open - open or create a file for reading or writing
Standard C Library (libc, -lc)
#include <fcntl.h>
int
open(const char *path, int flags, mode_t mode);
The file name specified by path is opened for reading and/or writing as
specified by the argument flags and the file descriptor returned to the
calling process. The flags are specified by or'ing the following values.
Applications must specify exactly one of the first three values (file
access modes):
O_RDONLY Open for reading only.
O_WRONLY Open for writing only.
O_RDWR Open for reading and writing.
Any combination of the following may be used:
O_NONBLOCK Do not block on open or for data to become available.
O_APPEND Append to the file on each write.
O_CREAT Create the file if it does not exist, in which case the
file is created with mode mode as described in chmod(2)
and modified by the process' umask value (see
umask(2)).
O_TRUNC Truncate size to 0.
O_EXCL Error if O_CREAT and the file already exists.
O_SHLOCK Atomically obtain a shared lock.
O_EXLOCK Atomically obtain an exclusive lock.
O_DSYNC If set, write operations will be performed according to
synchronized I/O data integrity completion: each write
will wait for the file data to be committed to stable
storage.
O_SYNC If set, write operations will be performed according to
synchronized I/O file integrity completion: each write
will wait for both the file data and file status to be
committed to stable storage.
O_RSYNC If set, read operations will complete at the same level
of integrity which is in effect for write operations:
if specified together with O_SYNC, each read will wait
for the file status to be committed to stable storage.
Combining O_RSYNC with O_DSYNC only, or specifying it
without any other synchronized I/O integrity completion
flag set, has no further effect.
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 implement a simple exclusive access locking
mechanism. If O_EXCL is set 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 making
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 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(2) system calls;
see close(2) and fcntl(2).
The system imposes a limit on the number of file descriptors open simultaneously
by one process. Calling getdtablesize(3) returns the current
system limit.
The named file is opened unless:
[ENOTDIR] A component of the path prefix is not a directory.
[ENAMETOOLONG] A component of a pathname exceeded {NAME_MAX} characters,
or an entire path name exceeded {PATH_MAX} characters.
[ENOENT] O_CREAT is not set and the named file does not exist,
or a component of the path name that must exist does
not exist.
[EACCES] Search permission is denied for a component of the
path prefix, the required permissions (for reading
and/or writing) are denied for the given flags, or
O_CREAT is specified, the file does not exist, and the
directory in which it is to be created does not permit
writing.
[ELOOP] Too many symbolic links were encountered in translating
the pathname.
[EISDIR] The named file is a directory, and the arguments specify
it is to be opened for writing.
[EROFS] The named file resides on a read-only file system, and
the file is to be modified.
[EMFILE] The process has already reached its limit for open
file descriptors.
[ENFILE] The system file table is full.
[ENXIO] The named file is a character special or block special
file, and the device associated with this special file
does not exist, or the named file is a FIFO,
O_NONBLOCK and O_WRONLY is set and no process has the
file open for reading.
[EINTR] The open() operation was interrupted by a signal.
[EOPNOTSUPP] O_SHLOCK or O_EXLOCK is specified but the underlying
filesystem does not support locking.
[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.
[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 directory
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.
[EIO] An I/O error occurred while making the directory entry
or allocating the inode for O_CREAT.
[ETXTBSY] The file is a pure procedure (shared text) file that
is being executed and the open() call requests write
access.
[EFAULT] path points outside the process's allocated address
space.
[EEXIST] O_CREAT and O_EXCL were specified and the file exists.
[EOPNOTSUPP] An attempt was made to open a socket (not currently
implemented).
chmod(2), close(2), dup(2), lseek(2), read(2), umask(2), write(2),
getdtablesize(3)
The open() function conforms to ISO/IEC 9945-1:1990 (``POSIX.1''). The
flags values O_DSYNC, O_SYNC and O_RSYNC are extensions defined in IEEE
Std 1003.1b-1993 (``POSIX.1'').
An open() function call appeared in Version 6 AT&T UNIX.
BSD November 16, 1993 BSD
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