The open() function establishes the connection between a file and a file descriptor. It creates an open file description that refers to a file and a file descriptor that refers to that open file description. The file descriptor is used by other I/O functions to refer to that file. The path argument points to a pathname naming the file.

The openat() function is identical to the open() function except that the path argument is interpreted relative to the starting point implied by the fd argument. If the fd argument has the special value AT_FDCWD, a relative path argument will be resolved relative to the current working directory. If the path argument is absolute, the fd argument is ignored.

The open() function returns a file descriptor for the named file that is the lowest file descriptor not currently open for that process. The open file description is new, and therefore the file descriptor does not share it with any other process in the system. The FD_CLOEXEC file descriptor flag associated with the new file descriptor is cleared.

The file offset used to mark the current position within the file is set to the beginning of the file.

The file status flags and file access modes of the open file description are set according to the value of oflag. The mode argument is used only when O_CREAT is specified (see below.)

Values for oflag are constructed by a bitwise-inclusive-OR of flags from the following list, defined in <fcntl.h>. Applications must specify exactly one of the first three values (file access modes) below in the value of oflag:

O_RDONLY

Open for reading only.

O_WRONLY

Open for writing only.

O_RDWR

Open for reading and writing. The result is undefined if this flag is applied to a FIFO.

Any combination of the following may be used:

O_APPEND

If set, the file offset is set to the end of the file prior to each write.

O_CREAT

Create the file if it does not exist. This flag requires that the mode argument be specified.

If the file exists, this flag has no effect except as noted under O_EXCL below. Otherwise, the file is created with the user ID of the file set to the effective user ID of the process. The group ID of the file is set to the effective group IDs of the process, or if the S_ISGID bit is set in the directory in which the file is being created, the file's group ID is set to the group ID of its parent directory. If the group ID of the new file does not match the effective group ID or one of the supplementary groups IDs, the S_ISGID bit is cleared. The access permission bits (see <sys/stat.h>) of the file mode are set to the value of mode, modified as follows (see creat(2)): a bitwise-AND is performed on the file-mode bits and the corresponding bits in the complement of the process's file mode creation mask. Thus, all bits set in the process's file mode creation mask (see umask(2)) are correspondingly cleared in the file's permission mask. The "save text image after execution bit" of the mode is cleared (see chmod(2)). O_SYNC Write I/O operations on the file descriptor complete as defined by synchronized I/O file integrity completion (see fcntl(3HEAD) definition of O_SYNC.) When bits other than the file permission bits are set, the effect is unspecified. The mode argument does not affect whether the file is open for reading, writing or for both.

O_DSYNC

Write I/O operations on the file descriptor complete as defined by synchronized I/O data integrity completion.

O_EXCL

If O_CREAT and O_EXCL are set, open() fails if the file exists. The check for the existence of the file and the creation of the file if it does not exist is atomic with respect to other processes executing open() naming the same filename in the same directory with O_EXCL and O_CREAT set. If O_CREAT is not set, the effect is undefined.

O_LARGEFILE

If set, the offset maximum in the open file description is the largest value that can be represented correctly in an object of type off64_t.

O_NOCTTY

If set and path identifies a terminal device, open() does not cause the terminal device to become the controlling terminal for the process.

O_NONBLOCK or O_NDELAY

These flags may affect subsequent reads and writes (see read(2) and write(2)). If both O_NDELAY and O_NONBLOCK are set, O_NONBLOCK takes precedence.

When opening a FIFO with O_RDONLY or O_WRONLY set:

If O_NONBLOCK or O_NDELAY is set:

An open() for reading only returns without delay. An open() for writing only returns an error if no process currently has the file open for reading.

If O_NONBLOCK and O_NDELAY are clear:

An open() for reading only blocks until a process opens the file for writing. An open() for writing only blocks until a process opens the file for reading.

After both ends of a FIFO have been opened, there is no guarantee that further calls to open() O_RDONLY (O_WRONLY) will synchronize with later calls to open() O_WRONLY (O_RDONLY) until both ends of the FIFO have been closed by all readers and writers. Any data written into a FIFO will be lost if both ends of the FIFO are closed before the data is read.

When opening a block special or character special file that supports non-blocking opens:

If O_NONBLOCK or O_NDELAY is set:

The open() function returns without blocking for the device to be ready or available. Subsequent behavior of the device is device-specific.

If O_NONBLOCK and O_NDELAY are clear:

The open() function blocks until the device is ready or available before returning.

Otherwise, the behavior of O_NONBLOCK and O_NDELAY is unspecified.

O_RSYNC

Read I/O operations on the file descriptor complete at the same level of integrity as specified by the O_DSYNC and O_SYNC flags. If both O_DSYNC and O_RSYNC are set in oflag, all I/O operations on the file descriptor complete as defined by synchronized I/O data integrity completion. If both O_SYNC and O_RSYNC are set in oflag, all I/O operations on the file descriptor complete as defined by synchronized I/O file integrity completion.

O_SYNC

Write I/O operations on the file descriptor complete as defined by synchronized I/O file integrity completion.

O_TRUNC

If the file exists and is a regular file, and the file is successfully opened O_RDWR or O_WRONLY, its length is truncated to 0 and the mode and owner are unchanged. It has no effect on FIFO special files or terminal device files. Its effect on other file types is implementation-dependent. The result of using O_TRUNC with O_RDONLY is undefined.

O_XATTR

If set in openat(), a relative path argument is interpreted as a reference to an extended attribute of the file associated with the supplied file descriptor. This flag therefore requires the presence of a legal fildes argument. If set in open(), the implied file descriptor is that for the current working directory. Extended attributes must be referenced with a relative path; providing an absolute path results in a normal file reference.

