Get GNU GMP test suite fully passing

- Fix stdio fmemopen() buffer behaviors
- Fix scanf() to return EOF when appropriate
- Prefer fseek/ftell names over fseeko/ftello
- Ensure locale field is always set in the TIB
- Fix recent regression in vfprintf() return count
- Make %n directive in scanf() have standard behavior

libc/stdio/ftell.c

@@ -0,0 +1,59 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2020 Justine Alexandra Roberts Tunney                              │
+│                                                                              │
+│ Permission to use, copy, modify, and/or distribute this software for         │
+│ any purpose with or without fee is hereby granted, provided that the         │
+│ above copyright notice and this permission notice appear in all copies.      │
+│                                                                              │
+│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
+│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
+│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
+│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
+│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
+│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
+│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
+│ PERFORMANCE OF THIS SOFTWARE.                                                │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/calls/calls.h"
+#include "libc/errno.h"
+#include "libc/runtime/runtime.h"
+#include "libc/stdio/internal.h"
+#include "libc/stdio/lock.internal.h"
+#include "libc/stdio/stdio.h"
+#include "libc/sysv/consts/o.h"
+
+static inline int64_t ftell_unlocked(FILE *f) {
+  int64_t pos;
+  uint32_t skew;
+  if (f->fd != -1) {
+    if (__fflush_impl(f) == -1) return -1;
+    if ((pos = lseek(f->fd, 0, SEEK_CUR)) != -1) {
+      if (f->beg < f->end) pos -= f->end - f->beg;
+      return pos;
+    } else {
+      f->state = errno == ESPIPE ? EBADF : errno;
+      return -1;
+    }
+  } else {
+    return f->beg;
+  }
+}
+
+/**
+ * Returns current position of stream.
+ *
+ * @param stream is a non-null stream handle
+ * @returns current byte offset from beginning, or -1 w/ errno
+ * @threadsafe
+ */
+int64_t ftell(FILE *f) {
+  int64_t rc;
+  flockfile(f);
+  rc = ftell_unlocked(f);
+  funlockfile(f);
+  return rc;
+}
+
+__strong_reference(ftell, ftello);