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According to C99 \[[ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999]\], Section 6.7.3, "Type qualifiers," Paragraph 5 (see also [undefined behavior 61 | CC. Undefined Behavior#unb_61] of Appendix J): |
If an attempt is made to modify an object defined with a
const-qualified type through use of an lvalue with non-const-qualified type, the behavior is undefined.
(See also undefined behavior 61 of Appendix J.)
There are existing compiler implementations that allow const-qualified values to be modified without generating a warning message.
It is also a recommended practice not to cast away a const qualification (see as in guideline EXP05-C. Do not cast away a const qualification) because doing so makes it easier to modify a const-qualified value without warning.
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The first assignment is unsafe because it would allow the valid code that follows to attempt to change the value of the const object c.
Noncompliant Code Example (
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Modifying a
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String Literal)
Similarly to the previous example, the well-formed but noncompliant code example below modifies a constant object after casting away its constness. Compiling the program on a Linux/x64 system doesn't produce any diagnostics even at high warning levels but the generated executable program fails at runtime with SIGESGV.
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