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comp.lang.c FAQ list · Question 20.20

Q: Why don't C comments nest? How am I supposed to comment out code containing comments? Are comments legal inside quoted strings?

A: C comments don't nest mostly because PL/I's comments, which C's are borrowed from, don't either. Therefore, it is usually better to ``comment out'' large sections of code, which might contain comments, with #ifdef or #if 0 (but see question 11.19).

The character sequences /* and */ are not special within double-quoted strings, and do not therefore introduce comments, because a program (particularly one which is generating C code as output) might want to print them. (It is hard to imagine why anyone would want or need to place a comment inside a quoted string. It is easy to imagine a program needing to print "/*".)

Note also that // comments have only become legal in C as of C99.

References: K&R1 Sec. A2.1 p. 179
K&R2 Sec. A2.2 p. 192
ISO Sec. 6.1.9, Annex F
Rationale Sec. 3.1.9
H&S Sec. 2.2 pp. 18-9
PCS Sec. 10 p. 130

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