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The book The C Programming Language by Kernighan and Ritchie, second edition states on page 43 in the chapter about Type Conversions:

Another example of char to int conversion is the function lower, which maps a single character to lower case for the ASCII character set. If the character is not an upper case letter, lower returns returns it unchanged.

/* lower: convert c to lower case; ASCII only */

int lower(int c)

{

    if (c >= 'A' && c <= 'Z')

        return c + 'a' - 'A';

    else

        return c;

}

It isn't mentioned explicitly in the text so I'd like to make sure I understand it correctly: The conversion happens when you call the lower function with a variable of type char, doesn't it? Especially, the expression

c >= 'A'

has nothing to do with a conversion from int to char since a character constant like 'A'
is handled as an int internally from the start, isn't it? Edit: Or is this different (e.g. a character constant being treated as a char) for ANSI C, which the book covers?

K&R C is old. Really old. Many particulars of K&R C are no longer true in up-to-date standard C.

In stadard, up-to-date C11, there is no conversion to/from char in the function you posted:

/* lower: convert c to lower case; ASCII only */

int lower(int c)

{

    if (c >= 'A' && c <= 'Z')

        return c + 'a' - 'A';

    else

        return c;

}

The function accepts int arguments as int c, and per 6.4.4.4 Character constants of the C standard, character literals are of type int.

Thus the entire lower function, as posted, under C11 deals entirely with int values.

The conversion, if any, is may be done when the function is called:

char upperA = 'A`;



// this will implicitly promote the upperA char

// value to an int value

char lowerA = lower( upperA );

Note that this is one of the differences between C and C++. In C++, character literals are of type char, not int.

Character constants have type int, as you expected, so you are correct that there are no promotions to int in this function.

Any promotion that may occur would happen if a variable of type char is passed to this function, and this is most likely what the text is referring to.

The type of character constants is int in both the current C17 standard (section 6.4.4.4p10):

An integer character constant has type
int

And in the C89 / ANSI C standard (section 3.1.3.4 under Semantics):

An integer character constant has type int

The latter of which is what K&R Second Edition refers to.