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Can C Keywords Be Redefined Using the #define Preprocessor?

Barbara Streisand

Nov 23, 2024 pm 02:51 PM

Can C Keywords Be Redefined Using the #define Preprocessor?

Redefining Keywords with C Preprocessor #define

The heated debate surrounding the validity of redefining language keywords using #define in C standards has often sparked controversy. This article aims to shed light on this contentious issue.

According to the C standard (§17.4.3.1.1/2), preprocessor macros cannot redefine names declared or defined in standard library headers. Additionally, it prohibits the redefinition of macros that are lexically identical to C keywords within a translation unit that includes a standard header.

However, in C , the rules have undergone an apparent change. The C 0x draft (FCD §17.6.3.3.1) explicitly forbids the redefinition of keywords using #define or #undef. This move appears to align with the common consensus that manipulating keywords with the preprocessor can lead to unpredictable behavior and potential standards violations.

Nevertheless, it's worth noting that the statement "A translation unit shall not #define or #undef names lexically identical to keywords" has since been revised in the final C 11 standard. The new wording (ISO/IEC 14882:2011 §17.6.4.1/4) reads: "A translation unit shall not define macros for names lexically identical to keywords."

This revision seems to suggest that the outright prohibition on redefining keywords with #define has been removed from the standard. However, the prohibition on redefining names declared or defined in standard headers remains in place (ISO/IEC 14882:2011 §17.6.4.1/6).

In light of this ongoing debate, it's advisable to exercise caution when attempting to redefine language keywords in C programs. While it may be technically possible under certain conditions, such practices can compromise the program's compatibility with various compilers and toolchains. Adhering to the standards and using alternative mechanisms, such as preprocessor macros or user-defined types, is generally considered the safer approach.

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