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All SCCS commands that modify an SCCS file do so by writing a copy called the `x-file'. This is done to ensure that the SCCS file is not damaged if processing terminates abnormally. SCCS names the x-file by replacing the s. prefix of the SCCS filename with an x. prefix. The x-file is created in the same directory as the SCCS file, given the same mode (see chmod(C)) and owned by the effective user. When processing is complete, the old SCCS file is destroyed, and the modified x-file is renamed (with x. replaced by s.) and becomes the new SCCS file.
To prevent simultaneous updates to an SCCS file, the same modifying commands also create a lock-file called the `z-file'. SCCS forms its name by replacing the s. prefix of the SCCS filename with a z. prefix. The z-file contains the process number of the command that created it, and its existence prevents other commands from processing the SCCS file. The z-file is created with access permission mode 444 (read only) in the same directory as the SCCS file, and is owned by the effective user. Like the x-file, the z-file exists only for the duration of the execution of the command that creates it.
In general, users can ignore x-files and z-files. They are useful only in the event of system crashes or similar situations.