What do inline and static inline actually mean in C language? [duplicate]

Question

I am just confused by the inline keyword in C Programming language.

In my practice, an inline function has internal scope, so how does it differ with the static keyword?
inline function has internal scope, but I saw some inline function plus the static keyword — what`s the use?

//temp.c
inline void exit_aio(){
    puts("inline");
}

void test(){
    exit_aio();//error,undefined reference to `exit_aio'
}

If I add the static keyword in the definition of function exit_aio(), then the program will work normally.

Best to make sure your code sample compiles cleanly. minimal reproducible example For now it is at least missing #include files. — chux
– chux, Commented Oct 27, 2024 at 19:22
Can't reproduce (after adding #include <stdio.h> and a main() function). — Weather Vane
– Weather Vane, Commented Oct 27, 2024 at 19:22
See Is "inline" without "static" or "extern" ever useful in C99?. There are numerous other questions about inline functions in C. Note that inline is a hint; the compiler can ignore the hint — and can inline functions not explicitly marked inline. As a simple rule of thumb, make functions static inline in a header and include the header where you use the function. — Jonathan Leffler
– Jonathan Leffler, Commented Oct 28, 2024 at 1:52

Jonathan Leffler · Accepted Answer · 2024-10-28 01:39:45Z

An inline function has internal scope, but I saw some inline function plus the static keyword — what`s the use?

It is to place inline functions in headers without risking linker problems.

I am just confused by the inline keyword

Modern compilers take it only as a hint and may decide whether to inline function or not. If the function is not static and is defined in a header file included by many modules, the linker will complain.

Some compilers have special attributes to force inlining - for example gcc's __attribute__((always_inline))

 //error,undefined reference to `exit_aio'
If I add the static keyword in the definition of function exit_aio(), then the program will work normally.

If optimizations are disabled, the compiler will never inline the inline function.
Even if optimizations are enabled, the compiler can decide not inline the function and it will generate the same problem https://godbolt.org/z/W6eP1a5qY
if you use static or extern, the function will have external or internal linkage and it will not be ignored https://godbolt.org/z/cqrW8zWrr https://godbolt.org/z/esoa6qrq7

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