Inline Functions In C

Introduction

GNU C (and some other compilers) had inline functions long before standard C introduced them (in the 1999 standard); this page summarizes the rules they use, and makes some suggestions as to how to actually use inline functions.

The point of making a function "inline" is to hint to the compiler that it is worth making some form of extra effort to call the function faster than it would otherwise - generally by substituting the code of the function into its caller. As well as eliminating the need for a call and return sequence, it might allow the compiler to perform certain optimizations between the bodies of both functions.

Sometimes it is necessary for the compiler to emit a stand-alone copy of the object code for a function even though it is an inline function - for instance if it is necessary to take the address of the function, or if it can't be inlined in some particular context, or (perhaps) if optimization has been turned off. (And of course, if you use a compiler that doesn't understand "inline", you'll need a stand-alone copy of the object code so that all the calls actually work at all.)

There are various ways to define inline functions; any given kind of definition might definitely emit stand-alone object code, definitely not stand-alone emit object code, or only emit stand-alone object code if it is known to be needed. Sometimes this can lead to duplication of object code, which is a potential problem for following reasons:

It wastes space.
It can cause pointers to what is apparently the same function to compare not equal to one another.
It might reduce the effectiveness of the instruction cache. (Although inlining might do that in other ways too.)

If any of these are a problem for you then you will want to use a strategy that avoids duplication. These are discussed below.

GNU C inline rules

The GNU C rules are described in the GNU C manual, which is included with the compiler. This is freely available if you follow links from e.g. http://gcc.gnu.org.

A function defined with "inline" on its own. Stand-alone object code is always emitted. You can only write one definition like this in your entire program. If you want to use it from other translation units to the one where it is defined, you put a declaration in a header file; but it would not be inlined in those translation units.

This is of rather limited use: if you only want to use the function from one translation unit then "static inline" below makes more sense, if not the you probably want some form that allows the function to be inlined in more than one translation unit.

However it does have the advantage that by defining away the "inline" keyword, the program reduces to a portable program with the same meaning (provided no other non-portable constructions are used).
A function defined with "extern inline". Stand-alone object code is never emitted. You can have multiple such definitions and your program will still work. However, you should add a non-inline definition somewhere too, in case the function is not inlined everywhere.

This provides sensible semantics (you can avoid duplicate copies of the functions' object code) but is a bit inconvenient to use.

One approach to using this would be to put the definitions in a header file, surrounded by a #if that expands to true either when using GNU C, or when the header has been included from the file that is contain the emitted definitions (whether or not using GNU C). In the latter case the "extern" is omitted (for instance writing "EXTERN" and #define-ing that to either "extern" or nothing). The "#else" branch would contain just declarations of the functions, for non-GNU compilers.
A function defined with "static inline". Stand-alone object code may be emitted if required. You can have multiple definitions in your program, in different translation units, and it will still work. Just dropping the "inline" reduces the program to a portable one (again, all other things being equal).

This is probably useful primarily for small functions that you might otherwise use macros for. If the function isn't always inlined then you get duplicate copies of the object code, with the problems described above.

A sensible approach would be to put the "static inline" functions in either a header file if they are to be widely used or just in the source files that use them if they are only ever used from one file.
Summary:

If you don't use the 'static' prefix, gcc assumes there may be calls from other source files (as in the case of a normal function). Therefore, 'inline' without the 'static' or 'extern' prefix is essentially the equivalent of a normal function.

'static inline' is defined as "inline wherever you can, and if you can't in any place, create a static version for those instances". Reasons for the static version include calls that precede the function's definition, recursive calls within the definition, not using -O, referring to the function by address, etc..

'extern inline' is defined as "inline, and if you can't, expect an external definition".

C99 inline rules

The specification for "inline" is section 6.7.4 of the C99 standard (ISO/IEC 9899:1999). This isn't freely available, but you can buy a PDF of it from ISO relatively cheeply.

A function where all the declarations (including the definition) mention "inline" and never "extern". There must be a definition in the same translation unit. No stand-alone object code is emitted. You can (must?) have a separate (not inline) definition in another translation unit, and the compiler might choose either that or the inline definition (but must document how it chooses).

Such functions may not contain modifiable static variables, and may not refer to static variables or functions elsewhere in the source file where they are declared.
A function where at least one declaration mentions "inline", but where some declaration doesn't mention "inline" or does mention "extern". There must be a definition in the same translation unit. Stand-alone object code is emitted (just like a normal function) and can be called from other translation units in your program.

The same constraint about statics above applies here, too.
A function defined "static inline". A local definition may be emitted if required. You can have multiple definitions in your program, in different translation units, and it will still work. This is the same as the GNU C rules.

main is not allowed to be an inline function.

(If you think I've misinterpreted these rules, please let me know!)

(C++ is stricter: a function which is inline anywhere must be inline everywhere and must be defined identically in all the translation units that use it.)

Recent versions of GNU C have a -std=c99 option, but this doesn't enable C99 inline rules yet. The manual recommends sticking to "static inline", that being the portable subset, and promises C99 semantics in a future release.

Strategies for using inline functions

These rules suggest several possible models for using inline functions in more or less portable ways.

A simple portable model. Use "static inline" (either in a common header file or just in one file). If the compiler needs to emit a definition (e.g. to take its address, or because it doesn't want to inline some call) then you waste a bit of space; if you take the address of the function in two translation units then the result won't compare equal.

For instance, in a header file:
```
static inline int max(int a, int b) {
  return a > b ? a : b;
}
```
You can support legacy compilers (i.e. anything without "inline") via -Dinline="", although this wastes space.
A GNU C model. Use "extern inline" in a common header and provide a definition in a .c file somewhere, perhaps using macros to ensure that the same code is used in each case. For instance, in the header file:
```
#ifndef INLINE
# define INLINE extern inline
#endif
INLINE int max(int a, int b) {
  return a > b ? a : b;
}
```
...and in exactly one source file:
```
#define INLINE
#include "header.h"
```
Supporting legacy compilers is awkward unless you don't mind wasting space and having multiple addresses for the same function; you need to restrict the definitions to a in single translation unit (with INLINE defined to the empty string) and add some external declarations in the header file.
A C99 model. Use "inline" in a common header, and provide definitions in a .c file somewhere, via "extern" declarations. For instance, in the header file:
```
inline int max(int a, int b) {
  return a > b ? a : b;
}
```
...and in exactly one source file:
```
#include "header.h"
extern int max(int a, int b);
```
To support legacy compilers, you have to swap the whole thing around so that the declarations are visible in the common header and the definitions are restricted to a single translation unit, with inline defined away.
A complicated portable mode. Use macros to choose either "extern inline" for GNU C, "inline" for C99, or neither for a definition. For instance, in the header:
```
#ifndef INLINE
# if __GNUC__
#  define INLINE extern inline
# else
#  define INLINE inline
# endif
#endif
INLINE int max(int a, int b) {
  return a > b ? a : b;
}
```
...and in exactly one source file:
```
#define INLINE
#include "header.h"
```
Supporting legacy compilers has the same issues as the GNU C model.

(This model won't work properly when GNU C gains C99 inline support - instead it'll be necessary to determine which version of the compiler is in use and whether it is a C99 mode.)

Please report any errors.