OPEN(2) BSD System Calls Manual OPEN(2)NAMEopen — open or create a file for reading or writing
SYNOPSIS
#include <fcntl.h>
int
open(const char *path, int oflag, ...);
DESCRIPTION
The file name specified by path is opened for reading and/or writing, as
specified by the argument oflag; the file descriptor is returned to the
calling process.
The oflag argument may indicate that the file is to be created if it does
not exist (by specifying the O_CREAT flag). In this case, open requires
a third argument mode_t mode; the file is created with mode mode as
described in chmod(2) and modified by the process' umask value (see
umask(2)).
The flags specified are formed by or'ing the following values:
O_RDONLY open for reading only
O_WRONLY open for writing only
O_RDWR open for reading and writing
O_NONBLOCK do not block on open or for data to become available
O_APPEND append on each write
O_CREAT create file if it does not exist
O_TRUNC truncate size to 0
O_EXCL error if O_CREAT and the file exists
O_SHLOCK atomically obtain a shared lock
O_EXLOCK atomically obtain an exclusive lock
O_NOFOLLOW do not follow symlinks
O_SYMLINK allow open of symlinks
O_EVTONLY descriptor requested for event notifications only
O_CLOEXEC mark as close-on-exec
Opening a file with O_APPEND set causes each write on the file to be
appended to the end. If O_TRUNC is 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 imple‐
ment a simple exclusive-access locking mechanism. If O_EXCL is set with
O_CREAT 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 mak‐
ing all subsequent I/O on the open file non-blocking.
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 O_NOFOLLOW is used in the mask and the target file passed to open() is
a symbolic link then the open() will fail.
If O_SYMLINK is used in the mask and the target file passed to open() is
a symbolic link then the open() will be for the symbolic link itself, not
what it links to.
The O_EVTONLY flag is only intended for monitoring a file for changes
(e.g. kqueue). Note: when this flag is used, the opened file will not
prevent an unmount of the volume that contains the file.
The O_CLOEXEC flag causes the file descriptor to be marked as close-on-
exec, setting the FD_CLOEXEC flag. The state of the file descriptor
flags can be inspected using the F_GETFD fcntl. See fcntl(2).
If successful, open() returns a non-negative integer, termed a file
descriptor. It returns -1 on failure. 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 system calls; see
close(2) and fcntl(2).
The system imposes a limit on the number of file descriptors that can be
held open simultaneously by one process. Getdtablesize(2) returns the
current system limit.
RETURN VALUES
If successful, open() returns a non-negative integer, termed a file
descriptor. It returns -1 on failure, and sets errno to indicate the
error.
ERRORS
The named file is opened unless:
[EACCES] Search permission is denied for a component of the
path prefix.
[EACCES] The required permissions (for reading and/or writing)
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.
[EACCES] O_TRUNC is specified and write permission is denied.
[EAGAIN] path specifies the slave side of a locked pseudo-ter‐
minal device.
[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 direc‐
tory 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.
[EEXIST] O_CREAT and O_EXCL are specified and the file exists.
[EFAULT] Path points outside the process's allocated address
space.
[EINTR] The open() operation is interrupted by a signal.
[EINVAL] The value of oflag is not valid.
[EIO] An I/O error occurs while making the directory entry
or allocating the inode for O_CREAT.
[EISDIR] The named file is a directory, and the arguments spec‐
ify that it is to be opened for writing.
[ELOOP] Too many symbolic links are encountered in translating
the pathname. This is taken to be indicative of a
looping symbolic link.
[EMFILE] The process has already reached its limit for open
file descriptors.
[ENAMETOOLONG] A component of a pathname exceeds {NAME_MAX} charac‐
ters, or an entire path name exceeded {PATH_MAX} char‐
acters.
[ENFILE] The system file table is full.
[ELOOP] O_NOFOLLOW was specified and the target is a symbolic
link.
[ENOENT] O_CREAT is not set and the named file does not exist.
[ENOENT] A component of the path name that must exist does not
exist.
[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 directory.
[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.
[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.
[ENXIO] O_NONBLOCK and O_WRONLY are set, the file is a FIFO,
and no process has it open for reading.
[EOPNOTSUPP] O_SHLOCK or O_EXLOCK is specified, but the underlying
filesystem does not support locking.
[EOPNOTSUPP] An attempt is made to open a socket (not currently
implemented).
[EOVERFLOW] The named file is a regular file and its size does not
fit in an object of type off_t.
[EROFS] The named file resides on a read-only file system, and
the file is to be modified.
[ETXTBSY] The file is a pure procedure (shared text) file that
is being executed and the open() call requests write
access.
COMPATIBILITYopen() on a terminal device (i.e., /dev/console) will now make that
device a controlling terminal for the process. Use the O_NOCTTY flag to
open a terminal device without changing your controlling terminal.
SEE ALSOchmod(2), close(2), dup(2), getdtablesize(2), lseek(2), read(2),
umask(2), write(2)HISTORY
An open() function call appeared in Version 6 AT&T UNIX.
4th Berkeley Distribution November 10, 2010 4th Berkeley Distribution
