It Xr fchroot 2 .It Xr fexecve 2 .It Xr funlinkat 2 can be passed as the third argument .It Dv SCM_RIGHTS can be passed over a .Xr unix 4 socket using a .Dv SCM_RIGHTS message .It Xr kqueue 2 only with .Dv EVFILT_VNODE .It Xr __acl_get_fd 2 .It Xr __acl_aclcheck_fd 2 .It Xr extattr 2 .It Xr capsicum 4 can be passed to .Fn cap_*_limit and .Fn cap_*_get system calls (such as .Xr cap_rights_limit 2 ) . .El .Pp Other operations like .Xr read 2 , .Xr ftruncate 2 , and any other that operate on file and not on file descriptor (except .Xr fstat 2 ) , are not allowed. .Pp A file descriptor created with the .Dv O_PATH flag can be opened as a normal (operable) file descriptor by specifying it as the .Fa fd argument to .Fn openat with an empty .Fa path and the .Dv O_EMPTY_PATH flag. Such an open behaves as if the current path of the file referenced by .Fa fd is passed, except that path walk permissions are not checked. See also the description of .Dv AT_EMPTY_PATH flag for .Xr fstatat 2 and related syscalls. .Pp If successful, .Fn open 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. .Pp If a sleeping open of a device node from .Xr devfs 4 is interrupted by a signal, the call always fails with .Er EINTR , even if the .Dv SA_RESTART flag is set for the signal. A sleeping open of a fifo (see .Xr mkfifo 2 ) is restarted as normal. .Pp When a new file is created, it is assigned the group of the directory which contains it. .Pp Unless .Dv O_CLOEXEC flag was specified, the new descriptor is set to remain open across .Xr execve 2 system calls; see .Xr close 2 , .Xr fcntl 2 and the description of the .Dv O_CLOEXEC flag. .Pp The system imposes a limit on the number of file descriptors open simultaneously by one process. The .Xr getdtablesize 2 system call returns the current system limit. .Sh RETURN VALUES If successful, .Fn open and .Fn openat return a non-negative integer, termed a file descriptor. They return \-1 on failure, and set .Va errno to indicate the error. .Sh ERRORS The named file is opened unless: .Bl -tag -width Er .It Bq Er ENOTDIR A component of the path prefix is not a directory. .It Bq Er ENAMETOOLONG A component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 characters. .It Bq Er ENOENT .Dv O_CREAT is not set and the named file does not exist. .It Bq Er ENOENT A component of the path name that must exist does not exist. .It Bq Er EACCES Search permission is denied for a component of the path prefix. .It Bq Er EACCES The required permissions (for reading and/or writing) are denied for the given flags. .It Bq Er EACCES .Dv O_TRUNC is specified and write permission is denied. .It Bq Er EACCES .Dv O_CREAT is specified, the file does not exist, and the directory in which it is to be created does not permit writing. .It Bq Er EPERM .Dv 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 .Xr chflags 2 manual page for more information. .It Bq Er EPERM The named file has its immutable flag set and the file is to be modified. .It Bq Er EPERM The named file has its append-only flag set, the file is to be modified, and .Dv O_TRUNC is specified or .Dv O_APPEND is not specified. .It Bq Er ELOOP Too many symbolic links were encountered in translating the pathname. .It Bq Er EISDIR The named file is a directory, and the arguments specify it is to be modified. .It Bq Er EISDIR The named file is a directory, and the flags specified .Dv O_CREAT without .Dv O_DIRECTORY . .It Bq Er EROFS The named file resides on a read-only file system, and the file is to be modified. .It Bq Er EROFS .Dv O_CREAT is specified and the named file would reside on a read-only file system. .It Bq Er EMFILE The process has already reached its limit for open file descriptors. .It Bq Er ENFILE The system file table is full. .It Bq Er EMLINK .Dv O_NOFOLLOW was specified and the target is a symbolic link. POSIX specifies a different error for this case; see the note in .Sx STANDARDS below. .It Bq Er ENXIO The named file is a character special or block special file, and the device associated with this special file does not exist. .It Bq Er ENXIO .Dv O_NONBLOCK is set, the named file is a fifo, .Dv O_WRONLY is set, and no process has the file open for reading. .It Bq Er EINTR The .Fn open operation was