|OPEN(2)||Linux Programmer's Manual||OPEN(2)|
NAME¶open, openat, creat - open and possibly create a file
#include <sys/types.h> #include <sys/stat.h> #include <fcntl.h>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);int openat(int dirfd, const char *pathname, int flags); int openat(int dirfd, const char *pathname, int flags, mode_t mode);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
- Since glibc 2.10:
- _POSIX_C_SOURCE >= 200809L
- Before glibc 2.10:
DESCRIPTION¶Given a pathname for a file, open() returns a file descriptor, a small, nonnegative integer for use in subsequent system calls (read(2), write(2), lseek(2), fcntl(2), etc.). The file descriptor returned by a successful call will be the lowest-numbered file descriptor not currently open for the process. By default, the new file descriptor is set to remain open across an execve(2) (i.e., the FD_CLOEXEC file descriptor flag described in fcntl(2) is initially disabled); the O_CLOEXEC flag, described below, can be used to change this default. The file offset is set to the beginning of the file (see lseek(2)). A call to open() creates a new open file description, an entry in the system-wide table of open files. The open file description records the file offset and the file status flags (see below). A file descriptor is a reference to an open file description; this reference is unaffected if pathname is subsequently removed or modified to refer to a different file. For further details on open file descriptions, see NOTES. The argument flags must include one of the following access modes: O_RDONLY, O_WRONLY, or O_RDWR. These request opening the file read-only, write-only, or read/write, respectively.
- The file is opened in append mode. Before each
write(2), the file offset is positioned at the end of the file, as
if with lseek(2). The modification of the file offset and the write
operation are performed as a single atomic step.
- Enable signal-driven I/O: generate a signal (SIGIO by default, but this can be changed via fcntl(2)) when input or output becomes possible on this file descriptor. This feature is available only for terminals, pseudoterminals, sockets, and (since Linux 2.6) pipes and FIFOs. See fcntl(2) for further details. See also BUGS, below.
- O_CLOEXEC (since Linux 2.6.23)
- Enable the close-on-exec flag for the new file descriptor.
Specifying this flag permits a program to avoid additional fcntl(2)
F_SETFD operations to set the FD_CLOEXEC flag.
- If the file does not exist, it will be created.
The mode argument specifies the file mode bits be applied when a new file is created. This argument must be supplied when O_CREAT or O_TMPFILE is specified in flags; if neither O_CREAT nor O_TMPFILE is specified, then mode is ignored. The effective mode is modified by the process's umask in the usual way: in the absence of a default ACL, the mode of the created file is (mode & ~umask). Note that this mode applies only to future accesses of the newly created file; the open() call that creates a read-only file may well return a read/write file descriptor. The following symbolic constants are provided for mode:
- 00700 user (file owner) has read, write, and execute permission
- 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 permission
- 00001 others have execute permission
- According to POSIX, the effect when other bits are set in mode is unspecified. On Linux, the following bits are also honored in mode:
- 0004000 set-user-ID bit
- 0002000 set-group-ID bit (see stat(2))
- 0001000 sticky bit (see stat(2))
- O_DIRECT (since Linux 2.4.10)
- Try to minimize cache effects of the I/O to and from this
file. In general this will degrade performance, but it is useful in
special situations, such as when applications do their own caching. File
I/O is done directly to/from user-space buffers. The O_DIRECT flag
on its own makes an effort to transfer data synchronously, but does not
give the guarantees of the O_SYNC flag that data and necessary
metadata are transferred. To guarantee synchronous I/O, O_SYNC must
be used in addition to O_DIRECT. See NOTES below for further
- If pathname is not a directory, cause the open to fail. This flag was added in kernel version 2.1.126, to avoid denial-of-service problems if opendir(3) is called on a FIFO or tape device.
- Write operations on the file will complete according to the
requirements of synchronized I/O data integrity completion.
- Ensure that this call creates the file: if this flag is
specified in conjunction with O_CREAT, and pathname already
exists, then open() will fail.
- (LFS) Allow files whose sizes cannot be represented in an off_t (but can be represented in an off64_t) to be opened. The _LARGEFILE64_SOURCE macro must be defined (before including any header files) in order to obtain this definition. Setting the _FILE_OFFSET_BITS feature test macro to 64 (rather than using O_LARGEFILE) is the preferred method of accessing large files on 32-bit systems (see feature_test_macros(7)).
- O_NOATIME (since Linux 2.6.8)
- Do not update the file last access time (st_atime in
the inode) when the file is read(2).
