We've noticed this when switching to using the llvm-wasm backend. (currently using emscripten 1.39.4)

Here's a small testcase:

#include <stdio.h>
#include <errno.h>
#include <fcntl.h>

int main()
{
    int fd = 0;
    printf("errno is %d\n", errno);
    fd = open("doesnotexist.txt", O_RDONLY);
    printf("fd is %d, errno is %d\n", fd, errno);
    return 0;
}

When compiled with gcc, I get the expected "no such file":
errno is 0
fd is -1, errno is 2

When using emcc and running in a browser, I keep seeing errno being set to 44 "Channel number out of range"
(compiled with emcc testopen.c -o testopen.html)
errno is 0
fd is -1, errno is 44

I'm not sure if this is indicative of some other problem, but it's obviously causing some issues for us since moving off of fastcomp.