On Emscripten, when reading from a pipe created via pipe(), the read() call returns immediately with EAGAIN when there's no data in the pipe. This contradicts the standard behavior specified by POSIX:

  EAGAIN The file descriptor fd refers to a file other than a
         socket and has been marked nonblocking (O_NONBLOCK), and
         the read would block.  See open(2) for further details on
         the O_NONBLOCK flag.
  EAGAIN or EWOULDBLOCK
         The file descriptor fd refers to a socket and has been
         marked nonblocking (O_NONBLOCK), and the read would block.

https://man7.org/linux/man-pages/man2/read.2.html

  If a process attempts to read from an empty pipe, then read(2)
  will block until data is available. <...>
  Nonblocking I/O is possible by using the fcntl(2)
  F_SETFL operation to enable the O_NONBLOCK open file status flag.

https://man7.org/linux/man-pages/man7/pipe.7.html

Given that pipe() doesn't set the O_NONBLOCK flag, I believe the Emscripten's behavior with returning EAGAIN is wrong.

Sample program:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main(int argc, char** argv) {
  int pipefd[2];
  if (pipe(pipefd)) {
    perror("pipe() failed");
    exit(-1);
  }
  if (argc > 1) {
    char c = 0;
    if (write(pipefd[1], &c, 1) != 1) {
      perror("write() failed");
      exit(-1);
    }
  }
  char c;
  if (read(pipefd[0], &c, 1) != 1) {
    perror("read() failed");
    exit(-1);
  }
  exit(0);
}

This program, when run under Emscripten, fails with "Resource temporarily unavailable". When the same program is run on Linux using standard g++ and libc, it correctly blocks in read().
P.S. The program also demonstrates that, in case write() was previously called, the data is successfully read from it under Emscripten.