yuyz-cyber left a comment (kamailio/kamailio#4563)
You are correct that the prototype takes an int, but the value domain is
strictly restricted.
The isalnum()/isalpha() argument must be either EOF or a value representable as
unsigned char (0–255). Anything else is undefined behavior.
On platforms where char is signed, a raw byte like 0x80 stored in a char
promotes to -128. If the code calls isalnum((int)*p), it passes -128. This is
not EOF (-1) and not in the unsigned char range, so it triggers UB.
Casting to unsigned char forces the value into the 0–255 range, which is
exactly what the ctype APIs require.
The GNU libc manual (v2.42) explicitly states this requirement:
"If the argument c is of type char, it must be cast to unsigned char, as in the
following example: isalpha ((unsigned char) c);" Source:
https://sourceware.org/glibc/manual/2.42/html_node/Classification-of-Characters.html
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