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Linker User Guide

Preface Overview of the Linker Linking Models Supported by armlink Image Structure and Generation Linker Optimization Features Getting Image Details Accessing and Managing Symbols with armlink Scatter-loading Features The scatter-loading mechanism Overview of scatter-loading When to use scatter-loading Linker-defined symbols that are not defined when s Specifying stack and heap using the scatter file Scatter-loading command-line options Scatter-loading images with a simple memory map Scatter-loading images with a complex memory map Scatter file with link to bit-band objects Root execution regions Root execution region and the initial entry point Root execution regions and the ABSOLUTE attribute Root execution regions and the FIXED attribute Methods of placing functions and data at specific Placement of code and data with __attribute__((sec Placement of __at sections at a specific address Restrictions on placing __at sections Automatic placement of __at sections Manual placement of __at sections Placement of a key in flash memory with an __at se Mapping a structure over a peripheral register wit Example of how to explicitly place a named section Placement of unassigned sections with the .ANY mod Placement rules when using multiple .ANY selectors Command-line options for controlling the placement Prioritization of .ANY sections Specify the maximum region size permitted for plac Examples of using placement algorithms for .ANY se Example of next_fit algorithm showing behavior of Examples of using sorting algorithms for .ANY sect Behavior when .ANY sections overflow because of li Placement of veneer input sections in a scatter fi Placement of sections with overlays Reserving an empty region Placement of ARM C and C++ library code Specifying ARM standard C and C++ libraries in a s Example of placing code in a root region Example of placing ARM C library code Example of placing ARM C++ library code Example of placing ARM library helper functions Creation of regions on page boundaries Overalignment of execution regions and input secti Preprocessing of a scatter file Example of using expression evaluation in a scatte Equivalent scatter-loading descriptions for simple Command-line options for creating simple images Type 1 image, one load region and contiguous execu Type 2 image, one load region and non-contiguous e Type 3 image, multiple load regions and non-contig How the linker resolves multiple matches when proc How the linker resolves path names when processing Scatter file to ELF mapping Scatter File Syntax Linker Command-line Options Linker Steering File Command Reference Via File Syntax

Placement of code and data with __attribute__((section("name")))

7.2.5 Placement of code and data with __attribute__((section("name")))

You can place code and data by separating them into their own objects without having to use toolchain-specific pragmas or attributes.

However, you can also use __attribute__((section("name"))) to place an item in a separate ELF section. You can then use a scatter file to place the named sections at specific locations.

Example

To use __attribute__((section("name"))) to place a variable in a separate section:
  1. Use __attribute__((section("name"))) to specify the named section where the variable is to be placed, for example:
    Naming a section
    int variable __attribute__((section("foo"))) = 10;
  2. Use a scatter file to place the named section, for example:
    Placing a section
    FLASH 0x24000000 0x4000000
    {
        …                                ; rest of code
        ADDER 0x08000000
        {
            file.o (foo)                  ; select section foo from file.o
        }
    }
The following example shows the memory map for the FLASH load region:
…
  Load Region FLASH (Base: 0x24000000, Size: 0x00000004, Max: 0x04000000, ABSOLUTE)
    Execution Region ADDER (Base: 0x08000000, Size: 0x00000004, Max: 0xffffffff, ABSOLUTE)
    Base Addr    Size         Type   Attr      Idx    E Section Name        Object
    0x08000000   0x00000004   Data   RW           16    foo                 file.o
…
Be aware of the following:
  • Linking with --autoat or --no_autoat does not affect the placement.
  • If scatter-loading is not used, the section is placed in the default ER_RW execution region of the LR_1 load region.
  • If you have a scatter file that does not include the foo selector, then the section is placed in the defined RW execution region.
You can also place a function at a specific address using .ARM.__at_address as the section name. For example, to place the function sqr at 0x20000, specify:
int sqr(int n1) __attribute__((section(".ARM.__at_0x20000")));
int sqr(int n1)
{
    return n1*n1;
}
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