interrupted by a signal. .It Bq Er EOPNOTSUPP .Dv O_SHLOCK or .Dv O_EXLOCK is specified but the underlying file system does not support locking. .It Bq Er EOPNOTSUPP The named file is a special file mounted through a file system that does not support access to it (for example, NFS). .It Bq Er EWOULDBLOCK .Dv O_NONBLOCK and one of .Dv O_SHLOCK or .Dv O_EXLOCK is specified and the file is locked. .It Bq Er ENOSPC .Dv 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. .It Bq Er ENOSPC .Dv 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. .It Bq Er EDQUOT .Dv 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. .It Bq Er EDQUOT .Dv 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. .It Bq Er EIO An I/O error occurred while making the directory entry or allocating the inode for .Dv O_CREAT . .It Bq Er EINTEGRITY Corrupted data was detected while reading from the file system. .It Bq Er ETXTBSY The file is a pure procedure (shared text) file that is being executed and the .Fn open system call requests write access. .It Bq Er EFAULT The .Fa path argument points outside the process's allocated address space. .It Bq Er EEXIST .Dv O_CREAT and .Dv O_EXCL were specified and the file exists. .It Bq Er EOPNOTSUPP An attempt was made to open a socket (not currently implemented). .It Bq Er EINVAL An attempt was made to open a descriptor with an illegal combination of .Dv O_RDONLY , .Dv O_WRONLY , or .Dv O_RDWR , and .Dv O_EXEC or .Dv O_SEARCH . .It Bq Er EINVAL .Dv O_CREAT is specified, and the last component of the .Fa path argument is invalid on the file system on which the file is being created. .It Bq Er EBADF The .Fa path argument does not specify an absolute path and the .Fa fd argument is neither .Dv AT_FDCWD nor a valid file descriptor open for searching. .It Bq Er ENOTDIR The .Fa path argument is not an absolute path and .Fa fd is neither .Dv AT_FDCWD nor a file descriptor associated with a directory. .It Bq Er ENOTDIR .Dv O_DIRECTORY is specified and the file is not a directory. .It Bq Er ECAPMODE .Dv AT_FDCWD is specified and the process is in capability mode. .It Bq Er ECAPMODE .Fn open was called and the process is in capability mode. .It Bq Er ENOTCAPABLE .Fa path is an absolute path and the process is in capability mode. .It Bq Er ENOTCAPABLE .Fa path is an absolute path and .Dv O_RESOLVE_BENEATH is specified. .It Bq Er ENOTCAPABLE .Fa path contains a ".." component leading to a directory outside of the directory hierarchy specified by .Fa fd and the process is in capability mode. .It Bq Er ENOTCAPABLE .Fa path contains a ".." component leading to a directory outside of the directory hierarchy specified by .Fa fd and .Dv O_RESOLVE_BENEATH is specified. .It Bq Er ENOTCAPABLE .Fa path contains a ".." component, the .Dv vfs.lookup_cap_dotdot .Xr sysctl 3 is set, and the process is in capability mode. .El .Sh SEE ALSO .Xr chmod 2 , .Xr close 2 , .Xr dup 2 , .Xr fexecve 2 , .Xr fhopen 2 , .Xr getdtablesize 2 , .Xr getfh 2 , .Xr lgetfh 2 , .Xr lseek 2 , .Xr read 2 , .Xr umask 2 , .Xr write 2 , .Xr fopen 3 , .Xr capsicum 4 .Sh STANDARDS These functions are specified by .St -p1003.1-2008 . .Pp .Fx sets .Va errno to .Er EMLINK instead of .Er ELOOP as specified by POSIX when .Dv 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. .Pp The Open Group Extended API Set 2 specification, that introduced the .Fn *at API, required that the test for whether .Fa fd is searchable is based on whether .Fa fd is open for searching, not whether the underlying directory currently permits searches. The present implementation of the .Fa openat system call is believed to be compatible with .\" .St -p1003.1-2017 , .\" XXX: This should be replaced in the future when an appropriate argument to .\" the St macro is available: -p1003.1-2017 .No IEEE Std 1003.1-2008, 2017 Edition ("POSIX.1") , which specifies that behavior for .Dv O_SEARCH , in the absence of the flag the implementation checks the current permissions of a directory. .Sh HISTORY The .Fn open function appeared in .At v1 . The .Fn openat function was introduced in .Fx 8.0 . .Dv O_DSYNC appeared in 13.0. .Sh BUGS The .Fa mode argument is variadic and may result in different calling conventions than might otherwise be expected.