--- name: "openat" section: "2" platform: "macos" package: "xnu" version: "12377.121.6" release: "macOS 26.5" platforms: ["linux", "freebsd", "macos"] url: "https://new.carta.tech/open/2" --- # openat(2) open or create a file for reading or writing Part of [open(2)](/open/2.md). ## NAME **open**, **openat** – open or create a file for reading or writing ## SYNOPSIS **#include \** *int*\ **open**(*const char \*path*, *int oflag*, *...*); *int*\ **openat**(*int fd*, *const char \*path*, *int oflag*, *...*); ## DESCRIPTION The file name specified by *path* is opened for reading and/or writing, as specified by the argument *oflag*; the file descriptor is returned to the calling process. The *oflag* argument may indicate that 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*; the file is created with mode *mode* as described in [chmod(2)](/chmod/2.md) and modified by the process' umask value (see [umask(2)](/umask/2.md)). 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 *oflag* argument and the optional fourth argument correspond exactly to the arguments for **open**(). If **openat**() is passed the special value AT\_FDCWD in the *fd* argument, the current working directory is used and the behavior is identical to a call to **open**(). The flags specified for the *oflag* argument must include exactly one of the following file access modes: ``` O_RDONLY open for reading only O_WRONLY open for writing only O_RDWR open for reading and writing O_SEARCH open directory for searching O_EXEC open for execute only ``` In addition any combination of the following values can be *or*'ed in *oflag:* ``` O_NONBLOCK do not block on open or for data to become available 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 O_CREAT and the file exists O_SHLOCK atomically obtain a shared lock O_EXLOCK atomically obtain an exclusive lock O_DIRECTORY restrict open to a directory O_NOFOLLOW do not follow symlinks O_SYMLINK allow open of symlinks O_EVTONLY descriptor requested for event notifications only O_CLOEXEC mark as close-on-exec O_NOFOLLOW_ANY do not follow symlinks in the entire path O_RESOLVE_BENEATH path resolution must not escape the directory associated with the file descriptor O_UNIQUE ensure a file is opened only if it has a single hard link ``` 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 with O\_CREAT 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, do not wait for the device or file to be ready or available. If 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. This flag also has the effect of making all subsequent I/O on the open file non-blocking. When opening a file, a lock with [flock(2)](/flock/2.md) 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). If O\_DIRECTORY is used in the mask and the target file passed to **open**() is not a directory then the **open**() will fail. If O\_NOFOLLOW is used in the mask and the target file passed to **open**() is a symbolic link then the **open**() will fail. If O\_SYMLINK is used in the mask and the target file passed to **open**() is a symbolic link then the **open**() will be for the symbolic link itself, not what it links to. The O\_EVTONLY flag is only intended for monitoring a file for changes (e.g. kqueue). Note: when this flag is used, the opened file will not prevent an unmount of the volume that contains the file. The O\_CLOEXEC flag causes the file descriptor to be marked as close-on-exec, setting the FD\_CLOEXEC flag. The state of the file descriptor flags can be inspected using the F\_GETFD fcntl. See [fcntl(2)](/fcntl/2.md). If O\_NOFOLLOW\_ANY is used in the mask and any component of the path passed to **open**() is a symbolic link then the **open**() will fail. If O\_RESOLVE\_BENEATH is used in the mask and the specified path resolution escapes the directory associated with the *fd* then the **openat**() will fail. If O\_UNIQUE is used in the mask, the open operation will fail if the file has more than one hard link pointing to it. The operation succeeds only if the file is guaranteed to be uniquely accessible through the provided pathname. 