Why are islower() and friends required to handle EOF?

问题

Why are islower() and friends required to handle EOF, whereas putchar() and friends don't have to?

Why isn't islower() treating int as unsigned char, as it is the case in putchar()? This would make total sense, because we have to check for EOF first anyway. See also Why the argument type of putchar(), fputc(), and putc() is not char?

回答1:

because we have to check for EOF first anyway.

We absolutely don't.

int c;
while(isspace(c=fgetc(fp)));
if (c==EOF) ...

This is totally legitimate code to skip whitespaces. Checking each character for EOF separately is a waste of time.

The ctype functions are made to handle EOF specifically to enable code like this.

回答2:

None of character type functions are required to handle EOF, other than ignoring it (i.e. returning false). In fact, EOF marker is not even mentioned in <ctype.h> header documentation.

The most likely reason for character classification function signatures to use int in place of char, signed or unsigned, is to avoid implementation-defined behavior in loops like this:

int c;
while ((c =getchar()) != EOF) {
    if (islower(c)) {
        ...
    } else if (isdigi(c)) {
        ...
    }
}

This would compile and run with islower(char) instead of islower(int), but the result would be implementation defined, which is not desirable under such basic circumstances. Essentially, int in the signature of getchar became "contagious," getting into signatures of functions only marginally related to it.

来源：https://stackoverflow.com/questions/40716908/why-are-islower-and-friends-required-to-handle-eof

标签

glibc

c-standard-library