Should You Cast `char` to `unsigned char` Before Using `toupper()` and `tolower()` in C ?

Casting to Unsigned Char Before Calling Character Manipulation Functions

In C , the question arises whether it's necessary to cast char arguments to unsigned char before invoking functions like toupper() and tolower() from the header. The confusion stems from contrasting perspectives.

Some experts argue that casting is crucial to prevent undefined behavior. According to the C standard, the argument passed to toupper() must be representable as an unsigned char or equal to EOF. If the argument has any other value, the behavior is undefined.

Plain char can have either a signed or unsigned representation, and if it's signed, negative char values can cause undefined behavior when passed to toupper(). This occurs because toupper() expects an int argument, and implicit conversion of a negative signed char to int results in a negative value.

For example, given the initialization:

string name = "Niels Stroustrup";

The expression toupper(name[0]) is risky if plain char is signed because name[0] could be negative. To avoid this, casting to unsigned char is recommended:

char c = name[0];
c = toupper((unsigned char)c);

Other experts maintain that casting is unnecessary. They point out that the C standard guarantees non-negative values for members of the basic character set. Therefore, for strings initialized with valid characters, there's no risk of undefined behavior.

Bjarne Stroustrup himself demonstrates using toupper() without casting in his book, "The C Programming Language." He seems to assume that char is unsigned, but this is not always the case.

In the implementation, functions commonly use a lookup table to perform character manipulation. Passing a negative value to such a table could lead to index out-of-bounds errors. Nevertheless, toupper() could be implemented to tolerate negative values, but such behavior is not required.

Ultimately, the correct approach depends on the platform and compiler implementation. If in doubt, casting to unsigned char is a safe and conservative practice to avoid undefined behavior when calling character manipulation functions like toupper() and tolower().

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