C中的字符串连接？

Question

I am trying to understand string's behavior in C and it is bothering me since my following two code snippets result into different output: (For the sake of this question, Let us assume user enters 12)

int main(void)  
{
    char L_Red[2];
    char temp[] = "I";
    printf("Enter pin connected to red: ");
    scanf("%s", L_Red);
    strcat(temp,L_Red);
    printf("%s \n", temp);
    return 0;
}

this yields: 12 as output (and not I12) Why ?

int main(void)  
{
    char L_Red[2];
    printf("Enter pin connected to red: ");
    scanf("%s", L_Red);
    char temp[] = "I";
    strcat(temp,L_Red);
    printf("%s \n", temp);
    return 0;
}

This yields: I12I (and not, I12) Why ?

I have read about string in C and as per my understanding, neither am I allocating temp any fixed size and changing it later to get these vague outputs nor am I using strings like the way they are not supposed to. Is there any other concept at play here ?

Answer 1

The array temp is an array of two characters (the 'I' and the string terminator '\\0' ). That's it. Attempting to append more characters to that array will write out of bounds and lead to undefined behavior .

You need to make sure that the destination array temp have enough space to fit its original content plus the string you want to append (plus the terminator).

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Also, if you want to input more than one character for the "string" L_Red you need to increase its size as well.

I also recommend you use a limit in the format specifier so you can't write out of bounds:

char L_Red[3];  // Space for two characters, plus terminator
scanf("%2s", L_Red);  // Read at most two characters of input

Answer 2

You are getting strange answers because your destination string (ie the first argument to strcat) is not long enough to handle both strings plus a null termination character. Also the length of L_Red is too short as it does not have enough space for the null termination character either.