open(2) manual page
Table of Contents
open - open for reading or writing
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int open(const char *path, int oflag, /* mode_t mode */ ...);
Async-Signal-Safe
open() opens a file descriptor for the file with the path
name pointed to by path, and sets the file status flags according to the
value of oflag. oflag values are constructed by OR
-ing Flags from the following
list (only one of the first three flags below may be used):
- O_RDONLY
- Open
for reading only.
- O_WRONLY
- Open for writing only.
- O_RDWR
- Open for reading
and writing.
- O_NDELAY
- or O_NONBLOCK
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
will take precedence.
- When opening a
- FIFO
with O_RDONLY
or O_WRONLY
set:
- If
- O_NDELAY
or O_NONBLOCK
is set: An open for reading-only
will return without delay; an open for writing-only will return an error
if no process currently has the file open for reading.
- If
- O_NDELAY
and
O_NONBLOCK
are clear: An open for reading-only will block until a process
opens the file for writing; an open for writing-only will block until a
process opens the file for reading.
- When opening a file associated with
a terminal line:
- If
- O_NDELAY
or O_NONBLOCK
is set: The open will return
without waiting for the device to be ready or available; subsequent behavior
of the device is device specific.
- If
- O_NDELAY
and O_NONBLOCK
are clear:
The open will block until the device is ready or available.
- O_APPEND
- If
set, the file pointer will be set to the end of the file prior to each
write.
- O_DSYNC
- Write I/O
operations on the file descriptor complete as
defined by synchronized I/O
data integrity completion.
- 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
- When opening a regular file, this flag affects subsequent
writes. If set, each write(2)
will wait for both the file data and file
status to be physically updated. Write I/O
operations on the file descriptor
complete as defined by synchronized I/O
file integrity completion.
- O_NOCTTY
- If set and the file is a terminal, the terminal will not be allocated
as the calling process’s controlling terminal.
- O_CREAT
- If the file exists,
this flag has no effect, except as noted under O_EXCL
below. Otherwise,
the file is created and the owner ID
of the file is set to the effective
user ID
of the process, the group ID
of the file is set to the effective
group ID
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 ID
s,
the S_ISGID
bit is cleared. The access permission bits of the file mode
are set to the value of mode, modified as follows (see creat(2)
):
- All bits
set in the file mode creation mask of the process are cleared
- (see umask(2)
).
- 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(5)
definition
of O_SYNC
- O_EXCL
- If O_EXCL
and O_CREAT
are set, open() will fail 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.
- O_TRUNC
- If the file exists, its length is truncated
to 0 and the mode and owner are unchanged. O_TRUNC
has no effect on FIFO
special files or directories.
When opening a STREAMS
file, oflag may be
constructed from O_NDELAY
or O_NONBLOCK
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 of O_NDELAY
and O_NONBLOCK
affect
the operation of STREAMS
drivers and certain functions (see read(2)
, getmsg(2)
,
putmsg(2)
, and write(2)
). For drivers, the implementation of O_NDELAY
and O_NONBLOCK
is device specific. Each STREAMS
device driver may treat
these options differently.
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 is a symbolic link and O_CREAT
and O_EXCL
are set, the link is not followed.
The file pointer used to
mark the current position within the file is set to the beginning of the
file.
The new file descriptor is the lowest numbered file descriptor available
and is set to remain open across exec functions (see fcntl(2)
).
Certain
flag values can be set following open() as described in fcntl(2)
.
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.
Upon
successful completion, the file descriptor is returned. Otherwise, a value
of -1
is returned and errno is set to indicate the error.
The named
file is opened unless one or more of the following are true:
- EACCES
- The
file does not exist and write permission is denied by the parent directory
of the file to be created.
- O_TRUNC
- is specified and write permission is
denied
- A component of the
- path prefix denies search permission.
- oflag
- permission is denied for an existing file.
- EAGAIN
- If the file exists with
enforced record locking enabled, record locks are on the file (see chmod(2)
),
and O_TRUNC
is specified.
- EDQUOT
- The file does not exist, O_CREAT
is
specified, and 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.
- The file does not exist,
- O_CREAT
is specified, and
the user’s quota of inodes on the file system where the file is being created
has been exhausted.
- EEXIST
- O_CREAT
and O_EXCL
are set, and the named
file exists.
- EFAULT
- path points to an illegal address.
- EINTR
- A signal was
caught during the open() function.
- EIO
- A hangup or error occurred during
the open of the STREAMS
-based device.
- EISDIR
- The named file is a directory
and oflag is write or read/write.
- ELOOP
- Too many symbolic links were encountered
in translating path.
- EMFILE
- The process has too many open files (see getrlimit(2)
).
- 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 the length of a path component exceeds
{NAME_MAX} while {_POSIX_NO_TRUNC} is in effect.
- ENFILE
- The system file
table is full.
- ENOENT
- O_CREAT
is not set and the named file does not exist.
- O_CREAT
- is set and a component of the path prefix does not exist or
is the null pathname.
- ENOLINK
- path points to a remote machine, and the
link to that machine is no longer active.
- ENOMEM
- The system is unable
to allocate a send descriptor.
- ENOSPC
- O_CREAT
and O_EXCL
are set, and
the file system is out of inodes.
- O_CREAT
- cannot be extended.
- ENOSR
- Unable
to allocate a stream.
- ENOTDIR
- A component of the path prefix is not a directory.
- ENXIO
- The named file is a character special or block special file, and
the device associated with this special file does not exist.
- O_NDELAY
- or
O_NONBLOCK
is set, the named file is a FIFO
, O_WRONLY
is set, and no
process has the file open for reading.
- A STREAMS module or driver open routine
failed.
-
- EROFS
- The named file resides on a read-only file system and either
O_WRONLY,
O_RDWR,
O_CREAT,
or O_TRUNC
is set in oflag (if the file
does not exist).
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)
, fcntl(5)
, stat(5)
, connld(7M)
, streamio(7I)
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