[libc-commits] [libc] feat(filemode): implement class and helper functions to handle file modes for an opened file (PR #220906)

[David Dada via libc-commits]

https://github.com/obadafidii updated https://github.com/llvm/llvm-project/pull/220906

>From e3dc3a646ef6c422e75810881826e211aebb7a3b Mon Sep 17 00:00:00 2001
From: tdadadavid <davidtofunmidada at gmail.com>
Date: Thu, 3 Sep 2026 13:21:11 +0100
Subject: [PATCH] feat(filemode): implement class and helper functions to
 handle file modes for an opened file

Signed-off-by: tdadadavid <davidtofunmidada at gmail.com>
---
 libc/src/__support/File/file_mode.h | 125 ++++++++++++++++++++++++++++
 1 file changed, 125 insertions(+)
 create mode 100644 libc/src/__support/File/file_mode.h

diff --git a/libc/src/__support/File/file_mode.h b/libc/src/__support/File/file_mode.h
new file mode 100644
index 0000000000000..6b837ea46f652
--- /dev/null
+++ b/libc/src/__support/File/file_mode.h
@@ -0,0 +1,125 @@
+
+#ifndef LLVM_LIBC_SRC___SUPPORT_FILE_FILE_MODE_H
+#define LLVM_LIBC_SRC___SUPPORT_FILE_FILE_MODE_H
+#include <cstdint>
+
+namespace LIBC_NAMESPACE_DECL {
+
+// FileMode class handles everything regarding the mode of the file, be it's opening mode or content type.
+class FileMode {
+public:
+  // FileMode constructor accepts the mode string as an argument.
+  // It performs validation against several rules and records the `file_mode` property to
+  // the specific mode.
+  explicit FileMode(const char *mode) : file_mode_(0) {
+    // First character in |mode| should be 'a', 'r' or 'w'.
+    if (*mode != 'a' && *mode != 'r' && *mode != 'w')
+      return;
+
+    // There should be exactly one main mode ('a', 'r' or 'w') character.
+    // If there are more than one main mode characters listed, then
+    // we will consider |mode| as incorrect and return 0;
+    int main_mode_count = 0;
+
+    for (; *mode != '\0'; ++mode) {
+      switch (*mode) {
+      case 'r':
+        file_mode_ |= static_cast<Mode>(OpenMode::READ);
+        ++main_mode_count;
+        break;
+      case 'w':
+        file_mode_ |= static_cast<Mode>(OpenMode::WRITE);
+        ++main_mode_count;
+        break;
+      case '+':
+        file_mode_ |= static_cast<Mode>(OpenMode::PLUS);
+        break;
+      case 'b':
+        file_mode_ |= static_cast<Mode>(ContentType::BINARY);
+        break;
+      case 'a':
+        file_mode_ |= static_cast<Mode>(OpenMode::APPEND);
+        ++main_mode_count;
+        break;
+      case 'x':
+        file_mode_ |= static_cast<Mode>(CreateType::EXCLUSIVE);
+        break;
+      default:
+        file_mode_ = 0;
+      }
+    }
+
+    if (main_mode_count != 1)
+      file_mode_ = 0;
+  }
+
+  // helper function to show if file allows writing
+  bool write_allowed() const {
+    return (file_mode_ & static_cast<Mode>(OpenMode::WRITE)) != 0;
+  }
+
+  // helper function to show if file allows reading
+  bool read_allowed() const {
+    return (file_mode_ & static_cast<Mode>(OpenMode::READ)) != 0;
+  }
+
+  // helper function to show if file allows appending
+  bool append_allowed() const {
+    return (file_mode_ & static_cast<Mode>(OpenMode::APPEND)) != 0;
+  }
+
+  // helper function to denote if the file is in binary format.
+  bool is_binary_format() const {
+    return (file_mode_ & static_cast<Mode>(ContentType::BINARY)) != 0;
+  }
+
+  // '+' means update is allowed
+  // TODO: ask michael if I need to give it a better name like "update_allowed" or just continue with the
+  // old convention.
+  bool is_plus() const {
+    return (file_mode_ & static_cast<Mode>(OpenMode::PLUS)) != 0;
+  }
+
+  // checks if a file was created for writing
+  bool is_exclusive_create() const {
+    return (file_mode_ & static_cast<Mode>(CreateType::EXCLUSIVE)) != 0;
+  }
+
+private:
+  // Mode is a generic or abstract mode bit for all kinds of modes
+  // (open-mode, 'content-mode', 'create-modes')
+  using Mode = uint32_t;
+
+  // Denotes the mode of the file.
+  //
+  // The three different types of flags below are to be used with '|' operator.
+  // Their values correspond to mutually exclusive bits in a 32-bit unsigned
+  // integer value. A flag set can include both READ and WRITE if the file
+  // is opened in update mode (ie. if the file was opened with a '+' the mode
+  // string.)
+  enum class OpenMode: Mode {
+    READ = 0x1,
+    WRITE = 0x2,
+    APPEND = 0x4,
+    PLUS = 0x8,
+  };
+
+  // Denotes a file opened in binary mode (which is specified by including
+  // the 'b' character in teh mode string.)
+  enum class ContentType: Mode {
+    BINARY = 0x10,
+  };
+
+  // Denotes a file to be created for writing.
+  enum class CreateType: Mode {
+    EXCLUSIVE = 0x100,
+  };
+
+  // This property tracks the mode for the particular file instance (i.e currently opened file)
+  int file_mode_;
+};
+
+
+}
+
+#endif // LLVM_LIBC_SRC___SUPPORT_FILE_FILE_MODE_H