c syntax passing const pointer to const data to function

Question

In the function declaration below the second argument is a const pointer to const data.

ti_rc_t ti_uart_write_buffer(const ti_uart_t uart, const uint8_t *const data, uint32_t len);

Below is example code calling the function. Why is there a (uint8_t *) before BANNER_STR. Is this the usual syntax for passing a const pointer to a const data to a function? Are there other valid syntax?

#define BANNER_STR ("Hello, world!\n\n")
ti_uart_write_buffer(TI_UART_0, (uint8_t *)BANNER_STR, sizeof(BANNER_STR));

Thanks

the consts there are a promise by the function that it won't modify the pointer (not that that makes much sense) or the data pointed to - it is not a restriction on the caller (ie. the caller can pass in a const or non-const pointer to const or non-const data). — Sander De Dycker
– Sander De Dycker, Commented Feb 8, 2016 at 14:48
Thanks, but when you are calling the function what does (uint8_t *) before BANNER_STR refer to? — ultrasonic bananna
– ultrasonic bananna, Commented Feb 8, 2016 at 14:50
it casts BANNER_STR from const char* to uint8_t* before passing it to the function. — Sander De Dycker
– Sander De Dycker, Commented Feb 8, 2016 at 14:51
This is just bad code. Don't (miss)use macros that way. Just define a normal const variable array or pointer with the text. — too honest for this site
– too honest for this site, Commented Feb 8, 2016 at 15:12
char and uint8_t are usually incompatible types; a cast is necessary to do pointer-based aliasing like this. You are focusing on const but that is a red herring. They could have written (const uint8_t *) with the same end result. The uint8_t * value result of the cast will be implicitly converted to const uint8_t * to match the function parameter anyway — M.M
– M.M, Commented Jan 9, 2017 at 4:48

Frodo · Accepted Answer · 2016-02-08 15:13:56Z

2

The define BANNER_STR is an array of characters as its type, in other words a string. And therefore in your function call you need to cast this argument to avoid compiler warnings but this is a valid way to call this function.

An other valid syntax would be instead of using define directive to declare a constant variable which contains the string to write:

const char banner_str[] = "Hello, world!\n\n";
ti_uart_write_buffer(TI_UART_0, (uint8_t *)banner_str, sizeof(banner_str));

edited Feb 8, 2016 at 15:13

answered Feb 8, 2016 at 15:00

Frodo

76913 silver badges24 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Sander De Dycker Over a year ago

given that it's casting away const, I wouldn't call it "correct".

Frodo Over a year ago

you are right, maybe "valid" is more "correct" for this

Collectives™ on Stack Overflow

c syntax passing const pointer to const data to function

1 Answer 1

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related