If O_CREAT is set and the file did not previously exist, upon successful completion, open() marks for update the st_atime, st_ctime, and st_mtime fields of the file and the st_ctime and st_mtime fields of the parent directory.

If O_TRUNC is set and the file did previously exist, upon successful completion, open() marks for update the st_ctime and st_mtime fields of the file.

If path refers to a STREAMS file, oflag may be constructed from O_NONBLOCK or O_NODELAY OR-ed with either O_RDONLY, O_WRONLY, or O_RDWR. Other flag values are not applicable to STREAMS devices and have no effect on them. The values O_NONBLOCK and O_NODELAY affect the operation of STREAMS drivers and certain functions (see read(2), getmsg(2), putmsg(2), and write(2)) applied to file descriptors associated with STREAMS files. For STREAMS drivers, the implementation of O_NONBLOCK and O_NODELAY is device-specific.

When open() is invoked to open a named stream, and the connld module (see connld(7M)) has been pushed on the pipe, open() blocks until the server process has issued an I_RECVFD ioctl() (see streamio(7I)) to receive the file descriptor.

If path names the master side of a pseudo-terminal device, then it is unspecified whether open() locks the slave side so that it cannot be opened. Portable applications must call unlockpt(3C) before opening the slave side.

If path is a symbolic link and O_CREAT and O_EXCL are set, the link is not followed.

Certain flag values can be set following open() as described in fcntl(2).

The largest value that can be represented correctly in an object of type off_t is established as the offset maximum in the open file description.

Upon successful completion, the open() function opens the file and return a non-negative integer representing the lowest numbered unused file descriptor. Otherwise, -1 is returned, errno is set to indicate the error, and no files are created or modified.

The open() and openat() functions will fail if:

EACCES

Search permission is denied on a component of the path prefix, or the file exists and the permissions specified by oflag are denied, or the file does not exist and write permission is denied for the parent directory of the file to be created, or O_TRUNC is specified and write permission is denied.

EBADF

The file descriptor provided to openat() is invalid.

EDQUOT

The file does not exist, O_CREAT is specified, and either the directory where the new file entry is being placed cannot be extended because the user's quota of disk blocks on that file system has been exhausted, or the user's quota of inodes on the file system where the file is being created has been exhausted.

EEXIST

The O_CREAT and O_EXCL flags are set, and the named file exists.

EINTR

A signal was caught during open().

EFAULT

The path argument points to an illegal address.

EINVAL

The system does not support synchronized I/O for this file, or the O_XATTR flag was supplied and the underlying file system does not support extended file attributes.

EIO

The path argument names a STREAMS file and a hangup or error occurred during the open().

EISDIR

The named file is a directory and oflag includes O_WRONLY or O_RDWR.

ELOOP

Too many symbolic links were encountered in resolving path.

EMFILE

OPEN_MAX file descriptors are currently open in the calling process.

EMULTIHOP

Components of path require hopping to multiple remote machines and the file system does not allow it.

ENAMETOOLONG

The length of the path argument exceeds PATH_MAX or a pathname component is longer than NAME_MAX.

ENFILE

The maximum allowable number of files is currently open in the system.

ENOENT

The O_CREAT flag is not set and the named file does not exist; or the O_CREAT flag is set and either the path prefix does not exist or the path argument points to an empty string.

ENOLINK

The path argument points to a remote machine, and the link to that machine is no longer active.

ENOSR

The path argument names a STREAMS-based file and the system is unable to allocate a STREAM.

ENOSPC

The directory or file system that would contain the new file cannot be expanded, the file does not exist, and O_CREAT is specified.

ENOSYS

The device specified by path does not support the open operation.

ENOTDIR

A component of the path prefix is not a directory or a relative path was supplied to openat(), the O_XATTR flag was not supplied, and the file descriptor does not not refer to a directory.

ENXIO

The O_NONBLOCK flag is set, the named file is a FIFO, the O_WRONLY flag is set, and no process has the file open for reading; or the named file is a character special or block special file and the device associated with this special file does not exist.

EOPNOTSUPP

An attempt was made to open a path that corresponds to a AF_UNIX socket.

EOVERFLOW

The named file is a regular file and either O_LARGEFILE is not set and the size of the file cannot be represented correctly in an object of type off_t or O_LARGEFILE is set and the size of the file cannot be represented correctly in an object of type off64_t.

EROFS

The named file resides on a read-only file system and either O_WRONLY, O_RDWR, O_CREAT (if file does not exist), or O_TRUNC is set in the oflag argument.

The openat() function will fail if:

EBADF

The fildes argument is not a valid open file descriptor or is not AT_FTCWD.

The open() function may fail if:

EAGAIN

The path argument names the slave side of a pseudo-terminal device that is locked.

EINVAL

The value of the oflag argument is not valid.

ENAMETOOLONG

Pathname resolution of a symbolic link produced an intermediate result whose length exceeds PATH_MAX.

ENOMEM

The path argument names a STREAMS file and the system is unable to allocate resources.

ETXTBSY

The file is a pure procedure (shared text) file that is being executed and oflag is O_WRONLY or O_RDWR.

The open() function has a transitional interface for 64-bit file offsets. See lf64(5). Note that using open64() is equivalent to using open() with O_LARGEFILE set in oflag.

See attributes(5) for descriptions of the following attributes:

intro(2), chmod(2), close(2), creat(2), dup(2), exec(2), fcntl(2), getmsg(2), getrlimit(2), lseek(2), putmsg(2), read(2), stat(2), umask(2), write(2), attropen(3C), unlockpt(3C), attributes(5), fcntl(3HEAD), lf64(5), stat(3HEAD), connld(7M), streamio(7I)

Hierarchical Storage Management (HSM) file systems can sometimes cause long delays when opening a file, since HSM files must be recalled from secondary storage.