- The effective UID of the process matches the owner UID of the file.
- The calling process has the CAP_FOWNER capability in its user namespace and the owner UID of the file has a mapping in the namespace.
- This flag is intended for use by indexing or backup programs, where its use can significantly reduce the amount of disk activity. This flag may not be effective on all filesystems. One example is NFS, where the server maintains the access time.
- If pathname refers to a terminal device—see tty(4)—it will not become the process's controlling terminal even if the process does not have one.
- If pathname is a symbolic link, then the open fails,
with the error ELOOP. Symbolic links in earlier components of the
pathname will still be followed. (Note that the ELOOP error that
can occur in this case is indistinguishable from the case where an open
fails because there are too many symbolic links found while resolving
components in the prefix part of the pathname.)
- O_NONBLOCK or O_NDELAY
- When possible, the file is opened in nonblocking mode.
Neither the open() nor any subsequent operations on the file
descriptor which is returned will cause the calling process to wait.
- O_PATH (since Linux 2.6.39)
- Obtain a file descriptor that can be used for two purposes:
to indicate a location in the filesystem tree and to perform operations
that act purely at the file descriptor level. The file itself is not
opened, and other file operations (e.g., read(2), write(2),
fchmod(2), fchown(2), fgetxattr(2), mmap(2))
fail with the error EBADF.
- Getting and setting file descriptor flags (fcntl(2) F_GETFD and F_SETFD).
- Retrieving open file status flags using the fcntl(2) F_GETFL operation: the returned flags will include the bit O_PATH.
- Passing the file descriptor as the dirfd argument of openat() and the other "*at()" system calls. This includes linkat(2) with AT_EMPTY_PATH (or via procfs using AT_SYMLINK_FOLLOW) even if the file is not a directory.
- Passing the file descriptor to another process via a UNIX domain socket (see SCM_RIGHTS in unix(7)).
- When O_PATH is specified in flags, flag bits
other than O_CLOEXEC, O_DIRECTORY, and O_NOFOLLOW are
- Write operations on the file will complete according to the
requirements of synchronized I/O file integrity completion (by
contrast with the synchronized I/O data integrity completion
provided by O_DSYNC.)
- O_TMPFILE (since Linux 3.11)
- Create an unnamed temporary file. The pathname
argument specifies a directory; an unnamed inode will be created in that
directory's filesystem. Anything written to the resulting file will be
lost when the last file descriptor is closed, unless the file is given a
char path[PATH_MAX]; fd = open("/path/to/dir", O_TMPFILE | O_RDWR, S_IRUSR | S_IWUSR); /* File I/O on 'fd'... */ snprintf(path, PATH_MAX, "/proc/self/fd/%d", fd); linkat(AT_FDCWD, path, AT_FDCWD, "/path/for/file", AT_SYMLINK_FOLLOW);
- Improved tmpfile(3) functionality: race-free creation of temporary files that (1) are automatically deleted when closed; (2) can never be reached via any pathname; (3) are not subject to symlink attacks; and (4) do not require the caller to devise unique names.
- O_TMPFILE requires support by the underlying filesystem; only a subset of Linux filesystems provide that support. In the initial implementation, support was provided in the ext2, ext3, ext4, UDF, Minix, and shmem filesystems. Support for other filesystems has subsequently been added as follows: XFS (Linux 3.15); Btrfs (Linux 3.16); F2FS (Linux 3.16); and ubifs (Linux 4.9)
- If the file already exists and is a regular file and the access mode allows writing (i.e., is O_RDWR or O_WRONLY) it will be truncated to length 0. If the file is a FIFO or terminal device file, the O_TRUNC flag is ignored. Otherwise, the effect of O_TRUNC is unspecified.
creat()¶A call to creat() is equivalent to calling open() with flags equal to O_CREAT|O_WRONLY|O_TRUNC.
openat()¶The openat() system call operates in exactly the same way as open(), except for the differences described here.
RETURN VALUE¶open(), openat(), and creat() return the new file descriptor, or -1 if an error occurred (in which case, errno is set appropriately).
ERRORS¶open(), openat(), and creat() can fail with the following errors:
- The requested access to the file is not allowed, or search permission is denied for one of the directories in the path prefix of pathname, or the file did not exist yet and write access to the parent directory is not allowed. (See also path_resolution(7).)
- Where O_CREAT is specified, the file does not exist, and the user's quota of disk blocks or inodes on the filesystem has been exhausted.
- pathname already exists and O_CREAT and O_EXCL were used.
