Support TMP environment variables for tmp::file on POSIX

src/create_file.cpp

@@ -10,6 +10,10 @@
 #include <filesystem>
 #include <system_error>
 
+#if __has_include(<unistd.h>)
+#include <unistd.h>
+#endif
+
 namespace tmp::detail {
 
 namespace fs = std::filesystem;
@@ -18,11 +22,29 @@ namespace fs = std::filesystem;
 /// @returns A pointer to the file stream associated with the temporary file
 /// @throws fs::filesystem_error if cannot create a temporary file
 std::FILE* create_file() {
+#if __has_include(<unistd.h>)
+  std::string path = fs::temp_directory_path() / "XXXXXX";
+  int descriptor   = mkstemp(path.data());
+  if (descriptor == -1) {
+    std::error_code ec = std::error_code(errno, std::generic_category());
+    throw fs::filesystem_error("Cannot create a temporary file", ec);
+  }
+
+  unlink(path.data());
+
+  std::FILE* file = fdopen(descriptor, "wb+");
+  if (file == nullptr) {
+    std::error_code ec = std::error_code(errno, std::generic_category());
+    close(descriptor);
+    throw fs::filesystem_error("Cannot create a temporary file", ec);
+  }
+#else
   std::FILE* file = std::tmpfile();
   if (file == nullptr) {
     std::error_code ec = std::error_code(errno, std::generic_category());
     throw fs::filesystem_error("Cannot create a temporary file", ec);
   }
+#endif
 
   return file;
 }