中止陷阱：strncat（）出现6个错误

Question

I am trying to write code where I must implement versions of the library functions strncpy, strncat, and strncmp but it gives me Abort trap: 6 error while running. Any ideas are much appreciated:

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

int main() {

    char str1[400];

    printf ("Enter the first string: ");
    fgets (str1, 400, stdin);

    char str2[400];

    printf ("Enter the second string: ");
    fgets (str2, 400, stdin);

    int num;

    printf ("Enter the number: ");
    scanf ("%d", &num);

    char dest[num];

    strncpy(dest, str2, num);
    dest[num] = '\0';

    printf ("strncpy is %s \n", dest);

    int lengthStr1 = strlen (str1);

    char str1copy [lengthStr1];
    strncpy(str1copy, str1, lengthStr1);
    str1copy [lengthStr1] = '\0';

    printf ("str1copy is %s \n", str1copy);

    strncat(str1copy, dest, num);
    printf ("strncat is %s\n", str1copy);
}

I know that my strncpy section works.

Answer 1

An array of size n has indexes 0 to n-1 .

When you declare your array like this:

char dest[num];

Then do this:

dest[num] = '\0';

You are accessing an offset one byte past the end of the array. Doing so invokes undefined behavior , which in this case manifests in a crash.

Since you want to copy num bytes into this array, the size should be 1 more to make room for the null byte.

char dest[num+1];

Then setting dest[num] makes sense.

There's a similar error with str1copy . In this case however using lengthStr1-1 as the offset isn't enough. You copy in lengthStr bytes from str1 then an additional num bytes from dest . So the length has to be the sum of those, plus 1 for the null terminating byte.

char str1copy [lengthStr1+dest+1];
strncpy(str1copy, str1, lengthStr1);
str1copy [lengthStr1] = '\0';

printf ("str1copy is %s \n", str1copy);

strncat(str1copy, dest, num);
str1copy [lengthStr1+dest] = '\0';
printf ("strncat is %s\n", str1copy);