testcase for fseek's issue in BSD with buffered stdin

f.sh

#!/bin/sh

# set CC to a suitable compiler
# pass -DWORKAROUND (or equivalent) for testing "workaround" instead

if [ ! -n "$CC" ]; then
  for CC in cc gcc clang xlc cl; do
    if command -v $CC >/dev/null; then
      break
    fi
  done
fi

cat >f.c <<EOF
/*
   usage: a.out <f.in
          -DWORKAROUND for building POSIX workaround
 */
#include <stdio.h>
#include <assert.h>
#include <unistd.h>

int main(void)
{
  long pos;
  char buf[3] = {0};
  int rc;
  off_t o;

  rc = fread(buf, 1, 2, stdin); /* read 2 characters */
  assert(rc == 2);    /* also ensures !feof(stdin) and a seekable pos!=0 */
  pos = ftell(stdin); /* same behaviour if using ftello or fgetpos */
  assert(pos > 0);
  rc = fread(buf, 1, 2, stdin);
  printf("%s", buf); /* includes \n already */
  assert(rc == 2);
#ifndef WORKAROUND
  rc = fseek(stdin, pos, SEEK_SET); /* also BUG with fsetpos */
  assert(rc == 0);
#else
  o = lseek(0, pos, SEEK_SET);
  assert(o == pos);
#endif
  return 0;
}
EOF

cat >f.in <<EOF
1
2
3
4
EOF

$CC $1 f.c
exec 3<f.in
O1=`./a.out <&3`
O2=`head -1 <&3`
exec 3<&-
if [ "$O1" != "$O2" ]; then
  echo "BUG: $O1 != $O2"
fi