Embedded C essentials

BeginnerEmbedded C

Sample guide. Edit or replace it with your own material.

1. What does `volatile` do, and when do you need it?

It tells the compiler that the value can change outside the normal program flow, so every access must actually read or write memory. The compiler can’t cache the value in a register or remove the access.
Use it for: memory-mapped peripheral registers, variables shared between an ISR and main code, and memory changed by DMA.
It does not provide atomicity or ordering between CPU cores. Use critical sections, atomics, or barriers for that.
Follow-up question: what does const volatile mean? It is a read-only register whose value hardware can still change, such as a status register.

Section	Contents	Location at runtime
`.text`	Code and constants (often `.rodata` too)	Flash
`.data`	Initialised globals and statics	RAM, with initial values copied from flash at startup
`.bss`	Zero-initialised or uninitialised globals and statics	RAM, zeroed at startup
Stack	Local variables, return addresses	RAM
Heap	`malloc` allocations (often avoided in embedded)	RAM

On a local variable: it keeps its value between calls and lives in .data/.bss, not on the stack.
On a global variable or function: it has internal linkage and is visible only in that file.

Keep them short: set a flag or push to a buffer, and do the work in the main loop.
Don’t block or call non-reentrant functions (such as printf).
Mark shared variables volatile and protect multi-byte accesses.
Clear the interrupt source, or it will fire again immediately.

uint8_t a = 0xFF;
if (~a == 0x00) {
    /* Never reached */
}

a is promoted to int before ~ is applied, so ~a is 0xFFFFFF00 (on a 32-bit int), not 0x00. The fix is (uint8_t)~a == 0x00.

Write SET_BIT, CLEAR_BIT, and TOGGLE_BIT macros, and explain why the arguments need parentheses.
Explain why sizeof(struct { char c; int i; }) is usually 8, not 5.