# Julia.h internals

**URL:** <https://discourse.julialang.org/t/julia-h-internals/6829>\
**Category:** Internals & Design\
**Tags:** embedding\
**Created:** [November 1, 2017, 3:26pm UTC](https://discourse.julialang.org/t/julia-h-internals/6829 "2017-11-01T15:26:41Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![sysint64](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/sysint64/32/2398_2.png) [@sysint64](https://discourse.julialang.org/u/sysint64)\
**Post date:** [November 1, 2017, 3:26pm UTC](https://discourse.julialang.org/t/julia-h-internals/6829/1 "2017-11-01T15:26:42Z")

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Hello, in `julia.h` many structs have the `JL_DATA_TYPE` macro, what does this macro mean? this macro add some additional fields to structs? If so, which fields?

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**Author:** ![yuyichao](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yuyichao/32/20_2.png) [@yuyichao](https://discourse.julialang.org/u/yuyichao)\
**Post date:** [November 1, 2017, 3:35pm UTC](https://discourse.julialang.org/t/julia-h-internals/6829/2 "2017-11-01T15:35:10Z")

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Not anymore. They are there just for documentation now…

Also, you should be able to find the definition of it very easily…

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<div class="post-metadata">

**Author:** ![sysint64](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/sysint64/32/2398_2.png) [@sysint64](https://discourse.julialang.org/u/sysint64)\
**Post date:** [November 1, 2017, 3:36pm UTC](https://discourse.julialang.org/t/julia-h-internals/6829/3 "2017-11-01T15:36:02Z")

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Great, thanks!