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Clang header file intrin.h declares the _BitScanForward as below.

static unsigned char _BitScanForward(unsigned long *_Index, unsigned long _Mask);

When I include this header file and compile the source file, it is giving compilation error like below

error: inline function '_BitScanForward' is not defined [-Werror,-Wundefined-inline] _BitScanForward (

This is happening only in our code base but I am unable to reproduce this compilation error outside our environment.

Could somebody please explain what is this "-Wundefined-inline" mean?

Why is the question of "inline" coming into picture?

More Info: When put inline keyword just after static, its compiling without any issues

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    Does this answer your question? undefined reference when calling inline function Commented Nov 7, 2019 at 19:13
  • This is a C question, not a C++ question. If this was a C++ function, then _BitscanForward would surely have been declared as constexpr instead of static or static inline. But question and answers would still be the same. One thing to note though. C++ compilers (at least my current compiler clang-12) may print this error also, if you have overloads that do not agree on the same qualifications. I.e. all overloads must be constexpr or all must be not. I don't know if this is mandated by the standard, I believe not. Maybe a language lawyer can clarify. Commented Aug 3, 2022 at 4:59

1 Answer 1

Reset to default
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It is the compiler telling you which error flag(s) triggered the specific warning. There are many different warning flags you can enable in the compiler so reporting which flag triggered the warning can be helpful.

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7 Comments

But could you please tell me why its saying function _BitScanForward is inline?
@Pendyala Without a minimal reproducible example I can't say.
Actually I am unable to reproduce this issue outside my environment. So I have no clue when this kind of issue happens, and what is the cause.
@Pendyala Looking into it more, this should answer your question on why the warning is being generated. You can suppress it using -Wno-undefined-inline
Thanks Nathan, Here _BitScanForward is not declared as inline function. But why compiler is saying it is inline and has to be defined?
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