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GNU C (and some other compilers) supported inline functions before they were added to the C Standard and, as a result, have significantly different semantics. Richard Kettlewell provides a good explanation of differences between the C99 and GNU C rules [Kettlewell 2003].
Exceptions
PRE00-C-EX1: Macros can be used to implement local functions (repetitive blocks of code that have access to automatic variables from the enclosing scope) that cannot be achieved with inline functions.
PRE00-C-EX2: Macros can be used for concatenating tokens or performing stringification. For example,
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calculates only one of the two expressions depending on the selector's value. See PRE05-C. Understand macro replacement when concatenating tokens or performing stringification for more information.
PRE00-C-EX3: Macros can be used to yield a compile-time constant. This is not always possible using inline functions, as shown by the following example:
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In this example, the ADD_M(3,4) macro invocation yields a constant expression, but the add_f(3,4) function invocation does not.
PRE00-C-EX4: Macros can be used to implement type-generic functions that cannot be implemented in the C language without the aid of a mechanism such as C++ templates.
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Type-generic macros may also be used, for example, to swap two variables of any type, provided they are of the same type.
PRE00-C-EX5: Macro parameters exhibit call-by-name semantics, whereas functions are call by value. Macros must be used in cases where call-by-name semantics are required.
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