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OPEN(2) System Calls Manual OPEN(2) NAME open, openat -- open or create a file for reading or writing 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 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 which case the file is created with a mode specified by an additional argu- ment of type mode_t as described in chmod(2) and modified by the process' umask value (see umask(2)). The flags specified are a bitwise OR of the following values. Exactly one of the first three values (file access modes) must be specified: O_RDONLY Open for reading only. O_WRONLY Open for writing only. O_RDWR Open for reading and writing. Any combination of the following flags may additionally be used: O_NONBLOCK Do not block on open or for data to become avail- able. O_APPEND Append on each write. O_CREAT Create file if it does not exist. An additional ar- gument of type mode_t must be supplied to the call. O_TRUNC Truncate size to 0. O_EXCL Error if O_CREAT is set and file exists. O_SYNC Perform synchronous I/O operations. O_SHLOCK Atomically obtain a shared lock. O_EXLOCK Atomically obtain an exclusive lock. O_NOFOLLOW If last path element is a symlink, don't follow it. O_CLOEXEC Set FD_CLOEXEC (the close-on-exec flag) on the new file descriptor. O_DIRECTORY Error if path does not name a directory. Opening a file with O_APPEND set causes each write on the file to be appended to the end. If O_TRUNC and a writing mode are 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 mecha- nism. If either of O_EXCL or O_NOFOLLOW are set and the last component of the pathname is a symbolic link, open() will fail even if the sym- bolic 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 im- mediately. This flag also has the effect of making all subsequent I/O on the open file non-blocking. If the O_SYNC flag is set, all I/O op- erations on the file will be done synchronously. A FIFO should either be opened with O_RDONLY or with O_WRONLY. The be- havior for opening a FIFO with O_RDWR is undefined. 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 filesystem supports locking). If open() is successful, the file pointer used to mark the current po- sition 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(2) system calls; see close(2) and fcntl(2). The system imposes a limit on the number of file descriptors open si- multaneously by one process. getdtablesize(3) returns the current sys- tem limit. The openat() function is equivalent to open() except that where path specifies a relative path, the file to be opened is determined relative to the directory associated with file descriptor fd instead of the cur- rent working directory. If openat() is passed the special value AT_FDCWD (defined in <fcntl.h>) in the fd parameter, the current working directory is used and the be- havior is identical to a call to open(). RETURN VALUES If successful, open() returns a non-negative integer, termed a file de- scriptor. Otherwise, a value of -1 is returned and errno is set to in- dicate the error. ERRORS The open() and openat() functions will fail if: [ENOTDIR] A component of the path prefix is not a directory. [ENOTDIR] O_DIRECTORY is specified and path does not name a directory. [ENAMETOOLONG] A component of a pathname exceeded NAME_MAX charac- ters, or an entire pathname (including the terminat- ing NUL) exceeded PATH_MAX bytes. [ENOENT] O_CREAT is not set and the named file does not ex- ist. [ENOENT] A component of the pathname 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_CREAT is specified, the file does not exist, and the directory in which it is to be created does not permit writing. [ELOOP] Too many symbolic links were encountered in trans- lating the pathname, or the O_NOFOLLOW flag was specified and the target is a symbolic link. [EISDIR] The named file is a directory, and the arguments specify it is to be opened for writing. [EINVAL] The flags specified for opening the file are not valid. [EROFS] The named file resides on a read-only file system, and the file is to be modified. [EMFILE] The process has already reached its limit for open file descriptors. [ENFILE] The system file table is full. [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] The named file is a FIFO, the O_NONBLOCK and O_WRONLY flags are 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 filesystem does not support locking. [EWOULDBLOCK] O_NONBLOCK and one of O_SHLOCK or O_EXLOCK is speci- fied and the file is already 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. [ETXTBSY] The file is a pure procedure (shared text) file that is being executed and the open() call requests write access. [EFAULT] path points outside the process's allocated address space. [EEXIST] O_CREAT and O_EXCL were specified and the file ex- ists. [EPERM] The file named by path is flagged append-only but O_APPEND was not specified in flags. [EOPNOTSUPP] An attempt was made to open a socket (not currently implemented). [EBUSY] An attempt was made to open a terminal device that requires exclusive access and the specified device has already be opened. Additionally, the openat() function will fail if: [EBADF] The path argument specifies a relative path and the fd argument is neither AT_FDCWD nor a valid file de- scriptor. [ENOTDIR] The path argument specifies a relative path and the fd argument is a valid file descriptor but it does not reference a directory. [EACCES] The path argument specifies a relative path but search permission is denied for the directory which the fd file descriptor references. SEE ALSO chflags(2), chmod(2), close(2), dup(2), flock(2), lseek(2), read(2), umask(2), write(2), getdtablesize(3) STANDARDS The open() and openat() functions conform to IEEE Std 1003.1-2008 ("POSIX.1"). POSIX specifies three different flavors for synchronous I/O: O_SYNC, O_DSYNC, and O_RSYNC. In OpenBSD, these are all equivalent. The O_SHLOCK and O_EXLOCK flags are non-standard extensions and should not be used if portability is of concern. HISTORY An open() system call first appeared in Version 1 AT&T UNIX. The flags argument has been supported since 4.2BSD. Before that, a dedicated creat() system call had to be used to create new files; it appeared in Version 1 AT&T UNIX, was deprecated in 4.3BSD-Reno, and removed in OpenBSD 5.0. The openat() system call has been available since OpenBSD 5.0. CAVEATS The O_TRUNC flag requires that one of O_RDWR or O_WRONLY also be speci- fied, else EINVAL is returned. OpenBSD 6.9 January 3, 2021 OPEN(2)
NAME | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | STANDARDS | HISTORY | CAVEATS
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