What Is The Reason For Underscore In C Variable Name Definition?

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Learn more about Labs What is the reason for underscore in C variable name definition? Ask Question Asked 13 years ago Modified 6 years, 11 months ago Viewed 5k times 7

I am trying to understand when a developer needs to define a C variable with preceding '_'. What is the reason for it?

For example:

uint32_t __xyz_ = 0;

• c

Share Improve this question Follow edited Aug 2, 2014 at 1:32 Jonathan Leffler 751k145145 gold badges944944 silver badges1.3k1.3k bronze badges asked Nov 11, 2011 at 11:30 user1041588user1041588 8911 silver badge33 bronze badges 3

• 6 You'll have to ask the author – David Heffernan Commented Nov 11, 2011 at 11:31
• Welcome to StackOverflow! The first thing to realise is that this website is not a forum. It's a question/answer site. Come with a good question, and be prepared to accept an answer if you find one satisfactory. – Kerrek SB Commented Nov 11, 2011 at 11:32
• Possible duplicate of Why does C have keywords starting with underscore – klutt Commented Jun 17, 2019 at 18:17

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3 Answers 3

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Maybe this helps, from C99, 7.1.3 ("Reserved Identifiers"):

• All identifiers that begin with an underscore and either an uppercase letter or another underscore are always reserved for any use.

• All identifiers that begin with an underscore are always reserved for use as identifiers with file scope in both the ordinary and tag name spaces.

Moral: For ordinary user code, it's probably best not to start identifiers with an underscore.

(On a related note, I think you should also stay clear from naming types with a trailing _t, which is reserved for standard types.)

Share Improve this answer Follow answered Nov 11, 2011 at 11:34 Kerrek SBKerrek SB 476k9393 gold badges895895 silver badges1.1k1.1k bronze badges 5

• The _t reservation is from POSIX isn't it? – Flexo - Save the data dump ♦ Commented Nov 11, 2011 at 11:42
• @awoodland: I'm not 100% sure. I'd appreciate if someone could say where this rule comes from. That said, *_t types are so ubiquitous in the C standard that it's probably good to stick to that policy. – Kerrek SB Commented Nov 11, 2011 at 11:47
• 1 they key bit is the "any header" _t in the second table of: pubs.opengroup.org/onlinepubs/9699919799/functions/… – Flexo - Save the data dump ♦ Commented Nov 11, 2011 at 11:53
• 4 And corollary: a developer needs to define reserved names only when implementing the C language (and libraries). So if you're not contributing to a compiler or stdlib, that's not you. – Steve Jessop Commented Nov 11, 2011 at 11:59
• What does the second bullet point mean? Can we use those? – user129393192 Commented Jun 9, 2023 at 6:27

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It is a trick used in the header files of C implementations for global symbols, in order to prevent eventual conflicts with other symbols defined by the user.

Since C lacks a namespace feature, this is a rudimentary approach to avoid name collisions with the user.

Declaring such symbols in your own header and source files is not encouraged because it can introduce naming conflicts between your code and the C implementation. Even if that doesn't produce a conflict on your current implementation, you are still prone to strange conflicts across different/future implementations, since they are free to use other symbols prefixed with underscores.

Share Improve this answer Follow edited Nov 11, 2011 at 12:29 answered Nov 11, 2011 at 11:31 Blagovest BuyuklievBlagovest Buyukliev 43.4k1414 gold badges9595 silver badges134134 bronze badges 4

• 1 It's a reserved name, reserved for the implementation. Sounds like a good way of seeing a strange problem in the future. – Flexo - Save the data dump ♦ Commented Nov 11, 2011 at 11:43
• @awoodland: I fail to see a justification for the downvote. I'm not encouraging anyone to adopt this naming convention in their own header files. – Blagovest Buyukliev Commented Nov 11, 2011 at 11:51
• I disagreed with the "in order to prevent conflicts" and "a rudimentary approach to avoid name collisions" - it doesn't prevent conflicts at all, in fact it introduces a whole new class of possible conflicts with a perfectly legal implementation. I would prefer to see a clarification along the lines of "a flawed attempt at preventing conflicts" to make that explicit. Currently it reads like a weak recommendation. – Flexo - Save the data dump ♦ Commented Nov 11, 2011 at 11:58
• Upvoted now although some of the single underscore identifiers can also be reserved depending on where they're used. (you had a double underscore in the example at the end) – Flexo - Save the data dump ♦ Commented Nov 11, 2011 at 12:27

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whether its C or not, the leading underscore provides the programmer a status indication so he does not have to go look it up. In PHP, or any object oriented language where we deal with tens of thousands of properties and methods written by 1000's of authors, seeing an underscore prefix removes the need to go dig through the class andlook up whether its declared private, or protected or public. thats an immense time saver. the practice started before C, i am sure...

Share Improve this answer Follow answered Dec 21, 2017 at 6:08 James DanforthJames Danforth 81788 silver badges77 bronze badges Add a comment |

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