Automating Resource Embedding with Bin2C

Bin2C Best Practices: Efficient C Array Generation for Embedded SystemsEmbedding binary resources (images, fonts, firmware blobs, lookup tables) directly into C source as arrays is a common technique in embedded systems. The small utility “bin2c” — which converts a binary file into a C array — is a straightforward tool for this, but doing it well requires attention to efficiency, maintainability, portability, and build-system integration. This article covers best practices for generating and using C arrays produced by bin2c-like tools, with concrete examples and trade-offs that matter for constrained devices.

Why embed binary data as C arrays?

Embedding binary data into C source eliminates the need for a filesystem, simplifies deployment to ROM/flash, and avoids runtime parsing of external images or resources. It guarantees data is part of the firmware image produced by your compiler and linker, and makes it easy to place resources into specific linker sections. However, naive embedding can cause large object files, slow builds, and RAM/flash inefficiencies. The rest of this article explains how to avoid those pitfalls.

1. Choose the right format and tool options

• Prefer read-only data placement: Generate arrays declared const so the compiler places them in flash/ROM instead of RAM.
  • Example: const unsigned char my_data[] = { ... };
• Choose the proper integer type: Use uint8_t/unsigned char for byte data. For alignment-sensitive data, use uint16_t or uint32_t as appropriate.
• Name and length conventions: Include a companion length/sentinel symbol rather than relying on sizeof in external code to avoid accidental copying.
  • Example:

    
    extern const uint8_t my_image[]; extern const size_t my_image_len; 

• Tool options: Many bin2c tools allow options for:
  • Generating const qualifiers.
  • Producing a separate header file with extern declarations.
  • Choosing base (hex vs decimal) — prefer hex for clarity and compactness.
  • Aligning arrays to word boundaries.
  • Generating null termination if treating data as strings. Always inspect available options and use those that promote read-only placement and correct alignment.

2. Minimize object size and flash usage

• Compress before embedding: If the data is compressible, compress it (e.g., gzip, LZ4, zlib, or a lightweight algorithm suited to your device) and embed the compressed blob. Decompress at runtime into RAM only when needed.
  • Trade-off: CPU and RAM use for decompression vs flash savings.
• Use linker compression/support: Some toolchains support implicit compression or packaging — check your build system.
• Avoid large initializer lists in C when possible: Very large initializer arrays can dramatically increase compile time. Alternatives:
  • Use binary blobs linked directly into the firmware via the linker (see section 6).
  • Convert only moderately sized assets into C arrays; keep very large assets as separate linked binary sections.
• Prefer hex bytes over decimal: Hex notation is shorter and often produces smaller source files. Example: 0x3A vs 58.
• Split large resources: If build tools have limits, split big arrays into multiple smaller arrays and concatenate at runtime or link time.

3. Ensure correct alignment and access

• Align arrays for the access pattern: If the data will be read as 32-bit words, align the array on 4-byte boundaries and declare as const uint32_t when sensible.
  • Example for GCC:

    
    const uint8_t my_data[] __attribute__((aligned(4))) = { ... }; 

• Avoid unaligned access on strict platforms: Some MCUs fault on misaligned reads. Either copy to an aligned buffer before word access or access as bytes and assemble words manually.
• Use packed attributes only when necessary: They prevent unwanted padding but can produce unaligned fields; consider alignment implications.

4. Header and symbol management

• Generate a header file: Let bin2c produce a header with extern declarations for arrays and lengths. Keep the header clean and minimal.

  • Example header: “`c #ifndef MY_IMAGE_H #define MY_IMAGE_H #include
    #include

  extern const uint8_t my_image[]; extern const size_t my_image_len;

  #endif “`

• Use consistent naming: Pair array name + _len suffix (or _size) to avoid collisions and to make intent clear.

• Static vs global linkage: Prefer external linkage for resources used across modules. Use static only for truly private, local assets.

