Open (system call)

For most file systems, a program initializes access to a file in a file system using the open system call. This allocates resources associated to the file (the file descriptor), and returns a handle that the process will use to refer to that file. In some cases the open is performed by the first access.

The same file may be opened simultaneously by several processes, and even by the same process (resulting in several file descriptors for the same file) depending on the file organization and filesystem. Operations on the descriptors such as moving the file pointer or closing it are independent (they do not affect other descriptors for the same file). Operations on the file (like a write) can be seen by operations on the other descriptors (a posterior read can read the written data).

During the open, the filesystem may allocate memory for buffers (or it may wait until the first operation).

The absolute filename is resolved. This may include connecting to a remote host and notifying an operator that a removable medium is required. It may include the initialization of a communication device. At this point an error may be returned if the host or medium is not available. The first access to at least the directory within the filesystem is performed. An error will usually be returned if the higher level components of the path (directories) cannot be located or accessed. An error will be returned if the file is expected to exist and it does not or if the file should not already exist and it does.

If the file is expected to exist and it does, the file access, as restricted by permission flags within the file meta data or access control list, is validated against the requested type of operations. This usually requires an additional filesystem access although in some filesystems meta-flags may be part of the directory structure.

If the file is being created, the filesystem may allocate the default initial amount of storage or a specified amount depending on the file system capabilities. If this fails an error will be returned. Updating the directory with the new entry may be performed or it may be delayed until the close is performed.

Various other errors which may occur during the open include directory update failures, un-permitted multiple connections, media failures, communication link failures and device failures.

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