structural:summarization
Reserved words

The following reserved words are case sensitive.

C89 has 32 reserved words, also known as keywords, which are the words that cannot be used for any purposes other than those for which they are predefined:

C99 reserved five more words: (‡ is an alternative spelling alias for a C23 keyword)

C11 reserved seven more words: (‡ is an alternative spelling alias for a C23 keyword)

C23 reserved fifteen more words:

Most of the recently reserved words begin with an underscore followed by a capital letter, because identifiers of that form were previously reserved by the C standard for use only by implementations. Since existing program source code should not have been using these identifiers, it would not be affected when C implementations started supporting these extensions to the programming language. Some standard headers do define more convenient synonyms for underscored identifiers. Some of those words were added as keywords with their conventional spelling in C23 and the corresponding macros were removed.

Prior to C89, entry was reserved as a keyword. In the second edition of their book The C Programming Language, which describes what became known as C89, Kernighan and Ritchie wrote, "The... entry, formerly reserved but never used, is no longer reserved." and "The stillborn entry keyword is withdrawn."