Make more fixes and improvements

- Remove PAGESIZE constant
- Fix realloc() documentation
- Fix ttyname_r() error reporting
- Make forking more reliable on Windows
- Make execvp() a few microseconds faster
- Make system() a few microseconds faster
- Tighten up the socket-related magic numbers
- Loosen restrictions on mmap() offset alignment
- Improve GetProgramExecutableName() with getenv("_")
- Use mkstemp() as basis for mktemp(), tmpfile(), tmpfd()
- Fix flakes in pthread_cancel_test, unix_test, fork_test
- Fix recently introduced futex stack overflow regression
- Let sockets be passed as stdio to subprocesses on Windows
- Improve security of bind() on Windows w/ SO_EXCLUSIVEADDRUSE

libc/calls/mkstemp.c

@@ -0,0 +1,80 @@
+/*-*- 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/dce.h"
+#include "libc/errno.h"
+#include "libc/intrin/bits.h"
+#include "libc/stdio/rand.h"
+#include "libc/stdio/temp.h"
+#include "libc/str/str.h"
+#include "libc/sysv/consts/o.h"
+#include "libc/sysv/errfuns.h"
+
+/**
+ * Creates temporary file name and file descriptor.
+ *
+ * The best way to construct your path template is:
+ *
+ *     char path[PATH_MAX+1];
+ *     strlcat(path, kTmpDir, sizeof(path));
+ *     strlcat(path, "sauce.XXXXXX", sizeof(path));
+ *
+ * This usage pattern makes mkstemp() equivalent to tmpfd():
+ *
+ *     int fd;
+ *     fd = mkstemp(path);
+ *     unlink(path);
+ *
+ * This usage pattern makes mkstemp() equivalent to mktemp():
+ *
+ *     close(mkstemp(path));
+ *     puts(path);
+ *
+ * @param template is mutated to replace last six X's with rng
+ * @return open file descriptor r + w exclusive or -1 w/ errno
+ * @raise EINVAL if `template` didn't end with `XXXXXX`
+ * @see tmpfd() if you don't need a path
+ */
+int mkstemp(char *template) {
+  uint64_t w;
+  int i, n, e, fd;
+  if ((n = strlen(template)) < 6 ||
+      READ16LE(template + n - 2) != READ16LE("XX") ||
+      READ32LE(template + n - 6) != READ32LE("XXXX")) {
+    return einval();
+  }
+  for (;;) {
+    w = _rand64();
+    for (i = 0; i < 6; ++i) {
+      template[n - 6 + i] = "0123456789abcdefghijklmnopqrstuvwxyz"[w % 36];
+      w /= 36;
+    }
+    e = errno;
+    if ((fd = open(template,
+                   O_RDWR | O_CREAT | O_EXCL | (IsWindows() ? 0x00410000 : 0),
+                   0600)) != -1) {
+      return fd;
+    } else if (errno == EEXIST) {
+      errno = e;
+    } else {
+      template[0] = 0;
+      return fd;
+    }
+  }
+}