Question

Have a problem accessing some elements of a structure in a right way after typecasting.
Here my code:

void get_description(struct shmstruct *ptr/*, int number*/) {
    char buff[MESGSIZE];
    struct shmData *dparse;
    snprintf(buff, MESGSIZE, "%s", &ptr->msgdata[0]);
    dparse = (struct shmData *) buff;
    printf("Number: %s", dparse->number);
    printf("Description: %s", dparse->description);
}

Problem now is, that I get the number, like 123, but also the description in the first
line ->
printf(“Number: %s”, dparse->number);
like: 123 description

How can I get only the number?

(P.S.

struct shmData{
    char number[4];
    char description[1020];
};

)

Answer 1

It sounds like the number is not nul-terminated. You have the number field and then immediately following you have the description field. printf() assumes that you are giving it a nul-terminated string, and it will keep going until it hits the terminating nul character. It looks like in your case, there isn’t a nul terminating the number field, so printf() just keeps going and gets the description also.

Can all four characters from number be used for digits?

If the numbers are only 3-digits or fewer, then you can put a terminating nul right into the number buffer. If you might need to read a 4-digit number from there, you need to copy out the digits into a temp buffer of at least 5 chars, and then nul-terminate.

// max 3 digits:
dparse->number[3] = '\0'; // ensure nul-termination of number

// max 4 digits:
#define DPARSE_NUMBER 4  /* size of number field in dparse */
char temp[DPARSE_NUMBER + 1];
strncpy(temp, dparse->number, sizeof(temp));
temp[sizeof(temp) - 1] = '\0'; // strncpy() does not guarantee to nul-terminate, so we must do this

strncpy() doesn’t guarantee to nul-terminate, which is unfortunate. So we should always put in a terminating nul character in the last position so that no matter what, it is nul-terminated. Note that sometimes strncpy() will put a nul for us, but it does no harm to make sure one is there.