[BUG] Undefined behavior in ctype functions when processing non-ASCII characters in ffCharIsSpace and strutil #2721

General description of the bug

In src/util/FFstrutil.h and related string utility functions (e.g. ffCharIsSpace), characters of type char are passed directly to <ctype.h> functions such as isspace(), isblank(), and isalnum() without being cast to unsigned char.

According to the ISO C standard (C99 §7.4 paragraph 1, C11 §7.4 paragraph 1) and POSIX.1-2008:

In all cases the argument is an int, the value of which shall be representable as an unsigned char or shall equal the value of the macro EOF. If the argument has any other value, the behavior is undefined.

On architectures where char is signed (default on x86_64 Linux, macOS, and Windows), any non-ASCII character (e.g. byte values with the high bit set 0x80 to 0xFF, common in UTF-8 multibyte sequences for international text, hostnames, usernames, or distro names) is converted to a negative int due to sign extension.

When passed to standard library implementations such as GNU libc (glibc), musl, or MSVC, the negative index accesses memory before the *__ctype_b_loc() lookup table:

• In debug builds or with ASan/UBSan (-fsanitize=undefined), this triggers an undefined behavior diagnostic or crash.
• In production, it reads out-of-bounds memory from the ctype lookup array, potentially causing misclassification, crashes (SIGSEGV), or memory safety violations.

Version used

Fastfetch 2.38.0 (commit from dev branch)

Bug prevalence

Always (whenever non-ASCII UTF-8 characters or bytes with high-bit set are evaluated)

Regression

No

Installation

Built from source

Expected behavior

Characters passed to <ctype.h> functions should be safely cast to (unsigned char) before promotion to int:

static inline bool ffCharIsSpace(char c)
{
    return isspace((unsigned char) c);
}

Steps to Reproduce

1. Build with UndefinedBehaviorSanitizer (-fsanitize=undefined).
2. Pass characters with the high bit set (>= 0x80) to ffCharIsSpace or related string utility functions.
3. UBSan reports: index out of bounds for __ctype_b_loc.

Proposed Fix

Cast char arguments to (unsigned char) across ffCharIsSpace and related ctype helpers in src/util/FFstrutil.h (see PR #2716).