Writing a file

This section describes the write subroutines.

If you write to a file that does not contain a block corresponding to the byte offset resulting from the write process, the write subroutine allocates a new block. This new block is added to the i-node information that defines the file. Adding the new block might allocate more than one block if the underlying file system needs to add blocks for addressing the file blocks.

During the write operation, the i-node is locked. No other processes are allowed to modify the contents of the file while a write is in progress. However, the file is unlocked immediately on completion of the write operation. If another process changes the file between two write operations, the resulting data is different, but the integrity of the data structure is maintained.

The write subroutine loops in a way similar to the read subroutine, logically writing one block to disk for each iteration. At each iteration, the process either writes an entire block or only a portion of one. If only a portion of a data block is required to accomplish an operation, the write subroutine reads the block from disk to avoid overwriting existing information. If an entire block is required, it does not read the block because the entire block is overwritten. The write operation proceeds block by block until the number of bytes designated in the NBytes parameter is written.

Delayed write

You can designate a delayed write process with the O_DEFER flag. The data is then transferred to disk as a temporary file. The delayed write feature caches the data in case another process reads or writes the data sooner. Delayed write saves extra disk operations. Many programs, such as mail and editors, create temporary files in the /tmp directory and quickly remove them.

When a file is opened with the deferred update (O_DEFER) flag, the data is not written to permanent storage until a process issues an fsync subroutine call or a process issues a synchronous write to the file (opened with O_SYNC flag). The fsync subroutine saves all changes in an open file to disk. See the open subroutine for a description of the O_DEFER and O_SYNC flags.

Truncating files

The truncate or ftruncate subroutines change the length of regular files. The truncating process must have write permission to the file. The Length variable value indicates the size of the file after the truncation operation is complete. All measures are relative to the first byte of the file, not the current offset. If the new length (designated in the Length variable) is less than the previous length, the data between the two is removed. If the new length is greater than the existing length, zeros are added to extend the file size. When truncation is complete, full blocks are returned to the file system, and the file size is updated.