When reading an older development book, I found this:
However I have written C programs, that use "//" single line comments.
However I have written C programs, that use "//" single line comments.
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macOS Single Lines In C Comments
- Thread starter Marjamrob1
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Originally, the only style for commenting in C was /* ... */. This is still the case in C89, which is traditionally known as ANSI-C. C99 added support for one line comments using //. If your book was written before the C99 standard was approved, it was correct at the time, and still is for older systems.
Why does it matter?
There was a period of time when the C compilers used by Apple accepted // comments even though it wasn't in the ANSI/ISO C standard of the time. There was a command-line option for strict C conformance that would turn this off, so // was forbidden. IIRC, there was another option that would force strict compliance to the ISO C standard, EXCEPT that // comments were allowed. Other than as a historical footnote, none of this matters today, and hasn't since C99.
They originally came from BCPL, which was also the first brace programming language. But, this is just esoteric trivia.