ioctl(pipe_fd, FIONREAD, &n) fails on pipe file descriptors

[Ryan2026R]

Version of emscripten/emsdk:

5.0.6

ioctl(pipe_read, FIONREAD, &avail) should return 0 and set avail to the byte count buffered in the pipe. Under Emscripten it returns 28 and leaves avail = 0 even when the pipe has data ready to read.

Failing command line in full:

$ emcc -O0 -Wall fionread.c -o a.out.js
$ node a.out.js
FIONREAD: ret=28 avail=0

The same program on native Linux prints FIONREAD: ret=0 avail=5.

Example code snippet:

#include <unistd.h>
#include <sys/ioctl.h>
#include <stdio.h>

int main(void) {
    int pfd[2];
    pipe(pfd);
    write(pfd[1], "hello", 5);

    int avail = 0;
    int r = ioctl(pfd[0], FIONREAD, &avail);
    printf("FIONREAD: ret=%d avail=%d\n", r, avail);

    close(pfd[0]); close(pfd[1]);
    return (r == 0 && avail == 5) ? 0 : 1;
}