- pathname points outside your accessible address space.
- See EOVERFLOW.
- While blocked waiting to complete an open of a slow device (e.g., a FIFO; see fifo(7)), the call was interrupted by a signal handler; see signal(7).
- The filesystem does not support the O_DIRECT flag. See NOTES for more information.
- Invalid value in flags.
- O_TMPFILE was specified in flags, but neither O_WRONLY nor O_RDWR was specified.
- pathname refers to a directory and the access requested involved writing (that is, O_WRONLY or O_RDWR is set).
- pathname refers to an existing directory, O_TMPFILE and one of O_WRONLY or O_RDWR were specified in flags, but this kernel version does not provide the O_TMPFILE functionality.
- Too many symbolic links were encountered in resolving pathname.
- pathname was a symbolic link, and flags specified O_NOFOLLOW but not O_PATH.
- The per-process limit on the number of open file descriptors has been reached (see the description of RLIMIT_NOFILE in getrlimit(2)).
- pathname was too long.
- The system-wide limit on the total number of open files has been reached.
- pathname refers to a device special file and no corresponding device exists. (This is a Linux kernel bug; in this situation ENXIO must be returned.)
- O_CREAT is not set and the named file does not exist. Or, a directory component in pathname does not exist or is a dangling symbolic link.
- pathname refers to a nonexistent directory, O_TMPFILE and one of O_WRONLY or O_RDWR were specified in flags, but this kernel version does not provide the O_TMPFILE functionality.
- The named file is a FIFO, but memory for the FIFO buffer can't be allocated because the per-user hard limit on memory allocation for pipes has been reached and the caller is not privileged; see pipe(7).
- Insufficient kernel memory was available.
- pathname was to be created but the device containing pathname has no room for the new file.
- A component used as a directory in pathname is not, in fact, a directory, or O_DIRECTORY was specified and pathname was not a directory.
- O_NONBLOCK | O_WRONLY is set, the named file is a FIFO, and no process has the FIFO open for reading.
- The file is a device special file and no corresponding device exists.
- The filesystem containing pathname does not support O_TMPFILE.
- pathname refers to a regular file that is too large to be opened. The usual scenario here is that an application compiled on a 32-bit platform without -D_FILE_OFFSET_BITS=64 tried to open a file whose size exceeds (1<<31)-1 bytes; see also O_LARGEFILE above. This is the error specified by POSIX.1; in kernels before 2.6.24, Linux gave the error EFBIG for this case.
- The O_NOATIME flag was specified, but the effective user ID of the caller did not match the owner of the file and the caller was not privileged.
- The operation was prevented by a file seal; see fcntl(2).
- pathname refers to a file on a read-only filesystem and write access was requested.
- pathname refers to an executable image which is currently being executed and write access was requested.
- The O_NONBLOCK flag was specified, and an incompatible lease was held on the file (see fcntl(2)).
- dirfd is not a valid file descriptor.
- pathname is a relative pathname and dirfd is a file descriptor referring to a file other than a directory.
VERSIONS¶openat() was added to Linux in kernel 2.6.16; library support was added to glibc in version 2.4.
CONFORMING TO¶open(), creat() SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.
NOTES¶Under Linux, the O_NONBLOCK flag indicates that one wants to open but does not necessarily have the intention to read or write. This is typically used to open devices in order to get a file descriptor for use with ioctl(2). The (undefined) effect of O_RDONLY | O_TRUNC varies among implementations. On many systems the file is actually truncated.
Open file descriptions¶The term open file description is the one used by POSIX to refer to the entries in the system-wide table of open files. In other contexts, this object is variously also called an "open file object", a "file handle", an "open file table entry", or—in kernel-developer parlance—a struct file.
Synchronized I/O¶The POSIX.1-2008 "synchronized I/O" option specifies different variants of synchronized I/O, and specifies the open() flags O_SYNC, O_DSYNC, and O_RSYNC for controlling the behavior. Regardless of whether an implementation supports this option, it must at least support the use of O_SYNC for regular files.
NFS¶There are many infelicities in the protocol underlying NFS, affecting amongst others O_SYNC and O_NDELAY.
FIFOs¶Opening the read or write end of a FIFO blocks until the other end is also opened (by another process or thread). See fifo(7) for further details.
File access mode¶Unlike the other values that can be specified in flags, the access mode values O_RDONLY, O_WRONLY, and O_RDWR do not specify individual bits. Rather, they define the low order two bits of flags, and are defined respectively as 0, 1, and 2. In other words, the combination O_RDONLY | O_WRONLY is a logical error, and certainly does not have the same meaning as O_RDWR.
