C program produces weird output

Question

During my process of Learning CI decided to create a struct that gives me the size of fish I provide in it, my question is why when I write this small piece of code:

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

struct fish
    {
        char catfish[9]; //reserve memory for 9 chars
        char goldfish[10]; //reserve memory for 10 chars
        char mackrel;
        char oldfish;
    };

int main()
{
    struct fish typeof_fish;

    strcpy(typeof_fish.catfish, "Big Fish\n");
    strcpy(typeof_fish.goldfish, "Small Fish\n");
    printf("%s\n", typeof_fish.catfish);

    return 0;


}

the output produces "Big Fish Small Fish" Output here

yet when I rewrite the top piece of code and change char catfish[9] ; to char catfish[10] :

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

struct fish
    {
        char catfish[10]; //reserve memory for 10 chars
        char goldfish[10]; //reserve memory for 10 chars
        char mackrel;
        char oldfish;
    };

int main()
{
    struct fish typeof_fish;

    strcpy(typeof_fish.catfish, "Big Fish\n");
    strcpy(typeof_fish.goldfish, "Small Fish\n");
    printf("%s\n", typeof_fish.catfish);

    return 0;


}

it produces "Big Fish" output here

thank you in advance for an answer on this puzzling bug

Answer 1

You aren't leaving enough room for a null terminator in catfish[9] when you strcpy "Big Fish\\n" . That string is 9 characters long, which means you need a size-10 buffer to store the null-terminator.

If a string misses the null terminator the output has undefined behaviour since the program has no way to know where the string ends.

Answer 2

When you perform the first copy, strcpy() copies in total 10 bytes (9 from the string itself plus the terminator). As you have allocated only 9 bytes in catfish , the terminator goes into the first byte of goldfish , which then gets overwritten when you copy the second string. Hence, when you do a printf() of catfish , it doesn't stop at the end of catfish but keeps printing until it finds the terminator at the end of goldfish .

In the second case you add enough space for the terminator not to be overwritten so when you print it will just print the contents of catfish as expected.