Embedded C language question

[lawrence11]

How do I access, and give a value to, precise individual byte of a 16bit (word) array ??

Quickest possible code, on ARM cortex-M. ?

Like this? Well, I guess it dont compile, so this must not be it.

For example.

int16_t bufferk[6];

const uint8_t mask1=1;

.........

while(1){

.....
(uint8_t*)&bufferk[0]=mask1;

}

[ataradov]

Code: [Select]

((uint8_t *)bufferk)[0]=mask1;

Also, when you say that some thing does not compile, always include the exact error message.

[lawrence11]

A shit, thanks man, it compiles...

Its all about details, should have expected this.

Yeah I knew it was small and error code was not necessary in this case.

[brucehoult]

How do I access, and give a value to, precise individual byte of a 16bit (word) array ??

Quickest possible code, on ARM cortex-M. ?

Like this? Well, I guess it dont compile, so this must not be it.

For example.

int16_t bufferk[6];

const uint8_t mask1=1;

.........

while(1){

.....
(uint8_t*)&bufferk[0]=mask1;

}

Code: [Select]

#include <stdint.h>

const uint8_t mask1=1;

void foo(int16_t bufferk[6]){
  *(uint8_t*)&bufferk[0]=mask1;
}

Produces...

Code: [Select]

foo.c: In function ‘foo’:
foo.c:6:24: error: lvalue required as left operand of assignment
   (uint8_t*)&bufferk[0]=mask1;
                        ^

The message is helpful. You're trying to assign a uint8_t to a uint8_t*.

Code: [Select]

*(uint8_t*)&bufferk[0]=mask1;

Now you get...

Code: [Select]

00000000 <foo>:
   0: 2301      movs r3, #1
   2: 7003      strb r3, [r0, #0]
   4: 4770      bx lr

Note that the above code and ataradov's do different things if you change the 0 to another value.

When it is 0, both versions change the first half of bufferk[0], which on ARM is the least significant 8 bits.

If you change the 0 to a 1 then my code will change the least significant bits of bufferk[1], while ataradov's code will change the most significant bits of bufferk[0].

[SiliconWizard]

So yeah - this is not specifically an "embedded" C question, more like general C (and would probably be better asked in the Programming section, but anyway.)

You need to brush up on: C pointers, data types, casting and operator precedence. Then you'll be alright.
You got answers and fixes above - but take it as an opportunity to master the above rather than just apply a recipe blindly. Just a thought.