open, creat - open and possibly create a file or device
int open(const char *pathname, int flags));
int open(const char *pathname, int flags, mode_t mode
int creat(const char *pathname, mode_t mode));
open attempts to open a file and return a file descriptor (a
small, non-negative integer for use in read, write, etc.)
flags is one of O_RDONLY, O_WRONLY or O_RDWR which request
opening the file read-only, write-only or read/write,
flags may also be bitwise-or'd with one or more of the fol-
O_CREAT If the file does not exist it will be created.
O_EXCL When used with O_CREAT, if the file already exists
it is an error and the open will fail. O_EXCL is
broken on NFS file systems, programs which rely on
it for performing locking tasks will contain a race
condition. The solution for performing atomic file
locking using a lockfile is to create a unique file
on the same fs (e.g., incorporating hostname and
pid), use link(2) to make a link to the lockfile and
use stat(2) on the unique file to check if its link
count has increased to 2. Do not use the return
value of the link() call.
If pathname refers to a terminal device - see tty(4)
- it will not become the process's controlling ter-
minal even if the process does not have one.
O_TRUNC If the file already exists it will be truncated.
The file is opened in append mode. Initially, and
before each write, the file pointer is positioned at
the end of the file, as if with lseek. O_APPEND may
lead to corrupted files on NFS file systems if more
than one process appends data to a file at once.
This is because NFS does not support appending to a
file, so the client kernel has to simulate it, which
can't be done without a race condition.
O_NONBLOCK or O_NDELAY
The file is opened in non-blocking mode. Neither the
open nor any subsequent operations on the file
descriptor which is returned will cause the calling
process to wait.
O_SYNC The file is opened for synchronous I/O. Any writes
on the resulting file descriptor will block the cal-
ling process until the data has been physically
written to the underlying hardware. See RESTRIC-
TIONS below, though.
Some of these optional flags can be altered using fcntl
after the file has been opened.
mode specifies the permissions to use if a new file is
created. It is modified by the process's umask in the usual
way: the permissions of the created file are (mode &
The following symbolic constants are provided for mode:
00700 user (file owner) has read, write and execute
00400 user has read permission
00200 user has write permission
00100 user has execute permission
00070 group has read, write and execute permission
00040 group has read permission
00020 group has write permission
00010 group has execute permission
00007 others have read, write and execute permission
00004 others have read permission
00002 others have write permisson
00001 others have execute permission
mode should always be specified when O_CREAT is in the
flags, and is ignored otherwise.
creat is equivalent to open with flags equal to
open and creat return the new file descriptor, or -1 if an
error occurred (in which case, errno is set appropriately).
Note that open can open device special files, but creat can-
not create them - use mknod(2) instead.
On NFS file systems with UID mapping enabled, open may
return a file descriptor but e.g. read(2) requests are
denied with EACCES. This is because the client performs
open by checking the permissions, but UID mapping is per-
formed by the server upon read and write requests.
EEXIST pathname already exists and O_CREAT and O_EXCL were
EISDIR pathname refers to a directory and the access
requested involved writing.
ETXTBSY pathname refers to an executable image which is
currently being executed and write access was
EFAULT pathname points outside your accessible address
EACCES The requested access to the file is not allowed, or
one of the directories in pathname did not allow
search (execute) permission.
pathname was too long.
ENOENT A directory component in pathname does not exist or
is a dangling symbolic link.
ENOTDIR A component used as a directory in pathname is not,
in fact, a directory.
EMFILE The process already has the maximum number of files
ENFILE The limit on the total number of files open on the
system has been reached.
ENOMEM Insufficient kernel memory was available.
EROFS pathname refers to a file on a read-only filesystem
and write access was requested.
ELOOP Too many symbolic links were encountered in resolv-
ENOSPC pathname was to be created but the device containing
pathname has no room for the new file.
SVr4, SVID, POSIX, X/OPEN, BSD 4.3
There are many infelicities in the protocol underlying NFS,
affecting amongst others O_SYNC and O_NDELAY.
read(2), write(2), fcntl(2), unlink(2), mknod(2), stat(2),
mount(2), socket(2), socket(2), link(2).