Iterable Sections

This page contains the reference documentation for the iterable sections APIs, which can be used for defining iterable areas of equally-sized data structures, that can be iterated on using STRUCT_SECTION_FOREACH.

Usage

Iterable section elements are typically used by defining the data structure and associated initializer in a common header file, so that they can be instantiated anywhere in the code base.

struct my_data {
         int a, b;
};

#define DEFINE_DATA(name, _a, _b) \
         STRUCT_SECTION_ITERABLE(my_data, name) = { \
                 .a = _a, \
                 .b = _b, \
         }

...

DEFINE_DATA(d1, 1, 2);
DEFINE_DATA(d2, 3, 4);
DEFINE_DATA(d3, 5, 6);

Then the linker has to be setup to place the place the structure in a contiguous segment using one of the linker macros such as ITERABLE_SECTION_RAM or ITERABLE_SECTION_ROM. Custom linker snippets are normally declared using one of the zephyr_linker_sources() CMake functions, using the appropriate section identifier, DATA_SECTIONS for RAM structures and SECTIONS for ROM ones.

# CMakeLists.txt
zephyr_linker_sources(DATA_SECTIONS iterables.ld)

# iterables.ld
ITERABLE_SECTION_RAM(my_data, 4)

The data can then be accessed using STRUCT_SECTION_FOREACH.

STRUCT_SECTION_FOREACH(my_data, data) {
        printk("%p: a: %d, b: %d\n", data, data->a, data->b);
}

Note

The linker is going to place the entries sorted by name, so the example above would visit d1, d2 and d3 in that order, regardless of how they were defined in the code.

API Reference

group iterable_section_apis

Iterable Sections APIs.

Defines

ITERABLE_SECTION_ROM(struct_type, subalign)

Define a read-only iterable section output.

Define an output section which will set up an iterable area of equally-sized data structures. For use with STRUCT_SECTION_ITERABLE(). Input sections will be sorted by name, per ld’s SORT_BY_NAME.

This macro should be used for read-only data.

Note that this keeps the symbols in the image even though they are not being directly referenced. Use this when symbols are indirectly referenced by iterating through the section.

ITERABLE_SECTION_ROM_GC_ALLOWED(struct_type, subalign)

Define a garbage collectable read-only iterable section output.

Define an output section which will set up an iterable area of equally-sized data structures. For use with STRUCT_SECTION_ITERABLE(). Input sections will be sorted by name, per ld’s SORT_BY_NAME.

This macro should be used for read-only data.

Note that the symbols within the section can be garbage collected.

ITERABLE_SECTION_RAM(struct_type, subalign)

Define a read-write iterable section output.

Define an output section which will set up an iterable area of equally-sized data structures. For use with STRUCT_SECTION_ITERABLE(). Input sections will be sorted by name, per ld’s SORT_BY_NAME.

This macro should be used for read-write data that is modified at runtime.

Note that this keeps the symbols in the image even though they are not being directly referenced. Use this when symbols are indirectly referenced by iterating through the section.

ITERABLE_SECTION_RAM_GC_ALLOWED(struct_type, subalign)

Define a garbage collectable read-write iterable section output.

Define an output section which will set up an iterable area of equally-sized data structures. For use with STRUCT_SECTION_ITERABLE(). Input sections will be sorted by name, per ld’s SORT_BY_NAME.

This macro should be used for read-write data that is modified at runtime.

Note that the symbols within the section can be garbage collected.

STRUCT_SECTION_ITERABLE(struct_type, name)

Defines a new iterable section.

Convenience helper combining __in_section() and Z_DECL_ALIGN(). The section name is the struct type prepended with an underscore. The subsection is “static” and the subsubsection is the variable name.

In the linker script, create output sections for these using ITERABLE_SECTION_ROM() or ITERABLE_SECTION_RAM().

STRUCT_SECTION_ITERABLE_ALTERNATE(out_type, struct_type, name)

Defines an alternate data type iterable section.

Special variant of STRUCT_SECTION_ITERABLE(), for placing alternate data types within the iterable section of a specific data type. The data type sizes and semantics must be equivalent!

STRUCT_SECTION_FOREACH(struct_type, iterator)

Iterate over a specified iterable section.

Iterator for structure instances gathered by STRUCT_SECTION_ITERABLE(). The linker must provide a _<struct_type>_list_start symbol and a _<struct_type>_list_end symbol to mark the start and the end of the list of struct objects to iterate over. This is normally done using ITERABLE_SECTION_ROM() or ITERABLE_SECTION_RAM() in the linker script.