[libc-commits] [libc] [libc] Add tmpnam implementation (PR #204901)

[Jeff Bailey via libc-commits]

================
@@ -0,0 +1,79 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// Declaration of tmpnam, a POSIX function that generate a string that is a
+/// valid pathname that does not name an existing file.
+/// See:
+/// https://pubs.opengroup.org/onlinepubs/9799919799/functions/tmpnam.html
+///
+//===----------------------------------------------------------------------===//
+
+#include "src/stdio/tmpnam.h"
+#include "hdr/errno_macros.h"
+#include "hdr/stdio_macros.h"
+#include "hdr/unistd_macros.h"
+#include "src/__support/CPP/atomic.h"
+#include "src/__support/OSUtil/linux/syscall_wrappers/access.h"
+#include "src/__support/OSUtil/linux/syscall_wrappers/getrandom.h"
+#include "src/__support/macros/config.h"
+#include "src/stdio/snprintf.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+static char tmpbuf[L_tmpnam];
+static cpp::Atomic<size_t> tmpnam_budget = TMP_MAX;
+
+/* partially thread-safe */
+LLVM_LIBC_FUNCTION(char *, tmpnam, (char *s)) {
+  if (s == nullptr)
+    s = tmpbuf;
+
+  // here if the s is null then use tmpbuf and if sizeof
+  // POSIX portable filename character set, sorted by ASCII value.
+  // See
+  // https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap03.html#tag_03_265
+  const char charset[] = "-._0123456789"
+                         "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+                         "abcdefghijklmnopqrstuvwxyz";
+
+  int prefix_size = LIBC_NAMESPACE::snprintf(s, L_tmpnam, "%s/", P_tmpdir);
----------------
kaladron wrote:

We try to avoid using public entrypoints in functions.  But instead, I think you might be able to do something like this:

```c++
  // We want to construct: P_tmpdir / <14 random chars> \0
  // P_tmpdir is "/tmp" (length 4).
  // Total length must be L_tmpnam (20).
  // /tmp/ is 5 chars.
  // Random suffix is 14 chars.
  // Null terminator is 1 char.
  // Total: 5 + 14 + 1 = 20.
  
  constexpr cpp::string_view PREFIX = P_tmpdir "/";
  static_assert(PREFIX.size() + 14 + 1 == L_tmpnam, "L_tmpnam mismatch");
  inline_memcpy(buf.data(), PREFIX.data(), PREFIX.size());
```

https://github.com/llvm/llvm-project/pull/204901