open pathname flags &optional mode => file-descriptor, error
pathname -- A string.
flags -- A non-negative fixnum.
mode -- A non-negative fixnum.
file-descriptor -- A non-negative fixnum.
error -- A fixnum.
Given a pathname for a file, open returns a file descriptor, a small, nonnegative integer for use in subsequent system calls (read, write, lseek, fcntl, 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 (i.e., the FD_CLOEXEC file descriptor flag described in fcntl 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).
A call to open creates a new open file description, an entry in the system-wide table of open files. This entry records the file offset and the file status flags (modifiable via the fcntl F_SETFL operation). A file descriptor is a reference to one of these entries; this reference is unaffected if pathname is subsequently removed or modified to refer to a different file. The new open file description is initially not shared with any other process, but sharing may arise via fork.
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.
In addition, zero or more file creation flags and file status flags can be bitwise-or'd in flags. The file creation flags are +o-creat+, +o-excl+, +o-noctty+, and +o-trunc+. The file status flags are all of the remaining flags listed below. The distinction between these two groups of flags is that the file status flags can be retrieved and (in some cases) modified using fcntl. The full list of file creation flags and file status flags is as follows:
| +o-append+ |
The file is opened in append mode. Before each write, the file offset 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. |
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| +o-async+ |
Enable signal-driven I/O: generate a signal (SIGIO by default, but this can be changed via fcntl) when input or output becomes possible on this file descriptor. This feature is only available for terminals, pseudoterminals, sockets, and (since Linux 2.6) pipes and FIFOs. See fcntl for further details. |
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| +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 F_SETFD operations to set the FD_CLOEXEC flag. Additionally, use of this flag is essential in some multithreaded programs since using a separate fcntl F_SETFD operation to set the FD_CLOEXEC flag does not suffice to avoid race conditions where one thread opens a file descriptor at the same time as another thread does a fork plus execve. |
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| +o-creat+ |
If the file does not exist it will be created. The owner (user ID) of the file is set to the effective user ID of the process. The group ownership (group ID) is set either to the effective group ID of the process or to the group ID of the parent directory (depending on file system type and mount options, and the mode of the parent directory, see the mount options bsdgroups and sysvgroups described in mount(8)). mode specifies the permissions to use in case a new file is created. This argument must be supplied when +o-creat+ is specified in flags; if +o-creat+ is not specified, then mode is ignored. The effective permissions are modified by the process's umask in the usual way: The permissions of the created file are (mode & ~umask). Note that this mode only applies 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:
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| +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 discussion. A semantically similar (but deprecated) interface for block devices is described in raw(8). |
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| +o-directory+ |
If pathname is not a directory, cause the open to fail. This flag is Linux-specific, and 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, but should not be used outside of the implementation of opendir(3). |
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| +o-excl+ |
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. When these two flags are specified, symbolic links are not followed: if pathname is a symbolic link, then open fails regardless of where the symbolic link points to. In general, the behavior of +o-excl+ is undefined if it is used without +o-creat+. There is one exception: on Linux 2.6 and later, +o-excl+ can be used without +o-creat+ if pathname refers to a block device. If the block device is in use by the system (e.g., mounted), open fails with the error +ebusy+. On NFS, +o-excl+ is only supported when using NFSv3 or later on kernel 2.6 or later. In NFS environments where +o-excl+ support is not provided, programs that rely on it for performing locking tasks will contain a race condition. Portable programs that want to perform atomic file locking using a lockfile, and need to avoid reliance on NFS support for +o-excl+, can create a unique file on the same file system (e.g., incorporating hostname and PID), and use link to make a link to the lockfile. If link returns 0, the lock is successful. Otherwise, use stat on the unique file to check if its link count has increased to 2, in which case the lock is also successful. |
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| +o-largefile+ |
(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)). |
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| +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. 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 file systems. One example is NFS, where the server maintains the access time. |
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| +o-noctty+ |
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. |
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| +o-nofollow+ |
If pathname is a symbolic link, then the open fails. This is a FreeBSD extension, which was added to Linux in version 2.1.126. Symbolic links in earlier components of the pathname will still be followed. |
