Clobbering

In software engineering, clobbering a file or computer memory is overwriting its contents. The Jargon File defines clobbering as "overwrit[ing], usually unintentionally: 'I walked off the end of the array and clobbered the stack.' Compare mung, scribble, trash, and smash the stack."^[1]

Often this happens unintentionally, e.g., using the > redirection operator. To prevent unintentional clobbering, various means are used. For example, the setting shell parameter set -o noclobber (bash, ksh) or set noclobber (csh, tcsh) will prevent > from clobbering by making it issue an error message instead:^[2]

$ echo "Hello, world" >file.txt
$ echo "This will overwrite the first greeting." >file.txt
$ set -o noclobber
$ echo "Can we overwrite it again?" >file.txt
-bash: file.txt: cannot overwrite existing file
$ echo "But we can use the >| operator to ignore the noclobber." >|file.txt
$ # Successfully overwrote the contents of file.txt using the >| operator
$ set +o noclobber # Changes setting back

In makefiles, a common target clobber means complete cleanup of all unnecessary files and directories produced by previous invocations of the make command.^[3] It is a more severe target than clean and is commonly used to uninstall software. Some make-related commands invoke "make clobber" during their execution. They check the CLOBBER environment variable. If it is set to OFF then clobbering is not done.^[4]

In assembler programming, the term 'clobbered registers' is used to denote any registers whose value may be overwritten during the course of executing an instruction.

References

1. ^ "Clobber" entry, Jargon File (public domain)
2. ^ "Unix Power Tools", by Shelley Powers, Jerry Peek, Tim O'Reilly, Mike Loukides, p. 892
3. ^ UNIX System V, Release 4, Motorola Unix, Motorola, inc, inc Motorola, p. 28
4. ^ "Unix Unleashed", by Robin Burk, David B. Horvath