How Do Default Type Promotions Work in Variadic Argument Lists in C and C ?

Default Type Promotions in Variadic Argument Lists

In C and C , variadic functions can accept a variable number of arguments. However, the compiler may need to perform implicit type promotions on these arguments to ensure compatibility with the function signature. This article examines the default type promotions that occur within variadic argument lists.

Default Integer Promotions

For functions without prototypes or arguments matching the ellipsis "...", C99 specifies that default argument promotions are applied. These include:

• Default integer promotions: Every integer type of rank less than int is promoted to int or unsigned int.

This means that the following code is valid even though uint8_t is an 8-bit unsigned integer and printf expects a 16-bit signed integer:

<code class="c">uint8_t a = 5;
printf("%d", a);</code>

Float Promotion

In addition to default integer promotions, float arguments are promoted to double in variadic contexts. This ensures compatibility with functions expecting double arguments, such as printf's %f format specifier.

Other Analogous Types

The same default promotion rules apply to other analogous types:

• char to int
• short to int or unsigned int (depending on the implementation)
• long to long int or unsigned long int

Implications

These default promotions ensure that arguments passed to variadic functions are compatible with the function signature. However, it is important to be aware of the potential implications:

• Type promotions may cause precision loss if the promoted value is larger than the original type.
• Type mismatch errors can result if the promoted value is too large for the target type.

By understanding the default type promotions in variadic argument lists, programmers can use these functions safely and effectively.

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