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| +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. For the handling of FIFOs (named pipes), see also fifo(7). For a discussion of the effect of +o-nonblock+ in conjunction with mandatory file locks and with file leases, see fcntl. |
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| +o-sync+ |
The file is opened for synchronous I/O. Any writes on the resulting file descriptor will block the calling process until the data has been physically written to the underlying hardware. |
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| +o-trunc+ |
If the file already exists and is a regular file and the open 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. |
| +eacces+ | 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).) |
| +eexist+ | pathname already exists and +o-creat+ and +o-excl+ were used. |
| +efault+ | pathname points outside your accessible address space. |
| +efbig+ | See +eoverflow+. |
| +eintr+ | 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). |
| +eisdir+ | pathname refers to a directory and the access requested involved writing (that is, +o-wronly+ or +o-rdwr+ is set). |
| +eloop+ | Too many symbolic links were encountered in resolving pathname, or +o-nofollow+ was specified but pathname was a symbolic link. |
| +emfile+ | The process already has the maximum number of files open. |
| +enametoolong+ | pathname was too long. |
| +enfile+ | The system limit on the total number of open files has been reached. |
| +enodev+ | 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.) |
| +enoent+ | +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. |
| +enomem+ | Insufficient kernel memory was available. |
| +enospc+ | pathname was to be created but the device containing pathname has no room for the new file. |
| +enotdir+ | 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. |
| +enxio+ | +o-nonblock+ or +o-wronly+ is set, the named file is a FIFO and no process has the file open for reading. Or, the file is a device special file and no corresponding device exists. |
| +eoverflow+ | 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 (2<<31)-1 bits; see also +o-largefile+ above. This is the error specified by POSIX.1-2001; in kernels before 2.6.24, Linux gave the error +efbig+ for this case. |
| +eperm+ | 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 (CAP_FOWNER). |
| +erofs+ | pathname refers to a file on a read-only file system and write access was requested. |
| +etxtbsy+ | pathname refers to an executable image which is currently being executed and write access was requested. |
| +ewouldblock+ | The +o-nonblock+ flag was specified, and an incompatible lease was held on the file (see fcntl). |
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.
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+. Linux reserves the special, nonstandard access mode 3 (binary 11) in flags to mean: check for read and write permission on the file and return a descriptor that can't be used for reading or writing. This nonstandard access mode is used by some Linux drivers to return a descriptor that is only to be used for device-specific ioctl operations.
The (undefined) effect of +o-rdonly+ | +o-trunc+ varies among implementations. On many systems the file is actually truncated.
There are many infelicities in the protocol underlying NFS, affecting amongst others +o-sync+ and +o-ndelay+.
POSIX provides for three different variants of synchronized I/O, corresponding to the flags +o-sync+, +o-dsync+, and +o-rsync+. Currently (2.6.31), Linux only implements +o-sync+, but glibc maps +o-dsync+ and +o-rsync+ to the same numerical value as +o-sync+. Most Linux file systems don't actually implement the POSIX +o-sync+ semantics, which require all metadata updates of a write to be on disk on returning to user space, but only the +o-dsync+ semantics, which require only actual file data and metadata necessary to retrieve it to be on disk by the time the system call returns.
Note that open can open device special files, but creat() cannot create them; use mknod instead.
On NFS file systems with UID mapping enabled, open may return a file descriptor but, for example, read requests are denied with EACCES. This is because the client performs open by checking the permissions, but UID mapping is performed by the server upon read and write requests.
If the file is newly created, its st_atime, st_ctime, st_mtime fields (respectively, time of last access, time of last status change, and time of last modification; see stat) are set to the current time, and so are the st_ctime and st_mtime fields of the parent directory. Otherwise, if the file is modified because of the +o-trunc+ flag, its st_ctime and st_mtime fields are set to the current time.
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 file system and kernel version and might be absent entirely. However there is currently no file system-independent interface for an application to discover these restrictions for a given file or file system. Some file systems 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 file system. Under Linux 2.6, alignment to 512-byte boundaries suffices.
+o-direct+ I/Os should never be run concurrently with the fork system call, if the memory buffer is a private mapping (i.e., any mapping created with the mmap 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 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 or mmap with the MAP_SHARED flag. Nor does this restriction apply when the memory buffer has been advised as MADV_DONTFORK with madvise, ensuring that it will not be available to the child after fork.
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 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 file systems 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 file system 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 of files with direct I/O to the same files.
The behaviour of +o-direct+ with NFS will differ from local file systems. 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 only bypass the page cache 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.