When is "inline" ineffective? (in C)

Question

Some people love using inline keyword in C, and put big functions in headers. When do you consider this to be ineffective? I consider it sometime even annoying, because it is unusual.

My principle is that inline should be used for small functions accessed very frequently, or in order to have real type checking. Anyhow, my taste guide me, but I am not sure how to explain best the reasons why inline is not so useful for big functions.

In this question people suggest that the compiler can do a better job at guessing the right thing to do. That was also my assumption. When I try to use this argument, people reply it does not work with functions coming from different objects. Well, I don't know (for example, using GCC).

Thanks for your answers!

Answer 1

inline does two things:

1. gives you an exemption from the "one definition rule" (see below). This always applies.
2. Gives the compiler a hint to avoid a function call. The compiler is free to ignore this.

#1 Can be very useful (e.g. put definition in header if short) even if #2 is disabled.

In practice compilers often do a better job of working out what to inline themselves (especially if profile guided optimisation is available).

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[EDIT: Full References and relevant text]

The two points above both follow from the ISO/ANSI standard (ISO/IEC 9899:1999(E), commonly known as "C99").

In §6.9 "External Definition", paragraph 5:

An external definition is an external declaration that is also a definition of a function (other than an inline definition) or an object. If an identifier declared with external linkage is used in an expression (other than as part of the operand of a sizeof operator whose result is an integer constant), somewhere in the entire program there shall be exactly one external definition for the identifier; otherwise, there shall be no more than one.

While the equalivalent definition in C++ is explictly named the One Definition Rule (ODR) it serves the same purpose. Externals (i.e. not "static", and thus local to a single Translation Unit -- typically a single source file) can only be defined once only unless it is a function and inline.

In §6.7.4, "Function Specifiers", the inline keyword is defined:

Making a function an inline function suggests that calls to the function be as fast as possible.^[118] The extent to which such suggestions are effective is implementation-defined.

And footnote (non-normative), but provides clarification:

By using, for example, an alternative to the usual function call mechanism, such as ‘‘inline substitution’’. Inline substitution is not textual substitution, nor does it create a new function. Therefore, for example, the expansion of a macro used within the body of the function uses the definition it had at the point the function body appears, and not where the function is called; and identifiers refer to the declarations in scope where the body occurs. Likewise, the function has a single address, regardless of the number of inline definitions that occur in addition to the external definition.

Summary: what most users of C and C++ expect from inline is not what they get. Its apparent primary purpose, to avoid functional call overhead, is completely optional. But to allow separate compilation, a relaxation of single definition is required.

(All emphasis in the quotes from the standard.)

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EDIT 2: A few notes:

• There are various restrictions on external inline functions. You cannot have a static variable in the function, and you cannot reference static TU scope objects/functions.
• Just seen this on VC++'s "whole program optimisation", which is an example of a compiler doing its own inline thing, rather than the author.

Answer 2

The important thing about an inline declaration is that it doesn't necessarily do anything. A compiler is free to decide to, in many cases, to inline a function not declared so, and to link functions which are declared inline.