[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);
+
+  bool is_unique = false;
+  while (!is_unique) {
+    size_t curr_budget = tmpnam_budget.load(cpp::MemoryOrder::RELAXED);
+
+    do {
+      if (curr_budget == 0)
+        break;
+    } while (
+        !tmpnam_budget.compare_exchange_strong(curr_budget, curr_budget - 1));
+
+    if (curr_budget == 0)
+      break;
+
+    for (size_t i = prefix_size; i < L_tmpnam - 1; i++) {
+      uint8_t rand_byte;
+      auto ret = linux_syscalls::getrandom(&rand_byte, 1, 0);
+      if (!ret.has_value()) {
+        /* return nullptr when getrandom fails but consume tmpnam budget */
+        return nullptr;
+      }
+      s[i] = charset[rand_byte % (sizeof(charset) - 1)];
+    }
+    s[L_tmpnam - 1] = '\0';
+    auto res = linux_syscalls::access(s, F_OK);
----------------
kaladron wrote:

Some failures (like EACCES or ELOOP) might indicate that the file exists, which this would see as working.  This should explicitly look for ENOENT, and otherwise loop again.

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