вопрос про select()

mkfifo /tmp/test_pipe

#include <sys/select.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>

#include <assert.h>

int main(int argc, char **argv) {

  int fd = open("/tmp/test_pipe", O_RDONLY);
  char buf[1024];

  fd_set rset;
  FD_ZERO(&rset);
  FD_SET(fd, &rset);

  int rv = select(fd+1, &rset, NULL, NULL, NULL);
  if (rv > 0) {
    if(FD_ISSET(fd, &rset)) {
      int len = read(fd, buf, 1023);
      printf("rv=%d, read %d bytes\n", rv, len);
      buf[len] = '\0';
      printf("%s", buf);
    }
  } else {
    perror("select");
  }


  FD_ZERO(&rset);
  FD_SET(fd, &rset);

  rv = select(fd+1, &rset, NULL, NULL, NULL);
  if (rv > 0) {
    if(FD_ISSET(fd, &rset)) {
      int len = read(fd, buf, 1023);
      assert(len);
      printf("rv=%d, read %d bytes\n", rv, len);
      buf[len] = '\0';
      printf("%s", buf);
    }
  } else {
    perror("select");
  }

  return 0;
}

echo lalala > /tmp/test_pipe

rv=1, read 7 bytes
lalala
a.out: test.c:42: main: Assertion `len' failed.
Aborted