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OPEN(2) System Calls Manual OPEN(2)

open, openat —
open or create a file for reading or writing

#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 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 argument 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:

Open for reading only.
Open for writing only.
Open for reading and writing.

Any combination of the following flags may additionally be used:

Do not block on open or for data to become available.
Append on each write.
Create file if it does not exist. An additional argument of type mode_t must be supplied to the call.
Truncate size to 0.
Error if O_CREAT is set and file exists.
Perform synchronous I/O operations.
Atomically obtain a shared lock.
Atomically obtain an exclusive lock.
If last path element is a symlink, don't follow it.
Set FD_CLOEXEC (the close-on-exec flag) on the new file descriptor.
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 mechanism. 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 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. If the O_SYNC flag is set, all I/O operations on the file will be done synchronously.

A FIFO should either be opened with O_RDONLY or with O_WRONLY. The behavior 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 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(2) system calls; see close(2) and fcntl(2).

The system imposes a limit on the number of file descriptors open simultaneously by one process. getdtablesize(3) returns the current system 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 current 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 behavior is identical to a call to open().

If successful, open() returns a non-negative integer, termed a file descriptor. Otherwise, a value of -1 is returned and errno is set to indicate the error.

The open() and openat() functions will fail if:
[]
A component of the path prefix is not a directory.
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is specified and path does not name a directory.
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A component of a pathname exceeded NAME_MAX characters, or an entire pathname (including the terminating NUL) exceeded PATH_MAX bytes.
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is not set and the named file does not exist.
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A component of the path name that must exist does not exist.
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Search permission is denied for a component of the path prefix.
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The required permissions (for reading and/or writing) are denied for the given flags.
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is specified, the file does not exist, and the directory in which it is to be created does not permit writing.
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Too many symbolic links were encountered in translating the pathname, or the O_NOFOLLOW flag was specified and the target is a symbolic link.
[]
The named file is a directory, and the arguments specify it is to be opened for writing.
[]
The flags specified for opening the file are not valid.
[]
The named file resides on a read-only file system, and the file is to be modified.
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The process has already reached its limit for open file descriptors.
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The system file table is full.
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The named file is a character special or block special file, and the device associated with this special file does not exist.
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The named file is a FIFO, the O_NONBLOCK and O_WRONLY flags are set, and no process has the file open for reading.
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The open() operation was interrupted by a signal.
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or O_EXLOCK is specified but the underlying filesystem does not support locking.
[]
and one of O_SHLOCK or O_EXLOCK is specified and the file is already locked.
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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.
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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.
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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.
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An I/O error occurred while making the directory entry or allocating the inode for O_CREAT.
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The file is a pure procedure (shared text) file that is being executed and the open() call requests write access.
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path points outside the process's allocated address space.
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and O_EXCL were specified and the file exists.
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The file named by path is flagged append-only but O_APPEND was not specified in flags.
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An attempt was made to open a socket (not currently implemented).
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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:

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The path argument specifies a relative path and the fd argument is neither AT_FDCWD nor a valid file descriptor.
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The path argument specifies a relative path and the fd argument is a valid file descriptor but it does not reference a directory.
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The path argument specifies a relative path but search permission is denied for the directory which the fd file descriptor references.

chflags(2), chmod(2), close(2), dup(2), flock(2), lseek(2), read(2), umask(2), write(2), getdtablesize(3)

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.

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.

The O_TRUNC flag requires that one of O_RDWR or O_WRONLY also be specified, else EINVAL is returned.
January 19, 2015 OpenBSD-current