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OPEN(2) System Calls Manual OPEN(2) NAME open, openat -- open or create a file for reading, writing or executing LIBRARY Standard C Library (libc, -lc) 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 either execution or read- ing and/or writing as specified by the argument flags and the file de- scriptor returned to the calling process. The flags argument may indi- cate 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 de- scribed 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. For openat() and relative path, the file to be opened is determined relative to the di- rectory associated with the file descriptor fd instead of the current working directory. The flag parameter and the optional fourth parame- ter 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 openat() is called with an absolute path, it ignores the fd argu- ment. In capsicum(4) capability mode, open() is not permitted. The path ar- gument 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 hi- erarchy 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, ".." compo- nents 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 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_SEARCH open for search only (an alias for O_EXEC typically used with O_DIRECTORY) O_NONBLOCK do not block on open O_APPEND set file pointer to the end of the file before each write O_CREAT create file if it does not exist O_TRUNC truncate size to 0 O_EXCL fail if O_CREAT is set and the file exists O_SHLOCK atomically obtain a shared lock O_EXLOCK atomically obtain an exclusive lock O_DIRECT read and write directly from the backing store O_FSYNC synchronous data and metadata writes (historical synonym for O_SYNC) O_SYNC synchronous data and metadata writes O_DSYNC synchronous data 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 automatically close file on execve(2) O_CLOFORK automatically close file on any child process cre- ated with fork(2) O_VERIFY verify the contents of the file with mac_veriexec(4) O_RESOLVE_BENEATH (openat(2) only) path resolution must not cross the fd directory O_PATH record only the target path in the opened descriptor O_EMPTY_PATH (openat(2) only) open file referenced by fd if path is empty O_NAMEDATTR open a named attribute or named attribute directory 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 ex- cept when O_EXCL is set too, in this case open() fails with EEXIST. This may be used to implement a simple exclusive access locking mecha- nism. In all other cases, the file is created and the access permis- sion 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, cre- ated 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, open() will fail even if the symbolic link points to a non-exis- tent name. If O_NONBLOCK is specified and the open() 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 synchro- nously 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 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. 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 mini- mize the impact the data has on the cache. Use of this flag can dras- tically 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 open() system call will not assign controlling terminals on FreeBSD. O_TTY_INIT may be used to ensure the OS restores the terminal attrib- utes 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 el- evated 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_CLOFORK may be used to set FD_CLOFORK flag for the newly returned file descriptor. The file will be closed on any child process created with fork(2), vfork(2) or rfork(2) with the RFFDG flag, remaining open in the parent. Both the O_CLOEXEC and O_CLOFORK flags can be modified with the F_SETFD fcntl(2) command. 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 de- tails 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 de- scriptor is as the lookup descriptor for the *at() 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 openat() 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) fstatfs(2) fchdir(2) fchroot(2) fexecve(2) funlinkat(2) can be passed as the third argument SCM_RIGHTS can be passed over a unix(4) socket using a SCM_RIGHTS message kqueue(2) only with EVFILT_VNODE __acl_get_fd(2) __acl_aclcheck_fd(2) extattr(2) capsicum(4) can be passed to cap_*_limit() and cap_*_get() system calls (such as cap_rights_limit(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 nor- mal (operable) file descriptor by specifying it as the fd argument to openat() 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 de- scription of AT_EMPTY_PATH flag for fstatat(2) and related syscalls. Conversely, a file descriptor fd referencing a filesystem file can be converted to the O_PATH type of descriptor by using the following call opath_fd = openat(fd, "", O_EMPTY_PATH | O_PATH); If successful, open() returns a non-negative integer, termed a file de- scriptor. It returns -1 on failure. The file descriptor value re- turned 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(4) 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 re- main open across execve(2) system calls; see close(2), fcntl(2) and the description of the O_CLOEXEC flag. When the O_NAMEDATTR flag is specified for an