C pass macro as parameter pointer

Question

I have a function called ADS_Transmit, declared as follows

void ADS_Transmit(uint8_t* data, uint16_t size);

The function is supposed to take arrays of uint8_t as parameter, but in some conditions, I'd like to send macros when calling the function. Like this

#define ADS_CMD_RST 0x06U
ADS_Transmit(ADS_CMD_RST, 1); // Does not work
uint8_t data = ADS_CMD_RST;
ADS_Transmit(&data, 1); // Works

Now perhaps obvious to many, sending the macro in the parameter doesn't work, since the function will use what's on register 0x06. But is there a way that I could send the macro in the parameter without changing the function?

Thank you in advance!

A macro represents a piece of text. A bunch of source code characters. — n. m. could be an AI
– n. m. could be an AI, Commented Jul 16, 2021 at 21:02
That's right. So I presume that it's not possible to do it in the way I want. I could just as well change the parameter of the function and work around it. Just wanted to make sure I understood why it doesnt work. Thanks! — Krippkrupp
– Krippkrupp, Commented Jul 16, 2021 at 21:04
No, it just doesn't make sense to talk about macros in this context. ADS_CMD_RST represents characters that make up the number 0x06U. So you should be asking "how do I send a number to a function that expect an address without storing the number in a variable and sending the address of the variable". You actually can do that, macros only muddle the water. — n. m. could be an AI
– n. m. could be an AI, Commented Jul 16, 2021 at 21:08

Nate Eldredge · Accepted Answer · 2021-07-16 21:04:00Z

4

You can use a compound literal:

ADS_Transmit(&(uint8_t){ADS_CMD_RST}, 1);

This creates a temporary uint8_t variable, initialized with the value ADS_CMD_RST, and passes its address to ADS_Transmit.

Note that if ADS_Transmit modifies the memory pointed to by the first argument, you won't be able to access the updated values.

answered Jul 16, 2021 at 21:04

Nate Eldredge

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4 Comments

Krippkrupp Over a year ago

Nice! This is what I was looking for. My best effort was writing ADS_Transmit((uint8_t) &ADS_CMD_RST, 1) but that didn't work. Thank you for the help, this works!

Krippkrupp Over a year ago

Follow up question. If I'd like to use this method and send 2 bytes. Is there a nice way to do it? This is what I'm trying to do ADS_Transmit(&(uint8_t){(0x16U<<8)|(0x10U)}), though only the first 8 bits are being sent in this case

Nate Eldredge Over a year ago

@Krippkrupp: Yep, use a compound literal of array type. ADS_Transmit((uint8_t[]){0x10U, 0x16U}, 2);

Krippkrupp Over a year ago

Now that I see it, perhaps it should've been obvious! This should also work for transmitting one byte as well, i.e ADS_Transmit((uint8_t[]){0x10U},1); Thanks a lot for taking your time to help me out!

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