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OPEN(2) | System Calls Manual | OPEN(2) |
NAME¶
open, openat — open or create a file for reading, writing or executingLIBRARY¶
Standard C Library (libc, -lc)SYNOPSIS¶
#include <fcntl.h> intopen(const char *path, int flags, ...); int
openat(int fd, const char *path, int flags, ...);
DESCRIPTION¶
The file name specified by path is opened for either execution or reading and/or writing as specified by the argument flags and the file descriptor returned to the calling process. The flags argument may indicate the file is to be created if it does not exist (by specifying theO_CREAT
flag). In this case open()
and openat() require an additional 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 behavior 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_EXEC open for execute only 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_SYNC synchronous writes O_NOFOLLOW do not follow symlinks O_NOCTTY don't assign controlling terminal O_TTY_INIT restore default terminal attributes
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 and the open() system call would result in
the process being blocked for some reason (e.g., waiting for carrier on a
dialup line), open() returns immediately. The descriptor
remains in non-blocking mode for subsequent operations.
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.
O_SYNC
is a synonym for O_FSYNC
required by POSIX.
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 file system supports locking).
O_DIRECT
may be used to minimize or eliminate the cache
effects of reading 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.
O_NOCTTY
may be used to ensure the OS does not assign
this file as the controlling terminal when it opens a tty device. This is the
default on FreeBSD, but is present for POSIX
compatibility. The open() system call will not assign
controlling terminals on FreeBSD.
O_TTY_INIT
may be used to ensure the OS restores the
terminal attributes when initially opening a TTY. This is the default on
FreeBSD, but is present for POSIX compatibility. The
initial call to open() on a TTY will always restore default
terminal attributes on FreeBSD.
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. The getdtablesize(2) system call 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.
- [
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_TRUNC
is specified and write permission is denied.- [
EACCES
] O_CREAT
is specified, the file does not exist, and the directory in which it is to be created does not permit writing.- [
EPERM
] O_CREAT
is specified, the file does not exist, and the directory in which it is to be created has its immutable flag set, see the chflags(2) manual page for more information.- [
EPERM
] The named file has its immutable flag set and the file is to be modified.
- [
EPERM
] The named file has its append-only flag set, the file is to be modified, and
O_TRUNC
is specified orO_APPEND
is not specified.- [
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 modified.
- [
EROFS
] - The named file resides on a read-only file system, and the file is to be modified.
- [
EROFS
] O_CREAT
is specified and the named file would reside on a read-only file system.- [
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
] O_NONBLOCK
is set, the named file is a fifo,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
orO_EXLOCK
is specified but the underlying file system does not support locking.- [
EOPNOTSUPP
] - The named file is a special file mounted through a file system that does not support access to it (e.g. NFS).
- [
EWOULDBLOCK
] O_NONBLOCK
and one ofO_SHLOCK
orO_EXLOCK
is specified 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 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() system call requests write access.
- [
EFAULT
] - The path argument points outside the process's allocated address space.
- [
EEXIST
] O_CREAT
andO_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 illegal
combination of
O_RDONLY
,O_WRONLY
,O_RDWR
andO_EXEC.
- [Eq EBADF]
- The path argument does not specify an
absolute path and the fd argument is neither
AT_FDCWD
nor a valid file descriptor open for searching. - [Eq ENOTDIR]
- The path argument is not an absolute
path and fd is neither
AT_FDCWD
nor a file descriptor associated with a directory.
SEE ALSO¶
chmod(2), close(2), dup(2), fexecve(2), fhopen(2), getdtablesize(2), getfh(2), lgetfh(2), lseek(2), read(2), umask(2), write(2), fopen(3)HISTORY¶
The open() function appeared in Version 6 AT&T UNIX. The openat() function was introduced in FreeBSD 8.0.BUGS¶
The Open Group Extended API Set 2 specification requires that the test for whether fd is searchable is based on whether fd is open for searching, not whether the underlying directory currently permits searches. The present implementation of the openat checks the current permissions of directory instead.February 28, 2009 | Debian |