# Extra Clang Tools 21.1.6 documentation

## clang-tidy - altera-struct-pack-align

# altera-struct-pack-align

Finds structs that are inefficiently packed or aligned, and recommends packing and/or aligning of said structs as needed.

Structs that are not packed take up more space than they should, and accessing structs that are not well aligned is inefficient.

Fix-its are provided to fix both of these issues by inserting and/or amending relevant struct attributes.

Based on the Altera SDK for OpenCL: Best Practices Guide.

    // The following struct is originally aligned to 4 bytes, and thus takes up
    // 12 bytes of memory instead of 10. Packing the struct will make it use
    // only 10 bytes of memory, and aligning it to 16 bytes will make it
    // efficient to access.
    struct example {
      char a;    // 1 byte
      double b;  // 8 bytes
      char c;    // 1 byte
    };

    // The following struct is arranged in such a way that packing is not needed.
    // However, it is aligned to 4 bytes instead of 8, and thus needs to be
    // explicitly aligned.
    struct implicitly_packed_example {
      char a;  // 1 byte
      char b;  // 1 byte
      char c;  // 1 byte
      char d;  // 1 byte
      int e;   // 4 bytes
    };

    // The following struct is explicitly aligned and packed.
    struct good_example {
      char a;    // 1 byte
      double b;  // 8 bytes
      char c;    // 1 byte
    } __attribute__((packed)) __attribute__((aligned(16));

    // Explicitly aligning a struct to the wrong value will result in a warning.
    // The following example should be aligned to 16 bytes, not 32.
    struct badly_aligned_example {
      char a;    // 1 byte
      double b;  // 8 bytes
      char c;    // 1 byte
    } __attribute__((packed)) __attribute__((aligned(32)));