See Also

Syscall.chmod

Syscall.close

Syscall.dup

Syscall.getdtablesize

Syscall.lseek

Syscall.read

Syscall.umask

Syscall.write

Syscall.fopen

Parameters

pathname

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flags

Documentation for this section has not yet been entered. [Edit]

Returns

If successful, open() returns a non-negative integer, termed a file descriptor. It returns -1 on failure, and sets Stdlib.GetLastError to indicate the error.

Usage

The named file is opened unless:

Error	Details
Errno.ENOTDIR	A component of the path prefix is not a directory.
Errno.ENAMETOOLONG	A component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 characters.
Errno.ENOENT	OpenFlags.O_CREAT is not set and the named file does not exist.
Errno.ENOENT	A component of the path name that must exist does not exist.
Errno.EACCES	Search permission is denied for a component of the path prefix.
Errno.EACCES	The required permissions (for reading and/or writing) are denied for the given flags.
Errno.EACCES	OpenFlags.O_CREAT is specified, the file does not exist, and the directory in which it is to be created does not permit writing.
Errno.ELOOP	Too many symbolic links were encountered in translating the pathname.
Errno.EISDIR	The named file is a directory, and the arguments specify it is to be opened for writing.
Errno.EROFS	The named file resides on a read-only file system, and the file is to be modified.
Errno.EMFILE	The process has already reached its limit for open file descriptors.
Errno.ENFILE	The system file table is full.
Errno.EMLINK	OpenFlags.O_NOFOLLOW was specified and the target is a symbolic link.
Errno.ENXIO	The named file is a character special or block special file, and the device associated with this special file does not exist.
Errno.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.
Errno.EINTR	The open() operation was interrupted by a signal.
Errno.EOPNOTSUPP	OpenFlags.O_SHLOCK or OpenFlags.O_EXLOCK is specified but the underlying file system does not support locking.
Errno.EOPNOTSUPP	The named file is a special file mounted through a file system that does not support access to it (e.g. NFS).
Errno.EWOULDBLOCK	OpenFlags.O_NONBLOCK and one of OpenFlags.O_SHLOCK or OpenFlags.O_EXLOCK is specified and the file is locked.
Errno.ENOSPC	OpenFlags.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.
Errno.ENOSPC	OpenFlags.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.
Errno.EDQUOT	OpenFlags.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.
Errno.EDQUOT	OpenFlags.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.
Errno.EIO	An I/O error occurred while making the directory entry or allocating the inode for OpenFlags.O_CREAT .
Errno.ETXTBSY	The file is a pure procedure (shared text) file that is being executed and the open() system call requests write access.
Errno.EFAULT	The path argument points outside the process's allocated address space.
Errno.EEXIST	OpenFlags.O_CREAT and OpenFlags.O_EXCL were specified and the file exists.
Errno.EOPNOTSUPP	An attempt was made to open a socket (not currently implemented).
Errno.EINVAL	An attempt was made to open a descriptor with an illegal combination of OpenFlags.O_RDONLY , OpenFlags.O_WRONLY , and OpenFlags.O_RDWR .

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Remarks

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 argument may indicate the file is to be created if it does not exist (by specifying the OpenFlags.O_CREAT flag). In this case open() requires a third argument mode_t mode , and the file is created with mode mode as described in Syscall.chmod(2) and modified by the process' umask value (see Syscall.umask(2) ) .

The flags specified are formed by or Ns 'ing the following values

Value	Details
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 OpenFlags.O_APPEND set causes each write on the file to be appended to the end. If OpenFlags.O_TRUNC is specified and the file exists, the file is truncated to zero length. If OpenFlags.O_EXCL is set with OpenFlags.O_CREAT and the file already exists, open() returns an error. This may be used to implement a simple exclusive access locking mechanism. If OpenFlags.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 OpenFlags.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 OpenFlags.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 OpenFlags.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 Syscall.flock(2) semantics can be obtained by setting OpenFlags.O_SHLOCK for a shared lock, or OpenFlags.O_EXLOCK for an exclusive lock. If creating a file with OpenFlags.O_CREAT , the request for the lock will never fail (provided that the underlying file system supports locking).

OpenFlags.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.

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 Syscall.execve(2) system calls; see Syscall.close(2) and Syscall.fcntl(2) .

The system imposes a limit on the number of file descriptors open simultaneously by one process. The Syscall.getdtablesize(2) system call returns the current system limit.

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Requirements

Namespace: Mono.Unix.Native
Assembly: Mono.Posix (in Mono.Posix.dll)
Assembly Versions: 1.0.5000.0, 2.0.0.0