openat() where the fd ar- gument is for a file object, a named attribute for the file object is opened and not the file object itself. If the O_CREAT flag has been specified as well, the named attribute will be created if it does not exist. When the O_NAMEDATTR flag is specified for a open(), a named attribute for the current working directory is opened and not the cur- rent working directory. The path argument for this openat() or open() must be a single component name with no embedded `/'. If the path ar- gument is `.' then the named attribute directory for the file object is opened. (See named_attribute(7) for more information.) The system imposes a limit on the number of file descriptors open si- multaneously by one process. The getdtablesize(2) system call 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. [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 ex- ist. [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 writ- ing) are denied for the given flags. [EACCES] O_TRUNC is specified and write permission is denied. [EACCES] O_CREAT is specified, the file does not exist, and the directory in which it is to be created does not permit writing. [EPERM] 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 chflags(2) manual page for more information. [EPERM] The named file has its immutable flag set and the file is to be modified. [EPERM] 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. [ELOOP] Too many symbolic links were encountered in trans- lating the pathname. [EISDIR] The named file is a directory, and the arguments specify it is to be modified. [EISDIR] The named file is a directory, and the flags speci- fied O_CREAT without O_DIRECTORY. [EROFS] The named file resides on a read-only file system, and the file is to be modified. [EROFS] O_CREAT is specified and the named file would reside on a read-only file system. [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 sym- bolic link. POSIX specifies a different error for this case; see the note in "STANDARDS" below. [ENXIO] The named file is a character special or block spe- cial file, and the device associated with this spe- cial file does not exist. [ENXIO] O_NONBLOCK is set, the named file is a fifo, O_WRONLY is set, and no process has the file open for reading. [EINTR] The open() operation was interrupted by a signal. [EOPNOTSUPP] O_SHLOCK or O_EXLOCK is specified but the underlying file system does not support locking. [EOPNOTSUPP] The named file is a special file mounted through a file system that does not support access to it (for example, NFS). [EWOULDBLOCK] O_NONBLOCK and one of O_SHLOCK or O_EXLOCK is speci- fied 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 direc- tory. [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 en- try or allocating the inode for O_CREAT. [EINTEGRITY] Corrupted data was detected while reading from the file system. [ETXTBSY] The file is a pure procedure (shared text) file that is being executed and the open() system call re- quests write access. [EFAULT] The path argument points outside the process's allo- cated address space. [EEXIST] O_CREAT and O_EXCL were specified and the file ex- ists. [EOPNOTSUPP] An attempt was made to open a socket (not currently implemented). [EINVAL] An attempt was made to open a descriptor with an il- legal combination of O_RDONLY, O_WRONLY, or O_RDWR, and O_EXEC or O_SEARCH. [EINVAL] O_CREAT is specified, and the last component of the path argument is invalid on the file system on which the file is being created. [EBADF] 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. [ENOTDIR] The path argument is not an absolute path and fd is neither AT_FDCWD nor a file descriptor associated with a directory. [ENOTDIR] O_DIRECTORY is specified and the file is not a di- rectory. [ECAPMODE] AT_FDCWD is specified and the process is in capabil- ity mode. [ECAPMODE] open() was called and the process is in capability mode. [ENOTCAPABLE] path is an absolute path and the process is in capa- bility mode. [ENOTCAPABLE] path is an absolute path and O_RESOLVE_BENEATH is specified. [ENOTCAPABLE] path contains a ".." component leading to a direc- tory outside of the directory hierarchy specified by fd and the process is in capability mode. [ENOTCAPABLE] path contains a ".." component leading to a direc- tory outside of the directory hierarchy specified by fd and O_RESOLVE_BENEATH is specified. [ENOTCAPABLE] path contains a ".." component, the vfs.lookup_cap_dotdot sysctl(3) is set, and the process is in capability mode. [ENOATTR] O_NAMEDATTR has been specified and the file object is not a named attribute directory or named at- tribute. SEE ALSO 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), named_attribute(7) STANDARDS 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 direc- tory 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. HISTORY 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. O_NAMEDATTR appeared in 15.0. O_CLOFORK appeared in FreeBSD 15.0. BUGS The mode argument is variadic and may result in different calling con- ventions than might otherwise be expected. FreeBSD 16.0 CURRENT May 17, 2025 OPEN(2)
NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | STANDARDS | HISTORY | BUGS
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