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. When a new file is created, it is given the group of the directory which contains it. The new descriptor is set to remain open across execve system calls; see [close(2)](/close/2.md) and [fcntl(2)](/fcntl/2.md). The system imposes a limit on the number of file descriptors that can be held open simultaneously by one process. A file's metadata can be updated even if the file was opened in read-only mode. Getdtablesize(2) returns the current system limit. ## RETURN VALUES If successful, **open**() returns a non-negative integer, termed a file descriptor. It returns -1 on failure, and sets *errno* to indicate the error. ## ERRORS The named file is opened unless: - \[EACCES\] Search permission is denied for a component of the path prefix. - \[EACCES\] The required permissions (for reading and/or writing or search or executing) 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. - \[EACCES\] O\_TRUNC is specified and write permission is denied. - \[EAGAIN\] *path* specifies the slave side of a locked pseudo-terminal device. - \[EDQUOT\] 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. - \[EEXIST\] O\_CREAT and O\_EXCL are specified and the file exists. - \[EFAULT\] *Path* points outside the process's allocated address space. - \[EINTR\] The **open**() operation is interrupted by a signal. - \[EINVAL\] The value of *oflag* is not valid. - \[EIO\] An I/O error occurs while making the directory entry or allocating the inode for O\_CREAT. - \[EISDIR\] The named file is a directory, and the arguments specify that it is to be opened for writing. - \[EISDIR\] The named file is a directory, and the arguments specify that it is to be opened for executing. - \[ELOOP\] Too many symbolic links are encountered in translating the pathname. This is taken to be indicative of a looping symbolic link. - \[EMFILE\] The process has already reached its limit for open file descriptors. - \[ENAMETOOLONG\] A component of a pathname exceeds {NAME\_MAX} characters, or an entire path name exceeded {PATH\_MAX} characters. - \[ENFILE\] The system file table is full. - \[ENOTDIR\] O\_DIRECTORY was specified and the target is not a directory. - \[ENOTDIR\] O\_SEARCH was specified and the target is not a directory. - \[ELOOP\] O\_NOFOLLOW was specified and the target is a symbolic link. - \[ELOOP\] O\_NOFOLLOW\_ANY was specified and and a component of the path is a symbolic link. - \[ENOENT\] 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. - \[ENOSPC\] 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. - \[ENOTDIR\] A component of the path prefix is not a directory. - \[EDEADLK\] A component of the pathname refers to a “dataless” directory that requires materialization and the I/O policy of the current thread or process disallows dataless directory materialization (see [getiopolicy\_np(3)](/getiopolicy_np/3.md)). - \[ENXIO\] The named file is a character-special or block-special file and the device associated with this special file does not exist. - \[ENXIO\] O\_NONBLOCK and O\_WRONLY are set, the file is a FIFO, and no process has it open for reading. - \[EOPNOTSUPP\] O\_SHLOCK or O\_EXLOCK is specified, but the underlying filesystem does not support locking. - \[EOPNOTSUPP\] An attempt is made to open a socket (not currently implemented). - \[EOVERFLOW\] The named file is a regular file and its size does not fit in an object of type off\_t. - \[EROFS\] The named file resides on a read-only file system, and the file is to be modified. - \[ETXTBSY\] The file is a pure procedure (shared text) file that is being executed and the **open**() call requests write access. - \[EBADF\] 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. - \[EILSEQ\] The filename does not match the encoding rules. - \[EWOULDBLOCK\] O\_SHLOCK or O\_EXLOCK is specified, but the file is locked and the O\_NONBLOCK option was specified. - \[ENOTCAPABLE\] *path* resolution escapes the directory associated with *fd* and O\_RESOLVE\_BENEATH option was specified. - \[ENOTCAPABLE\] *path* has multiple hard links and O\_UNIQUE option was specified. ## COMPATIBILITY **open**() on a terminal device (i.e., /dev/console) will now make that device a controlling terminal for the process. Use the O\_NOCTTY flag to open a terminal device without changing your controlling terminal. ## SEE ALSO [chmod(2)](/chmod/2.md), [close(2)](/close/2.md), [dup(2)](/dup/2.md), [getdtablesize(2)](/getdtablesize/2.md), [lseek(2)](/lseek/2.md), [openat\_authenticated\_np(2)](/openat_authenticated_np/2.md), [read(2)](/read/2.md), [umask(2)](/umask/2.md), [write(2)](/write/2.md) ## HISTORY An **open**() function call appeared in Version 6 AT&T UNIX. The **openat**() function was introduced in OS X 10.10