• Avoid name collisions: If auto-generating many arrays, use a prefix based on the package or resource group.

5. Build-system integration

• Automate generation in the build: Run bin2c in your build system (Make/CMake/SCons/meson) to generate .c and .h files on demand. Example CMake snippet:

  
  add_custom_command( OUTPUT ${CMAKE_BINARY_DIR}/resources/my_image.c ${CMAKE_BINARY_DIR}/resources/my_image.h COMMAND bin2c -i ${CMAKE_SOURCE_DIR}/assets/image.bin -o ${CMAKE_BINARY_DIR}/resources/my_image.c -h ${CMAKE_BINARY_DIR}/resources/my_image.h --const --align=4 DEPENDS ${CMAKE_SOURCE_DIR}/assets/image.bin ) add_library(resources ${CMAKE_BINARY_DIR}/resources/my_image.c) 

• Track inputs for incremental builds: Ensure the build system only regenerates when the source binary changes to avoid unnecessary rebuilds.
• Use out-of-source generation: Keep generated files in the build directory to avoid polluting source control.

6. Consider linker-based alternatives

Embedding via C initializers is convenient, but many toolchains support embedding arbitrary binary files into the final image using the linker. Advantages:

• Faster incremental builds (no C compilation of large initializers).
• Smaller compile-time memory footprint.
• The linker can place the blob into a dedicated section and provide symbols for start/stop addresses.

Example linker usage (GNU ld):

• Link binary into section:
  • At build time: objcopy –input binary –output elf32-littlearm –binary-architecture arm image.bin image.o
  • Link image.o into the final firmware.
  • In C, declare extern symbols:

    
    extern const uint8_t _binary_image_bin_start[]; extern const uint8_t _binary_image_bin_end[]; const size_t image_size = _binary_image_bin_end - _binary_image_bin_start; 

    This approach avoids large C sources and leverages linker capabilities—useful for very large assets.

7. Runtime considerations

• Avoid unnecessary runtime copies: If the array is placed in flash and can be read in place, design code to read from it directly. For MCUs with execute-in-place (XIP) capability, avoid copying into RAM.
• Cache coherency: If DMA or peripherals read directly from flash/ROM, ensure caches and memory barriers are considered.
• Immutable data: Mark as const and put in read-only memory to protect against accidental modification and to reduce RAM use.

8. Testing and validation

• Checksum or hash: Include a checksum or hash of the embedded data to verify integrity at runtime.
• Unit tests: Add a test that checks the length symbol and a few data samples against the original file.
• Boundary tests: Verify alignment and access on all target architectures, especially those with strict alignment requirements.

9. Security and licensing

• Consider sensitive data: Embedding cryptographic keys or secrets in firmware increases risk if firmware is distributed. Use secure storage or hardware-backed key storage where possible.
• License compliance: Ensure embedded assets’ licenses allow distribution in your firmware.

10. Example workflows

• Small assets (icons, small fonts): Use bin2c to generate const uint8_t arrays with headers; include directly in source.
• Compressible medium assets (large fonts, audio clips): Compress with LZ4 or zlib, embed compressed blob, decompress on demand.
• Very large assets (filesystem images, big media): Use linker-based embedding (objcopy + linker) to avoid huge .c files and long compile times.

Summary checklist

• Declare arrays const so they live in flash.
• Use fixed-width types (uint8_t, uint32_t) and proper alignment.
• Prefer hex representation in generated code for compactness.
• Compress when feasible; balance CPU vs flash trade-offs.
• Consider linker-based embedding for very large files.
• Automate generation in the build system and keep generated files out of source control.
• Add length symbols, checksums, and tests.

Embedding binary data via bin2c is simple, but behavior at scale and on constrained hardware depends on careful choices about alignment, memory placement, build integration, and whether to compress or use linker-based methods. Following these best practices will keep your firmware lean, fast to build, and reliable across targets.