libos: Add File::seek() and File::tell()

libos/include/os/File.h

@@ -37,6 +37,13 @@ namespace os
             ReadAppend = O_RDWR | O_APPEND, // Open the file read-write, and append to it.
         };
 
+        enum class SeekMode
+        {
+            Beginning = SEEK_SET, // Seek from the beginning of the file.
+            Current = SEEK_CUR,   // Seek from the current file position.
+            End = SEEK_END,       // Seek from the end of the file.
+        };
+
         /**
          * @brief Create a new File object from a file path, opening the file.
          *
@@ -191,6 +198,22 @@ namespace os
          */
         void rewind();
 
+        /**
+         * @brief Change the file pointer's position.
+         *
+         * @param offset The number of bytes to move the file pointer (can be negative, to move backwards).
+         * @param mode Where to seek from.
+         * @return Result<void> Whether the operation succeeded.
+         */
+        Result<void> seek(off_t offset, SeekMode mode);
+
+        /**
+         * @brief Read the file pointer's current position.
+         *
+         * @return Result<off_t> An error, or the file pointer's position.
+         */
+        Result<off_t> tell();
+
         /**
          * @brief Buffering modes for a File.
          */

libos/src/File.cpp

@@ -227,6 +227,20 @@ namespace os
         fflush(m_file);
     }
 
+    Result<void> File::seek(off_t offset, SeekMode mode)
+    {
+        int rc = fseek(m_file, offset, (int)mode);
+        if (rc < 0) return err(errno);
+        return {};
+    }
+
+    Result<off_t> File::tell()
+    {
+        off_t off = ftell(m_file);
+        if (off < 0) return err(errno);
+        return off;
+    }
+
     void File::set_buffer(BufferingMode mode)
     {
         setvbuf(m_file, NULL, mode, 0);