Understand the Role of Linker Scripts in Firmware Projects
Before diving into CMake specifics, it's essential to understand the role linker scripts play in firmware projects. Linker scripts are used to instruct the linker on how to arrange the output file and place different sections (like .text for code, .data for initialized data, and so on) into specific memory areas. It's crucial in embedded systems where you have limited memory resources.
Integrating Linker Scripts with CMake
Once you have a clear grasp of linker scripts, you need to integrate them properly with CMake. Often, CMake projects don't directly support the nuances of embedded development out of the box, so you'll need some manual configuration.
- Create a Linker Script Template: Start by creating a generic linker script (let's call it
generic.ld) that can serve as a template. This script should define various memory sections, their base addresses, sizes, and how different parts of the code should lay out in memory.
MEMORY
{
FLASH (rx) : ORIGIN = 0x08000000, LENGTH = 256K
RAM (rwx) : ORIGIN = 0x20000000, LENGTH = 40K
}
SECTIONS
{
.text :
{
KEEP(*(.isr_vector))
*(.text*)
*(.rodata*)
} > FLASH
.data : AT(ADDR(.text) + SIZEOF(.text))
{
*(.data*)
*(.bss*)
} > RAM
}
- Configure CMake to Use the Linker Script: To instruct CMake to use your specific linker script, you need to set proper linker flags.
set(LINKER_SCRIPT ${CMAKE_SOURCE_DIR}/path/to/your/ldscript/generic.ld)
set(LINKER_FLAGS "-T${LINKER_SCRIPT}")
add_executable(your_target_name ${your_sources})
target_link_options(your_target_name PRIVATE ${LINKER_FLAGS})
CMake Variable Substitution in Linker Scripts
In some cases, you'll want to customize your linker script for different build types, such as specifying different memory sizes or addresses. CMake can help with this through a combination of configure_file and variable replacement.
- Using
configure_file: Create a linker script template with variables that CMake can replace.
MEMORY
{
FLASH (rx) : ORIGIN = ${FLASH_ORIGIN}, LENGTH = ${FLASH_LENGTH}
RAM (rwx) : ORIGIN = ${RAM_ORIGIN}, LENGTH = ${RAM_LENGTH}
}
SECTIONS
{
// Remaining section definitions...
}
- Configure CMakeLists.txt to Substitute These Variables:
set(FLASH_ORIGIN "0x08000000")
set(FLASH_LENGTH "256K")
set(RAM_ORIGIN "0x20000000")
set(RAM_LENGTH "40K")
configure_file(${CMAKE_SOURCE_DIR}/path/to/your/ldscript/template.ld
${CMAKE_BINARY_DIR}/output.ld
@ONLY)
set(LINKER_FLAGS "-T${CMAKE_BINARY_DIR}/output.ld")
add_executable(your_target_name ${your_sources})
target_link_options(your_target_name PRIVATE ${LINKER_FLAGS})
Debugging Linker Script Issues
If you encounter issues during linking, consider the following:
- Check Linker Script Syntax: Make sure your linker script does not contain syntax errors and conforms to the expected format for the target toolchain.
- Examine CMake Configuration: Verify that the linker flags are correctly set and that the
target_link_options() or target_link_libraries() are accurately applied.
- Verbose Linker Output: Increase verbosity of the linking process to better understand what CMake is passing to the underlying linker.
make VERBOSE=1
Tools and Resources
Finally, remember to regularly reference the documentation for the specific linker or toolchain you're using, as behaviors and capabilities can vary. Communities specialized in the firmware or your specific platform can be a valuable resource for guidance related to CMake and linker script integration.
