help using extern struct in c??

**ridgerunnersjw** · 04-02-2020

Hello....
I am using C89 compiler (CCS) and have an h file containing

Code:

extern struct s_rf{    char command[10];
    boolean ack;
}s_radioMssg;

In main I then try and initialize

Code:

int main(void){
    WDTCTL = WDTPW | WDTHOLD;   // stop watchdog timer
    s_radioMssg.command = {'F'};
    s_radioMssg.ack = T;

I am not sure but I was thinking that in main I am defining the structure components. I have tried even adding before main

Code:

struct s_rf s_radioMssg;

in the event that the header file was ONLY declaring s_radioMssg as an s_rf struct but that seems redundant to me to define before main??

The compiler errors as follows:
#138 expression must be a modifiable lvalue

**laserlight** · 04-02-2020

The problem you're having is this: the member named command is an array, and you cannot assign to an array because it is not "a modifiable lvalue". You can assign to the elements of this array instead, e.g.,

Code:

s_radioMssg.command[0] = 'F';

**ridgerunnersjw** · 04-02-2020

So there is no way to

Code:

s_radioMssg.command[] = {'F'}

such that all values get initialized at once??

Also is there a better way to declare, define, instantiate a structure such as the one I have? I figured I'd declare it in .h file (maybe I shouldn't be using extern here??), then define it in main and finally instantiate/initialize it in main and or other c files?

Thanks

**laserlight** · 04-02-2020

Is s_radioMssg really supposed to be a global variable, i.e., does it represent truly global state?

If not, then I suggest that in your header:

Code:

struct s_rf {
    char command[10];
    boolean ack;
};

Then in a source file:

Code:

int main(void) {
    WDTCTL = WDTPW | WDTHOLD;   // stop watchdog timer
    struct s_rf s_radioMssg = {{'F'}, T};

**ridgerunnersjw** · 04-02-2020

What you posted works but only for [0] of the array....after reading online I found that memcpy seems to work quite well....Ultimately I wanted to initialize the array but then I wanted to update the s_radioMssg.command = r_getFreq where r_getFreq is a char array

Thank you for your help

**laserlight** · 04-02-2020

Originally Posted by ridgerunnersjw

What you posted works but only for [0] of the array

No, it works for the entire array:

Code:

struct s_rf s_radioMssg = {{'F', '1', '2', '3', '4', '5', '6', '7', '8', '9'}, T};

Originally Posted by ridgerunnersjw

after reading online I found that memcpy seems to work quite well

It doesn't: your array does not store a null terminated string.

Originally Posted by ridgerunnersjw

Ultimately I wanted to initialize the array but then I wanted to update the s_radioMssg.command = r_getFreq where r_getFreq is a char array

Use a loop to copy.

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