What is the use of \a in C?

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I stumbled upon the \a escape sequence in a piece of text in ISO/IEC 9899:2018 (C18), Section 5.2.2/2 - "Character display semantics":

\a (alert) Produces an audible or visible alert without changing the active position.

I´ve never seen this used in any C code to indicate an error. By the way, I didn´t know that an escape sequence can/could trigger an audible signal on some systems.

  • What is/was the certain use of that \a escape sequence?
  • Is it a remain from the earlier days of C, that is obsolete now?
  • And in which situation it would be or were (if it is obsolete today) appreciated to use that? Can you provide an example?

Thank you very much.

alert warnings escaping c
2 Answers
  • What is/was the certain use of that \a escape sequence?

According to the same section of the standard you were reading,

Alphabetic escape sequences representing nongraphic characters in the execution character set are intended to produce actions on display devices as follows

The particular action that a \a sequence is intended to produce is what you quoted. This is in fact used rarely, if ever, in modern code, and typical C implementations just map that sequence to a single ASCII character that has the same significance, relying on the terminal driver to handle the actual alert. You're more likely to see the effect if you accidentally output a binary file to a terminal than just about any other way.

  • Is it a remain from the earlier days of C, that is obsolete now?

I am not aware of it being used for the purpose described in any code I have ever seen. In that sense, yes, it is obsolete, but from the perspective of the standard, that provision is not obsolete.

  • And in which situation it would be or were (if it is obsolete today) appreciated to use that? Can you provide an example?

You would include that sequence in a string that is printed to stdout. When the string is printed, you (maybe) get a bell sound and / or the screen flashes and / or something similar.

printf("Ring the bell...\a\n");
user_24022720
  • \a Isn't always used for an error
  • It is an ascii character and can be used anywhere.
  • \a Is used to display a sound. You can try by typing printf '\a' in bash.
user_129477760
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