type casting in K&R

Question

K&R provide this getchar() example:

int getchar(void)
{
    char c;

    return (read(0, &c, 1) == 1) ? (unsigned char) c : EOF;
}

c is cast to unsigned char here to avoid sign extension issues, but in the fputs() example...

int fputs(char *s, FILE *iop)
{
    int c;

    while (c = *s++)
        putc(c, iop);
    return ferror(iop) ? EOF : 0;
}

*s is assigned to an int without first casting to an unsigned char. Why is the cast unnecessary this time?

Answer 1

It is not about "sign extension issues". This implementation of getchar makes sure that all successfully read characters are returned as non-negative int values. This behavior is required by the specification of getchar, which literally says that the character read is returned as unsigned char values converted to int, even if char is signed on the given platform. What you see there is basically a direct implementation of getchar spec.

Meanwhile fputs does not return any specific character values. fputs does not return c to the user. That c is a purely internal variable. It should preserve the original value of char type on the given platform, since the value of c is then passed to putc. putc does not expect character values converted to non-negative range, it expects original character values, which could easily be negative if char is signed.

BTW, why did you look at fputs, and not fputc? If you look at fputc, which just like getchar returns a character value, you will probably see that it is implemented similarly to getchar in that regard.