Using pointers within structure for assigning string

Question

I am trying to assign a string to a pointer which is in the structure. Even though, I am able to initialise the string to pointer successfully, when I try to use the string(access the pointer), I get some random values. How should I solve the problem. Following is my code:

typedef struct
{

    uint8_t LOG_ID;
    uint8_t timestamp;
    uint8_t loglength;
    uint8_t checksum;
    uint8_t *payload;

} Log;
Log LB_t;

void main(){
LB_t.LOG_ID=1;
    LB_t.timestamp=3;
    LB_t.loglength=17;
    LB_t.checksum=89;

LB_t.payload="LED initialised";
log_item(&LB_t,17);
}
void log_item(uint8_t *logptr,uint8_t length){
    while(length!=0){
    CB_buffer_add_item(tx,*logptr);
    length=length-1;
logptr++;
    }
}

Is there any alternate way in which I can access the pointer?

main() returns int. uint8_t is not guaranteed to exist, nor to be compatible with char (i.e. a "string"). Please format your code properly if you expect others to read it. And please provide examples of input and output data. — underscore_d
– underscore_d, Commented Dec 1, 2017 at 16:48
why is payload not char*. Why is log_item arg not of type Log? What is CB_buffer_add_item — pm100
– pm100, Commented Dec 1, 2017 at 16:49
@underscore_d This looks like it could be embedded, in which case void main() is OK. — dbush
– dbush, Commented Dec 1, 2017 at 16:49
Where do you try to access the string? It looks like you're trying to manually reinvent the wheel of offsetof(). Is that right? It usually doesn't end well for people who try it. — underscore_d
– underscore_d, Commented Dec 1, 2017 at 16:53

pm100 · Accepted Answer · 2017-12-01 16:51:48Z

1

OK, I get it , you expect 'payload' to be inside the struct. Its not, just the pointer. You need to do this

typedef struct
{

    uint8_t LOG_ID;
    uint8_t timestamp;
    uint8_t loglength;
    uint8_t checksum;
    uint8_t payload[100];

} Log;

and then

strncpy(LB_t.payload, "LED initialised", 100);

you probaly want to make the 100 a const or #define somehwere. And change yr 17 to sizeof(Log)

answered Dec 1, 2017 at 16:51

pm100

50.6k25 gold badges95 silver badges158 bronze badges

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

3 Comments

vikrant waje Over a year ago

is there any way apart from string copy function to do the above operation, beacause I am not allowed to use string.h library

pm100 Over a year ago

use memcpy or write your own strncpy function that loops over the character array one byte at a time

alk Over a year ago

payload should be an array of char.

Collectives™ on Stack Overflow

Using pointers within structure for assigning string

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related