Editing Self-modifying code (section)

===High-level languages===
Some compiled languages explicitly permit self-modifying code. For example, the ALTER verb in [[COBOL]] may be implemented as a branch instruction that is modified during execution.<ref name="MicroFocus_ALTER"/> Some [[batch file|batch]] programming techniques involve the use of self-modifying code. [[Clipper (programming language)|Clipper]] and [[SPITBOL]] also provide facilities for explicit self-modification. The Algol compiler on [[Burroughs Large Systems|B6700 system]]s offered an interface to the operating system whereby executing code could pass a text string or a named disc file to the Algol compiler and was then able to invoke the new version of a procedure.

With interpreted languages, the "machine code" is the source text and may be susceptible to editing on-the-fly: in [[SNOBOL]] the source statements being executed are elements of a text array. Other languages, such as [[Perl]] and [[Python (programming language)|Python]], allow programs to create new code at run-time and execute it using an [[eval]] function, but do not allow existing code to be mutated. The illusion of modification (even though no machine code is really being overwritten) is achieved by modifying function pointers, as in this JavaScript example:
<syntaxhighlight lang="JavaScript">
    var f = function (x) {return x + 1};

    // assign a new definition to f:
    f = new Function('x', 'return x + 2');
</syntaxhighlight>
[[Lisp macro]]s also allow runtime code generation without parsing a string containing program code.

The Push programming language is a [[genetic programming]] system that is explicitly designed for creating self-modifying programs. While not a high level language, it is not as low level as assembly language.<ref name="Push"/>

====Compound modification====
Prior to the advent of multiple windows, command-line systems might offer a menu system involving the modification of a running command script. Suppose a [[DOS]] script (or "batch") file MENU.BAT contains the following:<ref name="Fosdal_2001"/><ref group="nb" name="NB_CHOICE"/> 
    :start
    SHOWMENU.EXE
Upon initiation of MENU.BAT from the command line, SHOWMENU presents an on-screen menu, with possible help information, example usages and so forth. Eventually the user makes a selection that requires a command ''SOMENAME'' to be performed: SHOWMENU exits after rewriting the file MENU.BAT to contain
    :start
    SHOWMENU.EXE
    CALL ''SOMENAME''.BAT
    GOTO start
Because the DOS command interpreter does not compile a script file and then execute it, nor does it read the entire file into memory before starting execution, nor yet rely on the content of a record buffer, when SHOWMENU exits, the command interpreter finds a new command to execute (it is to invoke the script file ''SOMENAME'', in a directory location and via a protocol known to SHOWMENU), and after that command completes, it goes back to the start of the script file and reactivates SHOWMENU ready for the next selection. Should the menu choice be to quit, the file would be rewritten back to its original state. Although this starting state has no use for the label, it, or an equivalent amount of text is required, because the DOS command interpreter recalls the byte position of the next command when it is to start the next command, thus the re-written file must maintain alignment for the next command start point to indeed be the start of the next command.

Aside from the convenience of a menu system (and possible auxiliary features), this scheme means that the SHOWMENU.EXE system is not in memory when the selected command is activated, a significant advantage when memory is limited.<ref name="Fosdal_2001"/><ref name="Paul_1996"/>