Pointer definition in preprocessor directive

Question

I was going through a programming manual for one of the microcontrollers I came across and it had the preprocessor definition as follows:

#define SCICTL1A (volatile unsigned int *)0x7051

and a statement in the source file as follows:

*SCICTL1A = 0X0003;

My question is, what is the pointer variable here and what is it pointing to, (I have never come across pointer definitions in preprocessor directives before since I am a beginner to C programming) and what does the assignment statement do?

The int at address `0x7051` will be assigned the value `0x0003` — Paul Ogilvie, Sep 11 '18 at 17:07
In this case you have memory mapped I/O and the device is at address 0x7051. Then you are writing a command or something else to that address. That's all I can tell from the #define statement. — nicomp, Sep 11 '18 at 17:08
Note: Good programming practice would have employed enclosing `()` --> `define SCICTL1A ((volatile unsigned int *)0x7051)`. — chux - Reinstate Monica, Sep 11 '18 at 17:46
That's completely unrelated to the preprocessor. Start understanding what the preprocessor actually does. The expression itself will become clear from every textbook. — too honest for this site, Sep 11 '18 at 19:32
@chux: … and using fixed-width types instead of standard integer types. — too honest for this site, Sep 11 '18 at 19:34
@toohonestforthissite True, Using fixed width types is best for mapping hardware registers. Aniruddh P Rao, are your aware of `uint16_t` and family as in `#define SCICTL1A ((volatile uint16_t *)0x7051)` ? — chux - Reinstate Monica, Sep 11 '18 at 19:37
@chux: I suppose that's some vendor-provided header. No beginner will ceate them theirselves. — too honest for this site, Sep 11 '18 at 20:02
uint16_t was defined in the header file which wasn't included in the question above. — Aniruddh P Rao, Sep 15 '18 at 13:35

Antti Haapala -- Слава Україні · Accepted Answer · 2018-09-11T18:26:53.000

3

There is no variables here. The macro expands as text in place, so the 2nd excerpt becomes

*(volatile unsigned int *)0x7051 = 0X0003;

It casts the unsigned integer 0x7051 into a pointer to volatile unsigned integer, then references this in assignment. Essentially it stores 0x0003 into the unsigned integer-wide piece of memory that starts from address 0x7051 (or, however the integer-to-pointer conversion happens to work on your target platform)

volatile is required so that the compiler does not just optimize the assignment out - it must be strictly evaluated and considered a side effect (see as-if rule).

As for the actual reason why this is done - it is probably some memory-mapped device, check the microcontroller datasheets for more information.

edited Sep 11 '18 at 18:26

answered Sep 11 '18 at 17:07

Antti Haapala -- Слава Україні

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`volatile` is required when you want to tell the compiler to not optimize out memory access, in this case to a register in a memory-mapped peripheral. This memory will update without the compiler knowing it. Normally it would predict when it needs to access memory and avoid unnecessary memory access to speed up the code (optimization). In this case, those predictions would almost always be wrong and your code would not behave how you would expect it. – bigwillydos Sep 11 '18 at 18:02
@bigwillydos: That's not correct. `volatile` does not enforce memory accesses. – too honest for this site Sep 11 '18 at 19:34

score 0 · Answer 2 · answered Sep 11 '18 at 17:07

There is no variable there. Only the pointer.

the *SCICTL1A = 0X0003; is replaced by the preprocessor by the:

*(volatile unsigned int *)0x7051 = 0x0003;

You just write the location with the address of the 0x07051. That does it mean depends on your implementation

Fiddling Bits · Answer 3 · 2018-09-11T17:15:07.337

I'm assuming you're using a TMS320F2803x Piccolo microcontroller: http://www.ti.com/lit/ds/sprs584l/sprs584l.pdf

According to this document, address 0x7051 is Control Register 1 for the Serial Communications Interface (SCI) Module.

According to this document, https://www.swarthmore.edu/NatSci/echeeve1/Ref/embedRes/DL/28069TechRefManual_spruh18d.pdf, you're able to do the following with this register:

SCICTL1 controls the receiver/transmitter enable, TXWAKE and SLEEP functions, and the SCI software reset.

Pointer definition in preprocessor directive

3 Answers3