Rationale for openat() and other directory file descriptor APIs¶openat() and the other system calls and library functions that take a directory file descriptor argument (i.e., execveat(2), faccessat(2), fanotify_mark(2), fchmodat(2), fchownat(2), fstatat(2), futimesat(2), linkat(2), mkdirat(2), mknodat(2), name_to_handle_at(2), readlinkat(2), renameat(2), symlinkat(2), unlinkat(2), utimensat(2), mkfifoat(3), and scandirat(3)) are supported for two reasons. Here, the explanation is in terms of the openat() call, but the rationale is analogous for the other interfaces.
O_DIRECT¶The O_DIRECT flag may impose alignment restrictions on the length and address of user-space buffers and the file offset of I/Os. In Linux alignment restrictions vary by filesystem and kernel version and might be absent entirely. However there is currently no filesystem-independent interface for an application to discover these restrictions for a given file or filesystem. Some filesystems provide their own interfaces for doing so, for example the XFS_IOC_DIOINFO operation in xfsctl(3). Under Linux 2.4, transfer sizes, and the alignment of the user buffer and the file offset must all be multiples of the logical block size of the filesystem. Since Linux 2.6.0, alignment to the logical block size of the underlying storage (typically 512 bytes) suffices. The logical block size can be determined using the ioctl(2) BLKSSZGET operation or from the shell using the command:
blockdev --getss O_DIRECT I/Os should never be run concurrently with the fork(2) system call, if the memory buffer is a private mapping (i.e., any mapping created with the mmap(2) MAP_PRIVATE flag; this includes memory allocated on the heap and statically allocated buffers). Any such I/Os, whether submitted via an asynchronous I/O interface or from another thread in the process, should be completed before fork(2) is called. Failure to do so can result in data corruption and undefined behavior in parent and child processes. This restriction does not apply when the memory buffer for the O_DIRECT I/Os was created using shmat(2) or mmap(2) with the MAP_SHARED flag. Nor does this restriction apply when the memory buffer has been advised as MADV_DONTFORK with madvise(2), ensuring that it will not be available to the child after fork(2). The O_DIRECT flag was introduced in SGI IRIX, where it has alignment restrictions similar to those of Linux 2.4. IRIX has also a fcntl(2) call to query appropriate alignments, and sizes. FreeBSD 4.x introduced a flag of the same name, but without alignment restrictions. O_DIRECT support was added under Linux in kernel version 2.4.10. Older Linux kernels simply ignore this flag. Some filesystems may not implement the flag and open() will fail with EINVAL if it is used. Applications should avoid mixing O_DIRECT and normal I/O to the same file, and especially to overlapping byte regions in the same file. Even when the filesystem correctly handles the coherency issues in this situation, overall I/O throughput is likely to be slower than using either mode alone. Likewise, applications should avoid mixing mmap(2) of files with direct I/O to the same files. The behavior of O_DIRECT with NFS will differ from local filesystems. Older kernels, or kernels configured in certain ways, may not support this combination. The NFS protocol does not support passing the flag to the server, so O_DIRECT I/O will bypass the page cache only on the client; the server may still cache the I/O. The client asks the server to make the I/O synchronous to preserve the synchronous semantics of O_DIRECT. Some servers will perform poorly under these circumstances, especially if the I/O size is small. Some servers may also be configured to lie to clients about the I/O having reached stable storage; this will avoid the performance penalty at some risk to data integrity in the event of server power failure. The Linux NFS client places no alignment restrictions on O_DIRECT I/O. In summary, O_DIRECT is a potentially powerful tool that should be used with caution. It is recommended that applications treat use of O_DIRECT as a performance option which is disabled by default.
"The thing that has always disturbed me about O_DIRECT is that the whole interface is just stupid, and was probably designed by a deranged monkey on some serious mind-controlling substances."—Linus
BUGS¶Currently, it is not possible to enable signal-driven I/O by specifying O_ASYNC when calling open(); use fcntl(2) to enable this flag.
SEE ALSO¶chmod(2), chown(2), close(2), dup(2), fcntl(2), link(2), lseek(2), mknod(2), mmap(2), mount(2), open_by_handle_at(2), read(2), socket(2), stat(2), umask(2), unlink(2), write(2), fopen(3), acl(5), fifo(7), path_resolution(7), symlink(7)
COLOPHON¶This page is part of release 4.10 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.