OPEN(2) BSD System Calls Manual OPEN(2)NAME
open, openat — open or create a file for reading or writing
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <fcntl.h>
int
open(const char *path, int flags, ...);
int
openat(int fd, const char *path, int flags, ...);
DESCRIPTION
The file name specified by path is opened for reading and/or writing as
specified by the argument flags and the lowest unused file descriptor in
the process' file descriptor table is returned. The flags argument may
indicate the file is to be created if it does not exist (by specifying
the O_CREAT flag). In this case open() and openat() require a third
argument mode_t mode, and the file is created with mode mode as described
in chmod(2) and modified by the process' umask value (see umask(2)).
The openat() function is equivalent to the open() function except in the
case where the path specifies a relative path. In this case the file to
be opened is determined relative to the directory associated with the
file descriptor fd instead of the current working directory. The flag
parameter and the optional fourth parameter correspond exactly to the
parameters of open(). If openat() is passed the special value AT_FDCWD
in the fd parameter, the current working directory is used and the behav‐
ior is identical to a call to open().
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
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 create and file exists
O_SHLOCK atomically obtain a shared lock
O_EXLOCK atomically obtain an exclusive lock
O_DIRECT eliminate or reduce cache effects
O_FSYNC synchronous writes
O_NOFOLLOW do not follow symlinks
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 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 and 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. The first time the process
attempts to perform I/O on the open file it will block (not currently
implemented).
If O_FSYNC is used in the mask, all writes will immediately be written to
disk, the kernel will not cache written data and all writes on the
descriptor will not return until the data to be written completes.
If O_NOFOLLOW is used in the mask and the target file passed to open() is
a symbolic link then the open() will fail.
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).
O_DIRECT may be used to minimize or eliminate the cache effects of read‐
ing and writing. The system will attempt to avoid caching the data you
read or write. If it cannot avoid caching the data, it will minimize the
impact the data has on the cache. Use of this flag can drastically
reduce performance if not used with care.
If successful, open() and openat() return 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 simul‐
taneously by one process. Getdtablesize(2) returns the current system
limit.
RETURN VALUES
If successful, open() and openat() return a non-negative integer, termed
a file descriptor. They return -1 on failure, and set errno to indicate
the error.
ERRORS
The named file is opened unless:
[ENOTDIR] A component of the path prefix is not a directory or
the path argument is not an absolute path and the fd
argument is neither AT_FDCWD nor a file descriptor
associated with a directory.
[ENAMETOOLONG] A component of a pathname exceeded 255 characters, or
an entire path name exceeded 1023 characters.
[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.
[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.
[ELOOP] Too many symbolic links were encountered in translat‐
ing the pathname.
[EISDIR] The named file is a directory, and the arguments spec‐
ify 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.
[EMLINK] O_NOFOLLOW was specified and the target is a symbolic
link.
[ENXIO] The named file is a character special or block special
file, and the device associated with this special file
does not exist.
[ENXIO] The named file is a fifo, no process has it open for
reading, and the arguments specify it is to be opened
for writing.
[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.
[EWOULDBLOCK] O_NONBLOCK and one of O_SHLOCK or O_EXLOCK is speci‐
fied and the file is locked.
[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 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.
[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).
[EINVAL] An attempt was made to open a descriptor with an ille‐
gal combination of O_RDONLY, O_WRONLY, and O_RDWR.
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. An openat()
function call appeared first in Solaris and was ported to DragonFly 2.3.
BUGS
The Open Group Extended API Set 2 specification requires that the test
for fd's searchability is based on whether it is open for searching, and
not whether the underlying directory currently permits searches. The
present implementation of openat() checks the current permissions of
directory instead.
BSD July 24, 2009 BSD
