What is the use of the `inline` keyword in C?

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A C code can be optimized in two ways: For Code size and for Execution Time.

inline functions:

gcc.gnu.org says,

By declaring a function inline, you can direct GCC to make calls to that function faster. One way GCC can achieve this is to integrate that function’s code into the code for its callers. This makes execution faster by eliminating the function-call overhead; in addition, if any of the actual argument values are constant, their known values may permit simplifications at compile time so that not all of the inline function’s code needs to be included. The effect on code size is less predictable; object code may be larger or smaller with function inlining, depending on the particular case.

So, it tells the compiler to build the function into the code where it is used with the intention of improving execution time.

If you declare Small functions like setting/clearing a flag or some bit toggle which are performed repeatedly, inline, it can make a big performance difference with respect to time, but at the cost of code size.

non-static inline and Static inline

Again referring to gcc.gnu.org,

When an inline function is not static, then the compiler must assume that there may be calls from other source files; since a global symbol can be defined only once in any program, the function must not be defined in the other source files, so the calls therein cannot be integrated. Therefore, a non-static inline function is always compiled on its own in the usual fashion.

extern inline?

Again, gcc.gnu.org, says it all:

If you specify both inline and extern in the function definition, then the definition is used only for inlining. In no case is the function compiled on its own, not even if you refer to its address explicitly. Such an address becomes an external reference, as if you had only declared the function, and had not defined it.

This combination of inline and extern has almost the effect of a macro. The way to use it is to put a function definition in a header file with these keywords, and put another copy of the definition (lacking inline and extern) in a library file. The definition in the header file causes most calls to the function to be inlined. If any uses of the function remain, they refer to the single copy in the library.

To sum it up:

  1. For inline void f(void){},
    inline definition is only valid in the current translation unit.
  2. For static inline void f(void) {}
    Since the storage class is static, the identifier has internal linkage and the inline definition is invisible in other translation units.
  3. For extern inline void f(void);
    Since the storage class is extern, the identifier has external linkage and the inline definition also provides the external definition.

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