Fix freopen so it resets stream buffer (#61)

Author: jart

libc/stdio/freopen.c

@@ -46,6 +46,8 @@ FILE *freopen(const char *pathname, const char *mode, FILE *stream) {
       dup3(fd, stream->fd, flags & O_CLOEXEC);
       close(fd);
       stream->iomode = flags;
+      stream->beg = 0;
+      stream->end = 0;
       return stream;
     } else {
       return NULL;

test/libc/stdio/freopen_test.c

@@ -0,0 +1,66 @@
+/*-*- 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 2021 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/stdio/stdio.h"
+#include "libc/testlib/testlib.h"
+
+/*
+ * This test was contributed by @ahgamut
+ * https://github.com/jart/cosmopolitan/issues/61#issuecomment-792214575
+ */
+
+char testlib_enable_tmp_setup_teardown;
+
+int writefile(const char* filename) {
+  int stat = 0;
+  FILE* fp = fopen(filename, "w");
+  stat = fputs("cosmopolitan libc\n", fp);
+  fclose(fp);
+  return stat;
+}
+
+int readfile(const char* filename) {
+  int stat = 0;
+  char buf1[30];
+  char buf2[30];
+  FILE *fp1, *fp2;
+  fp1 = fopen(filename, "r");
+  if (!fp1) {
+    printf("failed to read %s in r\n", filename);
+    return 1;
+  }
+  buf1[0] = fgetc(fp1);
+  buf1[1] = '\0';
+  fp2 = freopen(filename, "rb", fp1);
+  if (!fp2) {
+    printf("failed to read %s in rb\n", filename);
+    return 1;
+  }
+  stat = fread(buf2, sizeof(buf2[0]), 20, fp2);
+  ASSERT_EQ(18, stat);
+  buf2[stat] = '\0';
+  fclose(fp2);
+  ASSERT_STREQ("c", buf1);
+  ASSERT_STREQ("cosmopolitan libc\n", buf2);
+  return 0;
+}
+
+TEST(freopen, test) {
+  writefile("file.txt");
+  readfile("file.txt");
+}