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Editorial Team
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Asked: 2026-05-24T02:42:16+00:00

I have spotted something in C header files what I can’t figure out what

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I have spotted something in C header files what I can’t figure out what is for. For example in file bits/socket.h there is an enumeration type enum __socket_type, but after every enumerator there is a define macro which defines the same. Example:

enum __socket_type
{
   SOCK_STREAM = 1,
   #define SOCK_STREAM SOCK_STREAM 
   ...
};

I have been unable to find out what this is for. Please enlighten me. I don’t even know how to form right question for querying google nor this site search box.

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1 Answer

  1. Editorial Team

    A prepreprocessor macro will never expand recursively, so what such a #define does is leave the name in place whereever it is used. Such things are useful when you want to have a preprocessor feature test.

    #ifdef SOCK_STREAM
..
#endif

    can be used to conditionally compile some code afterwards.

    Edit: So this combines the cleaner approach of enumerations (implicit values without collisions and scoping) with preprocessor tests.

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