Okay, so there's nothing to worry about with those warnings, that wouldn't be why the library isn't working for me. And yeah, that's the one I'm trying to port.

I've further nailed down the issue to a read() call in editline.c:rl_getc(void).

int rl_getc(void)
{
    int r;
    char c;
 
    do {
        r = read(el_infd, &c, 1);
    } while (r == -1 && errno == EINTR);
 
    return r == 1 ? c : EOF;
}

It appears that read() returns 5 instead of the expected 1 (or -1 for error), leading to the EOF return which eventually percolates up to the readline() call in the program itself getting NULL and figuring there's no more to do. So it looks like the fix might be to change the check from r == 1 to r > 0. However now I'm concerned that it'll only return every fifth character typed! Time for more testing.

UPDATE: Looks like it's returning every character, not every fifth. I've found my fix.

The work's not done yet. While I've been able to package up the working library and upload it to Aminet, I still need to figure out how to maintain my patch, especially as it seems that in porting the library I've found a problem in newlib's implementation of read() which the developer of libeditline doesn't want to account for. When all's figured out, though, I do plan to write up about the journey.

Unfortunately it's no simple case, but the attached archive is my build of the library without changes save some debugging code in the rl_getc() function inside src/editline.c, clearly called out. Running any of the programs in the examples drawer from the shell will exhibit the debug error message on the first keystroke following the display of the prompt, and whenever a character is typed following use of the tab, backspace, or delete key.