在C中将字符串分配给uint8_t

Question

I am trying to assign string to uint8_t buffer in IAR, but I get a warning message.

rx_buffer.rx_struct.RESP.RESPOND is struct field of uint8_t type.

My code is:

strncpy(rx_buffer.rx_struct.RESP.RESPOND, (uint8_t *)'NS,', 3);

And the associated warning message is the following:

Warning[Pe1422]: multicharacter character literal (potential portability problem) 
Warning[Pe167]: argument of type "uint8_t *" is incompatible with parameter of type "char *",
Warning[Pe167]: argument of type "uint8_t *" is incompatible with parameter of type "char const"

I have written a workaround :

rx_buffer.rx_struct.RESP.RESPOND[0] = 'N';
rx_buffer.rx_struct.RESP.RESPOND[1] = 'S';
rx_buffer.rx_struct.RESP.RESPOND[2] = ',';

But I'm not satisfied with it. What is the correct way to do that?

Answer 1

Your code is incorrect in many ways:

strncpy(rx_buffer.rx_struct.RESP.RESPOND, (uint8_t *)'NS,',3);

• The destination array does not have type char *
• the source is not an array: 'NS,' is a multicharacter character constant, a non portable historic oddity that nobody would use in any decent code... casting it to (uint8_t *) does not fix this issue. You should just use double quotes: "NS," .
• strncpy() is not the proper tool for this job. In fact it is never the right tool for any job. This function is not a safe replacement for strcpy , it semantics are widely misunderstood, it is very error prone. You should avoid using this function. In this particular case, it would just copy the 3 bytes as expected, but why use strncpy() when memcpy() is a simpler solution?

You can achieve your goal with this:

memcpy(rx_buffer.rx_struct.RESP.RESPOND, "NS,", 3);

or alternately by assigning each byte separately as posted in the question. Both approaches are likely to produce the same machine code.

Answer 2

strncpy expects its first two arguments to be of type char * and const char * , respectively. Instead of casting to unit8_t * , you should be casting to char * , if at all.

Also, 'NS,' is a character literal, a string literal would be "NS," .

Answer 3

You are using single quotes but you need double quotes ( "NS," ) , and take a look to Stop using strncpy already! , in this case it should work as expected because you don't want the trailing '\\0' , but don't use it.

Use memcpy or memmove , also (as a question of style) don't use magic numbers like 3 :

#include <stdio.h>
#include <string.h>
#include <stdint.h>

#define MAX_SIZE 3

int main(void)
{
    uint8_t arr[MAX_SIZE];

    memmove(arr, "NS,", MAX_SIZE);
    for (int i = 0; i < MAX_SIZE; i++) {
        printf("%c", arr[i]);
    }
    printf("\n");
    return 0;
}