src/fsfw/filesystem/HasFileSystemIF.h

@@ -83,7 +83,11 @@ class HasFileSystemIF {
   virtual ReturnValue_t truncateFile(FilesystemParams params) = 0;
 
   /**
-   * @brief   Generic function to append to file.
+   * @brief   Generic function to write to a file.
+   *
+   * @details
+   * Implementations should not truncate the file. This is equivalent to opening a  file with "r+" on Unix systems
+   * or using ios::out | ios::in with the C++ API.
    * @param fileOpInfo General information: File name, size to write, offset, additional arguments
    * @param data The data to write to the file
    */

src/fsfw_hal/host/HostFilesystem.cpp

@@ -18,7 +18,9 @@ ReturnValue_t HostFilesystem::writeToFile(FileOpParams params, const uint8_t *da
   if (not exists(path)) {
     return HasFileSystemIF::FILE_DOES_NOT_EXIST;
   }
-  ofstream file(path, ios::binary | ios::out);
+  // This is equivalent to "r+" mode, which is what we need here. Only using ::out would truncate
+  // the file
+  ofstream file(path, ios::binary | ios::out | ios::in);
   if (file.fail()) {
     return HasFileSystemIF::GENERIC_FILE_ERROR;
   }