Default Argument Promotion

Default Argument Promotion describes the process by which function arguments of type char, short and float are sometimes promoted when passed to a function. This occurs when the compiler is unable to make any assumptions about the type of the corresponding parameter in the receiving function, as in the following situations:

• An argument is passed to an old style function (i.e. a function that, at the point of calling, has not been defined or declared with a full ISO C style prototype). Refer to Old and New Style Functions for a discussion of the different ways in which a function can be defined.
• An argument is passed to a new style function (i.e. a function that, at the point of calling, has been defined or declared with full ISO C style prototype), but the argument corresponds to a parameter defined with ellipsis notation (, ...), i.e. a function defined with a variable number of parameters. In other words the type of the parameter has not been explicitly defined.

When default argument promotion is applied, arguments of type char or short are converted to type int or unsigned int according to the rules of Integral Promotion; arguments of type float are converted to type double. The promoted type of an argument is required to be consistent with the promoted type of the corresponding parameter in the function definition; so, for example, it is legitimate to pass an argument of type float when the receiving parameter is of type double.

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