open(2)
NAME
open, openat - open a file
SYNOPSIS
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int open(const char *path, int oflag, /* mode_t mode */...);
int openat(int fildes, const char *path, int oflag, /*
mode_t mode */...);
DESCRIPTION
The open() function establishes the connection between a
file and a file descriptor. It creates an open file descrip-
tion 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 argu-
ment has the special value AT_FDCWD, a relative path argu-
ment will be resolved relative to the current working direc-
tory. 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 effec-
tive 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 syn-
chronized 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 comple-
tion.
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 descrip-
tion 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 con-
trolling 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 read-
ing.
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 comple-
tion.
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 inter-
preted 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 associ-
ated with STREAMS files. For STREAMS drivers, the implemen-
tation 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.
RETURN VALUES
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.
ERRORS
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 speci-
fied by oflag are denied, or the file does not exist
and write permission is denied for the parent direc-
tory of the file to be created, or O_TRUNC is speci-
fied 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 under-
lying 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 associ-
ated 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 descrip-
tor 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.
USAGE
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.
When a UFS file system is mounted with logging enabled, file
system transactions that free blocks from files might not
actually add those freed blocks to the file system's free
list until some unspecified time in the future. This
behavior improves file system performance but does not con-
form to the POSIX, Single UNIX Specification, SPARC Confor-
mance Definition, System V Application Binary Interface,
System V Interface Definition, and X/Open Portability Guide
Standards, which require that freed space be available
immediately. To enable standards conformance regarding file
deletions or to address the problem of not being able to
grow files on a relatively full UFS file system even after
files have been deleted, disable UFS logging (see
mount_ufs(1M).
ATTRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
____________________________________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|_____________________________|_____________________________|
| Interface Stability | See below. |
|_____________________________|_____________________________|
| MT-Level | Async-Signal-Safe |
|_____________________________|_____________________________|
The open() function is Standard;. The openat() function is
Evolving.
SEE ALSO
mount_ufs(1M), 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)
NOTES
Hierarchical Storage Management (HSM) file systems can some-
times cause long delays when opening a file, since HSM files
must be recalled from secondary storage.
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