EMBEDDED TOOLKIT

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gcc attributes compiler flags

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Key GCC Compiler Extensions for Embedded

__attribute__((packed))

Instructs the compiler NOT to insert any empty alignment bytes (padding) inside a struct. Often causes Unaligned Access.

Use Case: Mapping network headers (TCP/IP) or raw binary UART frames.

__attribute__((aligned(X)))

Forces alignment of a variable/struct to a boundary of X bytes in memory.

Use Case: Required e.g. by hardware DMA controller, which can only read aligned addresses in SRAM.

__attribute__((section(".name")))

Places the compiled function code or variable in a specific section defined by the programmer in the linker script (.ld).

Use Case: Placing critical ISR functions in fast RAM (ITCM) instead of Flash, or saving certificates in Flash.

__attribute__((weak))

Marks the function as 'weak'. If you write a function with the same name anywhere else in the code, this weak one will be seamlessly overridden.

Use Case: Default interrupt handlers (Default_Handler) or callbacks generated in CubeMX (HAL_UART_RxCpltCallback).

__attribute__((always_inline))

Forces the compiler to absolutely copy the function's code into the call site, even if the -O0 flag is enabled.

Use Case: Ultra-fast, low-latency code in interrupts, or macros replacing bitwise functions.

__attribute__((naked))

The compiler generates no prologue or epilogue for this function. It compiles C directly as assembly inside.

Use Case: Writing custom RTOS procedures (Context Switch), where you manually manage SP, LR, PC registers.

__attribute__((used))

Tells the linker (LTO) to absolutely not discard this code, even if it seems uncalled anywhere in the program.

Use Case: Interrupt vector tables (Vector Table) or functions called only from assembly.

Writing clean C code itself is often not enough on the smallest and most hardware-sensitive devices.

GCC attributes inform the linking and Assembly generation process on how to optimize data in memory (bypassing C99/C11 standard rules).
Even the most optimized code with -O0 flag may turn out sluggish, and with -O3 grow so much in size (e.g. through FOR loop unrolling) that it won't fit in Flash. Always balance using -Os and flags -flto.